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close this bookGuide to the Training of Supervisors - Trainees' Manual/Part 2 - For Labour-Based Road Construction and Maintenance (ILO, 1981, 254 p.)
View the document(introduction...)
View the documentPREFACE
View the documentACKNOWLEDGEMENTS
Open this folder and view contentsMODULE 10: DRAINAGE
Open this folder and view contentsMODULE 11: SOIL MECHANICS
Open this folder and view contentsMODULE 12: CONCRETE TECHNOLOGY
Open this folder and view contentsMODULE 13: STRUCTURES
Open this folder and view contentsMODULE 14: GRAVELLING
Open this folder and view contentsMODULE 15: MAINTENANCE

(introduction...)

L.S. Karlsson and J.J. de Veen


International Labour Office · Geneva

Copyright © International Labour Organisation 1981

This book enjoys copyright under Protocol 2 of the Universal Copyright Convention.

Purchasers may nevertheless freely photocopy the learning text for trainees for exclusive use within their company or training institute. However, no portion of this book may be reproduced for resale or mass publication without the express consent in writing of the ILO.

No part of this work may be stored in a retrieval system or transmitted in any form or by any means or translated without the prior authorisation of the Editorial and Translation Branch, International Labour Office, CH-1211 Geneva 22, Switzerland, to which all enquiries should be addressed.

Trainees' manual, Part 2: ISBN 92-2-102828-3
Complete set of trainees' manuals (two parts): ISBN 92-2-102829-1

First published 1981

ILO publications can be obtained through major booksellers or ILO local offices in many countries, or direct from ILO Publications, International Labour Office, CH-1211 Geneva 22, Switzerland. A catalogue or list of new publications will be sent free of charge from the above address.

Printed in Switzerland

PREFACE

This Guide has been produced for use in the general and technical training of supervisors for road construction and maintenance programmes. It is basically in two parts, an instructors manual and a trainees manual. The former contains information and advice for instructors on how to plan, design and implement a training course. The latter provides the detailed training material for the trainees. In its present form the trainees manual is incomplete in that it is not country specific. Thus, such aspects as project organisation and administration, programme objectives and design standards will vary from country to country. The instructor, therefore, uses the general modules presented and adapts them to the specific circumstances of the country.

The Instructors Manual is divided into three parts. The first discusses the various items which the trainer will have to consider when setting up a training course for a programme in a particular country. It shows how to identify training needs, how to evaluate the existing training facilities and how to plan the courses in relation to the resources available and the output required.

The second serves as an introduction to the training material, the main body of the Guide, and provides information on the modular system and its use. It briefly describes the contents of the 15 basic modules and outlines when and how the trainer should develop country/project-specific training materials, related to the particular country or project environment.

The third part is the instructors text of the training course. It provides information on methods of presentation of the course, advice on points to emphasise and the training aids that could be used. It is presented in the same modular format1 as the trainees text.

1 This system has been developed by the ILO to enable individual vocational training programmes to be compiled. More recently, the ILO has applied the system in a modular programme for supervisory development, aimed at a broad range of trainers, programme designers, training directors, personnel managers and supervisors.

The Trainees Manual contains the basic material for a complete training course for supervisors of labour-based road construction and maintenance programmes. It follows a format developed by the ILO for supervisory and vocational training: “The modular system”. In line with this format, the training material is presented in loose leaf form as a number of self-contained modules, subdivided into learning elements. This presentation allows the trainer to develop country/project-specific courses adapted to the level of different groups of trainees by simply selecting those learning elements he feels are relevant and by adding others reflecting the project situation and the local procedures and regulations.

In all, there are 15 modules. Each deals with a particular aspect of road construction and maintenance. The different modules cover such subjects as administration, planning, organisation, management and control as well as the most important technical topics such as survey, drainage, concrete technology, soil mechanics and structures. Modules 1-9 are contained in one folder, Modules 10-15 in another.

Field instructions are added at the end of each module providing the trainee with the nucleus of a site handbook after the completion of the course.

Setting up training courses is resource consuming, especially when the subject is new to the existing training institution. For this reason, training courses must be planned well in advance. To ensure maximum benefit, they should be integrated to the extent possible into the established training organisations. This would firstly ensure that labour-based methods are not set apart from other techniques, secondly that the career prospects of potential supervisors are not reduced and finally that existing facilities can be utilised.

ACKNOWLEDGEMENTS

This Guide has been produced with the financial assistance of the Swedish International Development Authority as part of its support to the ILO's programme on appropriate construction technology. The Guide is principally the work of Lars Karlsson and Jan de Veen, both engineers, who have worked in various labour-based road construction and maintenance programmes. Specifically, they were able to draw upon the experience they gained during the implementation of the Rural Access Roads Programme (RARP), a large-scale labour-based road construction and maintenance programme in Kenya. Geoff Edmonds1, an engineer who has been advocating and promoting the effective utilisation of labour in construction for the past eight years, was responsible for the initiation of the work on the Guide and provided valuable support, comments and suggestions throughout its preparation.

1 Messrs. Edmonds and de Veen are both members of the Technology and Employment Branch of the Employment and Development Department of the International Labour Office. Both work in the construction technology group of this branch. Mr. Edmonds is the co-ordinator of this group.

Andreas Wiederkehr, an engineer/trainer who has developed and implemented training courses for supervisors in several countries, provided the basic material on how to set up training programmes. The training material contained in the Guide is based upon existing material from many sources. However, the supervisory courses developed for the RARP by the Staff Training Department of the Kenya Ministry of Works deserve special mention. Finally, as mentioned in the Preface, the flexibility of the Guide is mainly due to the use of the modular system, which has been developed by the Training Department of the International Labour Office for use in managerial, supervisory and vocational training.

Learning Element: LE-0 Module learning objectives

After you have learned this module, you should:

- know why the drainage system is important;

- know the different types of drains, classify them by function and know how they are constructed;

- be able to recognise the symptoms of bad drainage and suggest ways of improvement

Learning Element: LE-1 Nature, definition and types of drainage

LEARNING ELEMENT OBJECTIVES

After you have learned LE-1, you should:

- know why a drainage system is necessary;
- recognise the different parts of the drainage system and be able to explain their function.

NATURE AND DEFINITION

Water contributes to the wear and damage of the road. The water can be in the form of ground water (inside the earth), surface water (ponds, streams), or rain (which will become surface water when it has reached and collected on the surface). Water can damage the road in two ways: by washing away the soil (erosion or scouring) or by making the road less strong to traffic (lowering the road bearing capacity).


Fig 1

It is therefore very important to have a good drainage system which allows for the water to flow off the road and away from it as quickly as possible. Such a system consists of several components:

- road surface drainage which makes the water flow off the road surface;
- side drains and mitre drains which lead the water away;
- catchwater drains which catch the surface water before it reaches the road;
- scour checks which prevent erosion in the ditches by slowing down the water;
- culverts which will lead the water in the side drains under the road to the other side;
- Water-table drainage which will lower the level of underground water.

All these drains have to work together if the results are to be good and it is you, as the construction supervisor, who have to set out, instruct labourers, control and decide if the drainage system is correct.


Fig 2

REMEMBER: THE DRAINAGE SYSTEM IS THE MOST IMPORTANT OF ALL THE DIFFERENT COMPONENTS IN EARTH ROAD CONSTRUCTION AND IF IT FAILS, THE WHOLE ROAD CAN BE WASHED AWAY OR MADE IMPASSABLE.

Learning Element: LE-2 Road surface drainage

LEARNING ELEMENT OBJECTIVES

After you have learned the LE-2, you should:

- know why an adequate surface drainage is important;
- know how to construct a proper surface drainage in various circumstances.

ROAD SURFACE DRAINAGE (CAMBER)

The purpose of the road surface drainage is to prevent water from eroding the road surface or penetrating into the road. In order to avoid such damage, it is necessary to lead the water away quickly and this is achieved by shaping the road so that the water will flow freely into the side drains. It is of course important that the surface of the roads is free from holes or ruts in which water could be trapped.

Camber

The camber is the slope from either side of the centre-line towards the sides. For earth and gravel roads this slope should be 5 per cent to 7 per cent.


Fig 3

Exceptions

In curves with a radius of less than 30 m, the camber is substituted by a super elevation which makes the water flow to the inside of the curve. At the same time the super elevation provides a certain resistance to a car skidding off the road due to the centrifugal force.

The centrifugal force also depends on the speed of the vehicle. Since access roads are generally not intended to be used at high speeds (usually less than 40 km/h) on this type of road the super elevation need only be used in curves with a radius of less than 30 m.

In hilly terrain where the road width is reduced, it can also be advantageous to build a super elevation sloping towards the hillside, instead of camber. This applies especially to sections which become slippery when wet.


Fig 4

Construction of camber

The camber is formed by spreading the soil which has been heaped along the centre-line during the ditching and sloping process towards the sides of the road. Naturally, the quantity of soil along the centre-line should be sufficient for the camber to be formed to the correct angle (7 per cent before compaction/settling, which is equivalent to approximately 5 per cent when settled).

IT IS MOST IMPORTANT THAT THE CORRECT MEASUREMENTS OF THE DITCH ARE KEPT AND THAT THE EXCAVATED SOIL IS PLACED IN THE MIDDLE OF THE ROAD ON THE CENTRE-LINE


Fig 5

The camber is controlled by using a camber board in conjunction with a spirit level. The camber should be controlled during the construction.

If found to be less than the required percentage, the camber has to be reshaped. Ensure, before the road is gravelled, that the road base has the correct camber.

Learning Element: LE-3 Side drains and mitre drains

LEARNING ELEMENT OBJECTIVES

When you have learned LE-3, you should:

- know the function of side drains and mitre drains;
- know the correct dimensions of the drains;
- know how to construct the side drain/mitre drain and how to control the depth and width.

SIDE DRAINS

The function of the side drains (or ditches) is to collect the water from the carriage way and surrounding areas. When they are excavated the outcoming material serves to form the very important road surface drain, the camber.

The size of the side ditches must therefore be sufficient to cope with the run-off water. The outcoming material should be sufficient to provide a compacted camber of 5 per cent. When a road with 4 1/2 m formation width is to be provided with a 5 per cent compacted camber (7 per cent loose) the size of the ditch should be as shown in figure 6 (area = 0.16 + 0.2 = 0.36 m2).


Fig 6

When there are ditches on both sides of the road the depth of each side ditch can be reduced to 0.3 m. The soil from the first ditch can partly serve to raise the elevation of the carriage way.

On flat or slightly undulating terrain you should aim to achieve a longitudinal gradient between 2 and 5 per cent (1:50 and 1:20). With gradients less than 2 per cent silting occurs easily while with gradients steeper than 5 per cent the ditches will erode easily.


Fig 7

Constructing the side ditches

The side ditches are constructed in two steps: first a rectangular trench is excavated (ditching) and then a slope is made from the bottom of the ditch up towards the carriage way (sloping). The soil coming out of the ditches is placed in the middle of the road.


Fig 8

The reason for excavating the ditch in this manner and shape is that it is easier to measure and control than a traditional V-shaped ditch made in one operation.

The correct width and depth is controlled with a stick which has been cut to correct length. Each worker who works on drainage should have his own stick.

Mitre drains

The mitre drains (or off-shoot drains) lead away the water from the side ditches to lower areas.

Where possible they should be placed at the intervals shown below. In principle, the more mitre drains the better!

Remember to make the gradient of the side drains a minimum of 2 per cent (even in flat terrain, see figure 7).

Road gradient

Mitre drain intervals

%

should not exceed metre

when discharging the water on to a piece of farmland metre

12

40

10

80

8

120

if exceeded

20 to 50 wherever possible onto a

6

160

scouring will occur

boundary

4

200


1-2

50

if exceeded silting will occur


Remember that these are maximum distances; the quicker the water is led off the road, the less damage (either from scouring or silting) will occur. Wherever possible, the discharged water from the mitre drains should be channelled to land boundaries in order to avoid damage to farm land. The soil coming out when the mitre drain is excavated should be deposited on the downhill side of the drain.

Also the mitre drains should have a minimum gradient of 2 per cent.


Fig 9

Learning Element: LE-4 Catchwater drains and scour checks

LEARNING ELEMENT OBJECTIVES

When you have learned this learning element you should be able to:

- describe the function of catchwater drains and scour checks;
- supervise their construction;
- calculate the distance between scour checks.

CATCHWATER DRAINS (CUT-OFF DRAINS)

Catch (water) drains are ditches more or less parallel to the road. Their function is to catch and lead away the surface water coming from higher lying areas before it reaches the road.

Wherever possible the catchwater drains should be diverted to natural water courses.

Catchwater drains usually have a trapezoidal cross-section and the excavated material should always be deposited on the downhill side of the drain.

Fig 10


Catch-water drains - Next to a cut


Catch-water drains - Next to an embankment

Scour checks

Where longitudinal gradients are steep the water gains high speed. Therefore, if no protective measures are taken, scouring is likely to occur. The simplest way of dealing with scouring, apart from reducing the volume of water by leading it off the road at frequent intervals (mitre drains) is to reduce the velocity of the water by building scour cheeks in the ditches.

The scour checks hold back the silt carried by the water flow and provide a series of stretches with gentle gradients interrupted by small “waterfalls”.

Scour checks are usually constructed in natural stone or with wooden stakes (figure 12). Masonry, or concrete scour checks can also be constructed (figure 11) but are often unnecessarily expensive.

Guideline for scour checks intervals: (scour checks height = 0.4 m)

Side or mitre drain gradient
%

scour checks interval
metre

12

5

10

10

8

15

6

40

4

not required!

2

not required!

Depending on the situation (the nature of terrain, type of soil) culverts will be necessary (LE-5). Use scour checks and culverts at regular intervals, especially where the road gradient is 6 per cent or more.


Fig 11

Construction of scour checks

1. The scour checks interval in the ditch is determined according to the slope, see table above.

2. At the place of each scour check the ditch is deepened and widened to provide room for securing the boulders or stakes. When stakes are used, they should be driven deeply into the ground and stones should be placed downstream of the stakes. See figure 12.

Fig 12


Scour checks made of stones


Wooden stake scour checks

Learning Element: LE-5 Culverts

LEARNING ELEMENT OBJECTIVES

When you have learned LE-5, you should:

- know the function of culverts;
- know how to construct culverts;
- recognise the various parts of an ordinary concrete pipe culvert.

DESCRIPTION AND FUNCTION

The culvert is a transversal drain under the road end and its function is to lead water from the higher ground on the side of the road to the lower ground on the other.

The most common type of culvert is a single line of concrete pipes. The diameter of the pipe should preferably not be less than 60 cm (24”) because 'of the tendency of blockage of smaller pipes.


Fig 13

Culvert rings

The rings are usually of concrete and can be manufactured in the field. The most common diameter is 60 cm (24”) but also 90 cm (36”) is frequently used. Smaller diameters than 60 cm are difficult to maintain and are easily blocked. The joints between the rings should not be cemented but covered with a strip of tar paper.

Depending on the circumstances, instead of using large diameters which require a high fill over the pipes (overfill), two or more rows of a smaller dimension can be used. The spacing between the rows should be at least one diameter of the rings and one row could be placed lower than the others to accommodate small flows without silting.


Fig 14

Culvert bed

The culvert bed has to be stable and at the correct level. Remove stones which might damage the pipes. If the natural material is not suitable, a bed of gravel should be made.

The bed should be constructed to the correct gradient (figure 13). A string, spirit level and folding ruler or measure tape can be used (M-7) to set out the right gradient.

The bed can, if the ground is swampy, be made “floating”. Such a bed should be of at least two layers (of round timber minimum diameter 7 1/2 cm, each layer across the other). The width of the “floating” bed should be at least one and half times the diameter of the rings. The timber should be covered with a layer of 20 cm gravel.

Apron

Aprons should be constructed at the inlets and outlets to protect the culvert bed and the ditch bottom from erosion. They can be made of handpacked stones, masonry or concrete. Their length should be at least one and a half times the pipe diameter for inlet and twice the diameter for the outlet.

Headwalls

The purpose of the headwalls is to support the road embankment and protect it from water damage. Headwalls should always be built parallel to the centre-line of the road in order to take the pressure from the traffic evenly. They can be built in handpacked stones, masonry or concrete. It is not always necessary to make headwalls and instructions to build them should come from the engineer who also would design them and specify materials.

Backfill

The backfill around the pipes has to be well compacted and should be made of gravel, sand or other suitable material (not expanding soils like “black cotton”). The backfill has to be well compacted., using hand-rammers and watering. The minimum compacted thickness of the layer on top of the pipe, the overfill, should not be less than three-quarters of the diameter of the pipe.

Gradients

The gradient of a culvert depends on the terrain situation but to prevent silting up and erosion the gradient should normally be kept within 3 and 5 per cent.

If the gradient exceeds 5 per cent, erosion at the outlet has to be prevented by an apron of stone or a paved waterway. Where possible the pipe inlet level should be at the same level as the original water course.

Spacing of culvert lines

Normally, natural water courses/low points in the terrain should determine where culverts should be placed. However, if no scour checks are made the following intervals should not be exceeded:

Road gradient
%

Culvert intervals
(m)

If these intervals are exceeded scour checks are required

12

40

After 40 m every 5 m

10

80

After 80 m every 10 m

8

120

After 120 m every 15 m

6

160

After 160 m every 40 m

4

200

Not required

2

240

Not required (max. interval)

When water is discharged on to farm land, the culvert intervals should not exceed 80 m; whenever possible it should be discharged at the place of a boundary between two farms.

In flat areas with drainage difficulties, it is often far better not to dig a culvert trench but instead to build up an embankment over the culvert.


Fig 15

Learning Element: LE-6 Water-table drainage

LEARNING ELEMENT OBJECTIVE

When you have learned this learning element, you should be able to:

- describe in which cases water-table drainage is necessary.

DESCRIPTION AND FUNCTION

The function of water-table drainage is:

1. to intercept, collect and lead away any flow of subsurface water in the subgrade of the road;

2. to lower the water-table;

3. to drain water “pockets”.

Water-table drainage can consist of various types of ditches and canals, open or subsurface.

The design of these will vary from case to case, and it is the job of an engineer to design various types of water-table drainage.

Embankment (valley crossing)

The most common method used is to build the road on an embankment so that the road base is elevated above the ground water and the zone of capillary rise.


Fig 16

The zone of capillary rise can be reduced by placing a layer of porous material like hard core, gravel, sand, etc. at the bottom of the fill. Another, but rarely used method, is to place an impervious membrane (plastic sheets) on the existing ground before the fill is made.

Drains (through swampy areas)

In order to lower a high water-table, drains are dug.


Fig 17

French drains

To drain water pockets French drains can be constructed.


Fig 18

Learning Element: LE-7 Module summary and checkpoint

LEARNING ELEMENT OBJECTIVES

After you have learned LE-7, you should:

- know why the drainage system is important;

- know the different types of drains;

- recognise some of the common shortcomings of poor drainage and suggest how to deal with them.

SUMMARY

The drainage system is a very important component of a good earth or gravel road. The different elements of the drainage system are surface drains, side drains, mitre drains, catchment drains, scour checks, culverts and water-table drains.

The road surface drain is more commonly known as the camber (in some cases super elevation).

Side drains are more commonly known as ditches from which the water is led away in mitre drains.

Catchwater drains collect and lead away water before it reaches the road.

Scour checks should be constructed whenever there is risk of erosion. They can be made of wood, concrete, handpacked stones or masonry.

Culverts are transversal drains carrying the water under the road.

Water-table drains are made to lower ground water levels or to drain water pockets (wells).

PROBLEMS

Discuss the following problems, their causes and how to solve them:

1. ponding of water on the road surface;
2. stagnant water in the side ditches;
3. erosion of the road surface along the road and erosion in the ditches;
4. erosion of a side cut;
5. blockage of a culvert.

MODULE CHECKPOINT

Indicate True (T) or False (F) in the blank.

1. It is essential that all components in a drainage system match each other.

.....

2. Too little camber can be the result of improper:



(a) levelling of ground;

.....


(b) ditching;

.....


(c) placing of camber material;

.....


(d) spreading;

.....


(e) width of the road.

.....

3. Scour checks are not needed when the soil is non-cohesive (e.g. sand).

.....

4. Catch-water drains are particularly useful in flat areas

.....

5. The correct overfill is not so important as long as the culvert rings are not cracked

.....

Answer briefly:

6. Why is the drainage system so important?
.................................................................................................................
.................................................................................................................
.................................................................................................................

7. Which are the different components in the drainage system?
.................................................................................................................
.................................................................................................................
.................................................................................................................

8. What is the purpose of the camber?
.................................................................................................................
.................................................................................................................
.................................................................................................................

9. What is the purpose of the side drain?
.................................................................................................................
.................................................................................................................
.................................................................................................................

10. How is the side drain constructed?
.................................................................................................................
.................................................................................................................
.................................................................................................................

11. What is the purpose of the mitre drain?
.................................................................................................................
.................................................................................................................
.................................................................................................................

12. In what way and why does the man-made ditch differ in shape from a V-shaped machine-made ditch?
.................................................................................................................
.................................................................................................................
.................................................................................................................

ACTION GUIDELINES FOR DRAINAGE

When doing the initial survey for the road, remember the question of drainage.

When you set out the ditch and slope, tie strings between the pegs to clearly mark the sides of the ditch.

Always explain to the workers why they have to do something in a particular way, e.g. the camber and why it is so important that it is correct.

Remember to observe how the drains work at the first rains following their completion and correct any inadequacies immediately.

Field Instructions

DRAINAGE

Remember that:

- drainage is one of the most important construction activities;

- drainage consists of road surface drainage (camber), side drains, mitre drains and cross-drains (culverts);

- drainage works should be carried out as soon as possible after the excavation. Catchwater drains should preferably be provided before the excavation is done;

- side drains have a double function:

(a) to collect and lead away the surface water; and

(b) to provide sufficient material for the formation of the camber of the road (road surface drainage);

- side drains are best done in two separate stages:

(a) trenching (digging of a rectangular or square ditch); and

(b) sloping (completing the ditch by providing the slope towards the centre-line of the road);

- scour checks will protect the drainage system and the embankments against erosion;

- observe closely how the drainage system performs during the first rains. Correct inadequacies immediately!

EXCAVATING THE TRENCH FOR THE SIDE DRAIN

Note: The soil must be thrown on to the centre-line.

Method

- Calculate the volume (v) of the excavation by multiplying the area (a) of the trench by the length (1).

- Calculate the number of men needed for the trenching by dividing the volume by the productivity rate for the type of soil concerned.

- Set out, together with the headman/gang leader, the width (w) of the trench with pegs and strings for the distance to be excavated during the day.

- Instruct the headman/gang leader on how many men are needed and which tools to use.

- Give the headman/gang leader measuring sticks to check the width and the depth of the trench.

Control

Before approving the work, ensure that the answer to the following questions is “Yes”:

- are the dimensions of the trench correct?
- has the soil been placed on the centre-line?

Tools

For excavation: hoe, forked hoe, mattock, fork, spade, pickaxe and shovel.

For setting out and control: strings, pegs, measuring tape and measuring sticks.

SLOPING OF SIDE DRAINS

Note: As for trenching, the soil should be thrown on to the centre-line.

Method

- Calculate the volume to be excavated by multiplying the area of the triangular section by the length to be covered during the day.

- Calculate the number of men required by dividing the volume by the appropriate task rate.

- Set out the width of the slope with pegs and strings, i.e. a string parallel to the excavated trench.

- Instruct the headman/gang leader and workers; the workers should loosen the soil into the trench from where it should be thrown on to the centre-line.

Control

Before approving the work, ensure that the answer to the following questions is “Yes”:

- are the measurements correct? Check with a wooden mallet having the shape of the ditch cross-section;

- is the surface between the string and the bottom of the slope straight and without a “hump”?

- has the soil been placed on the centre-line?

Tools

- As for trenching plus a mallet showing the shape of the completed ditch.

CAMBER FORMATION

The formation of the camber follows immediately after the sloping and consists of spreading the soil heaped along the centre-line towards the sides of the road. The spreading should be done in such a way that a camber of 5-7 per cent is obtained.

Method

- Calculate the volume to be spread by adding the volumes for the ditching and sloping over the distance in question.

- Calculate the number of men needed by dividing the volume of work by the task rate for spreading.

- Instruct the gang leader/headman on the tasks, the number of workers and the tools to be used.

- Instruct the workers that the soil has to be spread (raked) from the heap along the centre-line towards the edges of the road.

- Tell the workers why the camber is necessary, explaining that rain-water must run off the road surface quickly because standing water weakens the road and very quickly makes it impassable to traffic.

- Inspect the work and measure the camber with a camberboard (a straight-edge board of a predetermined shape). The camberboard is used in combination with a spirit-level.

Control

Before the camber formation can start:

- is the soil from the trenching and sloping placed as a ridge along the centre-line?

Before approving the work:
- is the gradient of the camber correct?

- are there any depressions in the road surface?

- is the crown (the highest point) of the road on the centre-line in the middle of the road?

Tools

For spreading: spreader (heavy rake, hoe or shovel).

For control: camberboard (straight-edge board and spirit-level).

SCOUR CHECKS

Scour checks reduce the speed of water in the ditches and thus reduce the scouring. They should be provided whenever the gradient of the side drain is greater than 6 per cent. Some soils erode more easily than others and on such soils scour checks might be necessary even at lower gradients. (You can see this when inspecting the road after the first rains.)

Method

- When the side drain has been excavated, measure the gradient with a dumpy level, abney level or a measuring tape/spirit-level.

- Find the scour check interval from the learning text table; the steeper the gradient, the shorter the distance between the scour checks.

- Deepen and widen the ditch at the place of the scour check to be able to properly place and secure the boulders or stakes used to construct the check.

- When stakes are used, they should be driven deep into the ground and supported with stones on the lower side.

Control

- Ensure that the scour checks have been placed at the correct intervals and that they are properly constructed.

LAYING CULVERTS

Method

- Set out the position of the culvert lines on the lowest points in the alignment and on additional places as required.

- Determine how deep the culvert trench will have to be (the diameter of the ring + 15 cm gravel bed if necessary + overfill). Do not forget that the outlet should be deeper than the inlet (gradient multiplied by length of trench).

- Calculate the number of rings needed by dividing the width of the road (at the level where the culvert will be laid!) by the length of one ring.

- Calculate the volume of the trench (length × width × depth).

- Transport the rings to the site, keep one ring in reserve on the site in case a ring breaks during the laying.

- Set out the outlet ditch (mitre drain) and the culvert trench with pegs and strings.

- Excavate the outlet ditch before the culvert trench is excavated to avoid the ponding of water.

- Excavate the trench; remember that not more than 3 to 4 people can work effectively at the same time in the trench. With trained workers, the work can be done as task work.

- Prepare the culvert bed, using gravel if necessary. Ensure that the culvert bed is laid to the correct gradient; check the gradient with a string or straight-edge board, combined with a spirit-level and measuring tape.

- Lower the pipes with ropes and place them in the correct position.

- Cover the joints with tar paper, backfill and compact in thin layers; add water if necessary to obtain proper compaction; the layers of backfill should not be thicker than 15 cm.

- Construct aprons and headwalls if required.

Control

- Before starting the work on the trench:

- is the outlet or mitre drain ready?
- are the rings on site and undamaged?

- Before laying the rings:

- is the gradient of the bed correct (3-5 per cent)?
- is the bed properly shaped and compacted?

- Before approving the total work:

- is the backfill properly done?
- is the compaction satisfactory?
- are both inlets and outlets cleared?

Tools

- For excavation: hoes, forked hoes, mattocks, forks, pickaxes and shovels.

- For lowering rings: strong ropes, car tyres as “cushions”.

- For properly placing the rings: wooden stakes (not crowbars!).

- For compaction: hand-rammers.

- For setting out and control: strings, straight-edge board, spirit-level, tape measure.

Learning Element: LE-0 Module learning objectives

After you have learned this module you should be able to:

- explain the principles of soil mechanics and describe when and how soil mechanics are applied in road construction;

- carry out simple field tests to determine the most important characteristics of soils;

- describe the functions of surface layers and the desirable composition of soils used for surface layers;

- explain the function of mechanical and chemical stabilisation and describe when and how mechanical and chemical stabilisation are applied in road construction.

Learning Element: LE-1 Nature, definition and of soil mechanics

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- explain the principles and function of soil mechanics;
- describe how soils can be characterised;
- describe for which purposes soil mechanics are applied in road construction.

GENERAL

Soil mechanics are applied to find out what different soils consist of and how they behave; also, to find out how soils (or mixtures of soils) can be improved so that they are suitable for various purposes (dam building, base layers, surface layers, etc.).

Soils can be characterised by a description of the following properties:

(i) the proportions of different grain sizes;
(ii) permeability;
(iii) cohesion;
(iv) plasticity;
(v) consolidation.

Simple tests are carried out to define these properties. It should be remembered that many soils are mixtures of e.g. gravel/sand/clay or silt/sand/clay. Therefore, although a series of simple field tests will provide an indication of the properties to be expected from a particular soil, laboratory tests will be necessary if a detailed soil classification is required.

Road construction

There are many different types of soil and some are not suitable for road construction. Before starting to build a road it is therefore important to find out which kinds of soil there are along the planned alignment. By taking samples of the soil and examining them, one can find the places which would cause problems, e.g. areas with black cotton soil or loose sand.

If the road cannot be taken around such areas of difficult soil, it is sometimes possible to improve the behaviour of the soil. Some ways of doing this are: (i) mixing it with another type of soil, (ii) adding chemicals to it, and (iii) providing a surface layer.

A knowledge of soil mechanics will help you in judging what to do in different circumstances.

Learning Element: LE-2 Simple field tests to determine soil properties

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- carry out three different preliminary tests to roughly determine the nature of the soil;

- carry out two tests to determine the proportions of the coarse and fine particles in a sample and one test to roughly determine the quantity of silt and clay present in the finest fraction;

- describe how fractions of soil are classified according to grain sizes;

- carry out two tests to determine whether a fine-grained soil contains mainly clay or silt.

It is possible to say a lot about a soil by only looking at it and touching it. Before any other tests are carried out the following preliminary identification should be made:

- Grain sizes. Is the soil mainly coarse or fine? Try to separate the coarse particles from the fine ones and roughly estimate which percentages are coarse/fine. Press the big particles between your fingers to find out if they consist of many small particles bound together (e.g. hard lumps of clay).

- Organic material. Does the soil contain many fibres or small roots? Does it look dull or dirty? When the soil smells earthy or of plants, it is likely to be organic. This odour will become more distinct when the sample is heated.

- Silt/clay or sand/gravel. Dry soils which contain a large percentage of sand/gravel feel coarse and gritty, while dry clay feels hard and smooth. Dry silt feels floury and will disintegrate into a fine powder when rubbed. When wet, sand/gravel will not stick to the fingers; while wet clays and silts feel sticky and will stain the fingers.

Since the particle sizes determine to a great extent the properties of a particular soil, the first tests we will discuss deal with this aspect. Particles are classified according to their size into fractions (gravels, sands, clays, or silt).

Table 1 shows the sizes and basic characteristics of these fractions.

Table 1

Fraction

Grain size (mm)

Shape

Cohesion

Plasticity

Permeability

Consolidation

Volume change

Effect in a soil mixture


Max.

Min.








Stones

60

2

Various

None

None

High

Little

None

Contributes to stability and strength

Sand











coarse

2.0

0.6

Angular

“Apparent”

None

High to

Slight

Slight

Contributes to


medium

0.6

0.2

and

when damp

medium

strength and


fine

0.2

0.06

rounded

stability

Silt

0.06

0.002

Angular and rounded

Very slight

Slight to medium

Medium to low

Slight

Medium

Contributes to instability especially when vibrated or wet

Clay

0.001

0.005

Plates, sometimes rods

Considerable

High

Low

High

Considerable

Contributes to strength by cohesion, but to instability due to plastic movement under pressure

Three simple tests to determine the proportions of gravel, sand and fine particles in a soil sample:

(1) Vibration test

Figure 1 shows an easy way to separate particles according to their respective sizes. First, place a dry sample on a board or a piece of cardboard. When you lift this board at one end and tap it slightly, the particles will be separated as the difference in weight will cause the finer ones to stay high and coarser ones to move downward.


Fig 1. Vibration test

If there are a lot of different sizes between the largest and the smallest, the sample is well graded. If only a few sizes can be seen, the sample is single-sized or poorly graded. Single-sized materials do not compact well, because there are no suitably sized small particles to fill the empty spaces between the bigger particles and to produce “mechanical interlock”.

(2) Settling test

An easily carried out method to define the proportions of the various soil fractions is the settling test (figure 2). A sample is placed in a glass jar with straight sides. Approximately half of the jar should be filled. Then water is added until the jar is three-quarters full. Add some salt to the water, as this will improve the settling of the finer materials. The jar is shaken vigorously and the soil is allowed to settle.


Fig 2

The gravel and coarse sand fractions will settle immediately. The finer sand and the coarse silt fractions settle more slowly, taking approximately half a minute. The clay and fine silt fractions will remain in suspension for some hours before they settle. The approximate quantities of each size can be seen as layers in the sample.

(3) Cohesion test

To determine whether a soil contains an important quantity of silts or clays, a handful of moistened soil should be moulded into a ball. When silts or clays are present, the ball will stay together and the hands will be stained. When the sample contains only fine sand the ball will stick together but will crumble at a touch. Then when the sample contains only coarse sand or gravel, the material cannot be moulded at all.

The above tests have shown how you can determine the proportions of coarse and fine particles and define whether silt or clay is present. We will now discuss three tests designed to define the clay and silt proportions within the fines fraction of the soil.

Silts and clays

Dry lumps of silt will tend to crumble and disintegrate into a fine powder when rubbed. Dry silt will rapidly absorb water.

Dry lumps of clay will be hard when a lump of clay is broken into smaller lumps; the exposed surfaces will have a glossy appearance. Dry clay will not rapidly absorb water. The following tests can be used to find out if a soil consists of clay:

Moulding test

When moistened, it will be possible to mould clay into a thread, Moist silt, however, will crumble or form small short threads (figure 3).


Fig 3

Drying test

Fill a matchbox with a moistened sample and allow the sample to dry out.

Fig 4


Moist clay


Dry clay

Figure 4 shows that clay after drying out will have shrunk and cracked. Silt will not shrink, but tends to crumble after it has dried.

As we have said before, many soils are mixtures of various fractions. This means that the results of the tests described above can only provide indications as to the behaviour which can be expected from these soils. Laboratory tests will always be necessary when a detailed soil classification is required.

To summarise the above tests, it can be said that:

(1) Coarse grain soils can be identified by the size of the particles.
(2) Fine grain soils can be identified by their behaviour when they are wet or dry.
(3) Organic soils can be identified by their smell and appearance.

Table 2 shows how you can describe different soils after testing.

Figure 5 shows the various steps to be taken when a soil sample is tested.

Table 2: Recognition of soils

Feature
(a)

Description of characteristics
(b)

Soil description
(c)

Strength

Cohesive

Mainly clay fraction, strength from “binder” action


Non-cohesive

Mainly sand gravel, strength from friction (mechanical interlock)

Particle or grain size

Coarse grain

Mainly sands and gravels (larger than 75 micron sieve)


Fine grain

Mainly silts and clays (passing through 75 micron sieve)

Particle size

Well graded

Wide range of sizes well distributed

Distribution or grading

Poorly graded

Too much of some sizes; shortages of others

(coarse grain soil fraction)

Uniformly graded (closely graded, single sized)

- limited range of sizes
- mainly one size

Contaminated by animal or vegetable remains

Organic or humic, peaty

Dull and dark in colour, has distinct odour. Topsoil is almost always organic Swamp soils

Texture

Gravelly or stony

Mainly large particles of rock may be dense or loose


Sandy

Coarse to fine gritty soil. May be loose or dense. Firm when damp


Loamy

Mixtures of silts, sands and clays, may be firm or soft


Silty

Fine soft soil, powdery when dry, very soft when wet


Clayey

Fine soil, hard lumps when dry and cracked surface Sticky and soft when wet


Clayey loam, silty clay, etc.

Any mixture in which one fraction, i.e., silt or clay is in the majority, giving its characteristic to the soil texture


Fig 5

Learning Element: LE-3 Function and composition of a surface layer

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- explain the function of a surface layer;

- describe when a soil is suitable to be used as surfacing material;

- describe the functions and desirable quantities of the various fractions in a surface layer soil.

A road can be built up of a number of different layers: and weakening the base and subgrade, subgrade, sub-base, base and surface layer. Roads which are built for traffic by a few light vehicles or on very good soils do not need all these different layers; it is usually sufficient to put on a good surface layer. Sometimes even the natural soil is strong enough to carry the projected number of vehicles. Surface layers serve to spread the traffic load (usually through a base), so that the subgrade (natural) soil is protected from overloading and deformation. Bituminous surface layers also seal the surface and prevent surface water (rain) from penetrating and weakening the base and subgrade.

In the case of a gravelled road this function is carried out partly by the gravel layer and partly by the camber of the road.


Fig 6

The type (bitumen, gravel, stabilised soil or natural soil) and thickness of the surface layer should ideally be determined by the expected traffic density and the type of the natural soil. However, usually other factors, such as available funds, the location of the road and the availability of suitable material in the area are more important when surface layers are designed.

The suitability of a soil for a surface layer depends on its behaviour in dry and wet weather. In dry weather a fairly high proportion of clay is desirable to bind the particles together and to prevent corrugation. However, in wet weather the presence of a lot of clay in the mixture is disadvantageous, because it makes the surface slippery and ruts are easily formed. Therefore, the specifications for a soil to be used as a surface layer have to be a compromise between the ideal requirements for wet and dry weather.

As a rough guide, a mixture of 35-65 per cent stones, 20-40 per cent sand and 10-25 per cent clay will produce an optimum soil for a surface layer. A higher proportion (up to 65 per cent) of suitably sized stones is preferable, as the strength and density of the hard stones will provide a stronger surface. The gravel will behave better if the stones in the mixture are of various sizes “well graded”, so that the voids in between the particles, are filled. Angular-shaped particles are preferable, because they will “lock” together better than round particles.

Pure clay and silts, mixtures of silt and clay, and organic soils are generally unsuitable for surface layers.

The stony particles to be used in a surface layer should not be bigger than 3 cm in order to obtain a reasonably smooth finish. Bigger particles should be crushed by hand or by roller.


Fig 7

A good surface layer should spread the traffic load evenly to the underlying base and prevent water from penetrating. It should also be resistant to corrugation and provide a non-slippery and dust-free surface.

Although such surfaces can hardly be made with earth or gravel, fair results can be achieved by avoiding pure clays and silts, mixtures of clays and silt, and organic soils.

A mixture of some 10-25 per cent clay and a good gravel (i.e. small stones of different sizes + sand) can provide a very good surface material.

Sand/clay mixtures also provide reasonable surfaces, although the strength and durability are less than a mixture in which stones (gravel) are present. Here again, not more than 25 per cent clay should be present in the mixture.

Learning Element: LE-4 Improvement of soils by mechanical means

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- describe what mechanical stabilisation is and when and why it is applied;
- describe two examples of mechanical stabilisation;
- carry out a test to determine the best mix of a clay and sandy/gravel soil;
- explain the function of compaction;
- describe various methods to compact soil;
- explain how compaction is specified in big road construction projects;
- describe a method of road construction in black cotton soil.

Instead of, or in combination with, the placing of a surface layer, the stability and strength of the soil can be improved by mechanical stabilisation. Mechanical stabilisation includes:

(1) the mixing of imported materials with in situ soil; and
(2) compaction.

The soil strength and stability is greatest when the soil contains the right quantities of hard, well-graded coarse particles and a binder of cohesive soil.

Four factors contribute to this strength and stability:

(1) the hardness and shape of the coarse particles. An angular shape is preferable, because the particles will lock together better;

(2) when the right quantities of coarse, medium and fine material of various sizes (well graded) are present and mixed in the soil, the empty space between the particles will be filled and the soil will have a high bearing capacity and density. The maximum desirable size of the coarse particles is 30 mm;

(3) when the right amount (10-25 per cent) of clayey material is present the stone and sand particles will be glued together by the cohesive qualities of the clay. Twenty-five per cent clay should be the maximum allowed; a lower proportion is desirable;

(4) the large particles within the soil will form a skeleton. When a load is applied to the soil, the resulting friction between the particles will contribute much to its stability.


Fig 8

When a soil does not have the qualities described above the soil composition can sometimes be improved by adding the missing components. For example, in dry desert areas, much can be done to improve the running surface of sand roads if clayey soil is mixed with the sand. In wet areas with a lot of clayey soil, the road can be improved by mixing in gravel or even decomposing rock or other material which contains a substantial proportion of hard angular particles.

To determine the best mix, the following test can be carried out:

Take a substantial (minimum about 50 kg) sample of the soils to be investigated. Try two or three mixtures, e.g. 1:1, 2:1 or 2:3. Thoroughly mix the soils and add approximately 10 per cent water. Pack the mixtures into suitable small containers and add more water. Observe which mix is the densest after the samples have dried.

Compaction

Compaction is used to pack the soil particles closer together, so that a more dense and stable soil results.

A volume of soil is composed of three components - solid soil particles, water and air. Air does not contribute to the strength and stability of the soil but, on the contrary, reduces its stability by allowing water movement within the soil. A certain optimum quantity of water (which is different for each soil type and varies usually between 8 and 20 per cent) simplifies the compaction and contributes to the soil's strength and stability, because it lubricates the particles and allows them to settle in a dense mass. Too much or too little water is not good. Less water is required for well-graded gravel/sand/clay mixtures.

Fig 9


Too much (Watermovement, no compaction possible)


Too little (High friction cannot be overcome by compaction)


Correct (dense mass)

There are several ways to compact soil:

(1) apply a dead weight or surcharge on top of the soil (non-vibrating rollers);

(2) use manually or mechanically operated tampers or rammers;

(3) apply a vibrating weight on top of the soil to overcome the frictions between the particles and to cause them to pack together more closely (vibrating rollers). This type of compaction is suited to granular soils, not to clayey/silty soils.

(4) leave the soil to settle naturally (indirect compaction) by leaving it for a period of time. Rainfall and the passing of traffic (which should be sensibly channelled to achieve an even compaction) will produce densities of the same values as achieved on normally compacted roads. The main disadvantage of this last method of compaction is that it requires a longer period (two-three months) to reach a good degree of density, while in the meantime, the soil is exposed to erosion.

The required state of compaction is normally specified relative to a laboratory compaction test. For example, compaction to 95 per cent means that the dry density of samples taken in the field should be 95 per cent of the dry density obtained in a specified laboratory compaction test.

If there is a choice of roller, rubber-tyred rollers are most flexible and can be used on most soil types. Loaded vehicles used for hauling (dump trucks, tractors and trailers) also compact very effectively. The drivers should be instructed to pass over the spread layer several times before they dump their load of material. Steel-wheeled rollers do not have the “kneeding” effect of rubber-tyred rollers and tend to compact unevenly, especially on soils spread by manual means. These rollers are most effective on the compaction of evenly spread surface material.

Sheep's-foot rollers are best suited to clayey soils. They are not very effective on sand and silt.

“Black cotton” soil

This type of soil is unsuitable to serve as a base for roads. When its moisture content changes it swells/shrinks rapidly. A good way of controlling these changes in volume is to provide a layer (minimum 15 cm thick) of sand between the black cotton soil and the surfacing layer. It is also recommended to increase the thickness of the gravel layer.

Learning Element: LE-5 Improvement of soils by chemical means

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- explain what chemical stabilisation is;
- describe when and why chemical stabilisation is applied;
- describe two methods to carry out chemical stabilisation.

Roads provided with a bituminous surface layer must have a base layer which:

(1) is strong enough to carry the traffic loads as they are being transmitted through the bitumen layer;

(2) is of uniform quality so that the traffic loads will be uniformly distributed to the subgrade;

(3) allows as little water as possible to seep through;

(4) does not change its properties when wet.

One method to provide a suitable base layer for a bitumen surfaced road is to mix the natural soil with cement or lime. The soil becomes more stable, because the cement or lime reacts chemically with the soil particles and binds them together.

Organic soils cannot be stabilised with cement or lime because they contain components (acid) which prevent this chemical reaction from taking place.

The soils to be chemically stabilised should have three characteristics:

(1) it should be possible to break them into fine particles, so that they can be mixed properly with the stabiliser (cement/lime);

(2) they should be well graded;

(3) they should be free from organic particles.

The important advantage of cement/lime-stabilised soils is that they have a high strength in both dry and wet conditions. The proportion of cement or lime required normally ranges between 3 and 7 per cent by weight of the dry soil. While cement can be used to stabilise both plastic (clay containing) and non-plastic soils, the presence of clay minerals is necessary when lime is used as the stabiliser.

When the thickness of the layer to be stabilised is known, the volume per square metre can be calculated. For example, when a 20 cm layer is to be stabilised, this means a volume of 0.2 × 1 × 1 = 0.2 m3 per square metre. The density of the dry soil can be determined in a laboratory test. For example, if this density is 1,000 kg/m3 this means that the weight of the layer per square metre is 200 kg. With a cement proportion of 5 per cent, this means that 10 kg per square metre are required (one bag per 5 square metres).

Learning Element: LE-6 Module summary and checkpoint

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- explain the principles of soil mechanics and describe when and how soil mechanics are applied in road construction;

- describe simple field tests to determine the most important characteristics of soils;

- describe the function of surface layers and the desirable composition of soils used for surface layers;

- explain the function of mechanical and chemical stabilisation and describe when and how mechanical and chemical stabilisation is applied in road construction.

SUMMARY

Soil mechanics serves:

(1) to determine the composition and the properties of the soils;

(2) to define how the strength and stability of soils can be improved, so that they are more suited for construction purposes.

- Coarse grain soils can be identified by the sizes of the particles.
- Fine grain soils can be identified by their behaviour when dry and wet.
- Organic soil has a distinct smell and appearance.

Table 1 on LE-2/3 shows the characteristics and properties of the various soil fractions.

Surface layers consisting of gravel or improved soil serve to spread the traffic load through to the natural soil, so that this natural soil is protected from overloading and deformation.

An optimum soil for a surface layer will contain 35-65 per cent well-graded angular-shaped gravel, 20-40 per cent well-graded sand and 10-25 per cent clay. Silt is not suitable for use in road construction.

Mechanical stabilisation includes (1) the mixing of soils in order to obtain a more suitable composition and (2) compaction to improve the density and stability.

Compaction can be done best when the soil contains a certain amount of water (the optimum moisture content). Compaction increases the bearing capacity of a soil and its resistance to water movement, because the particles are packed densely together.

Chemical stabilisation is the mixing of soils with cement or hydrated lime. Chemically stabilised soils serve as base layers for bituminised roads and are not suitable as surface layers.

PROBLEMS

Discuss the following problems and recommend a suitable solution:

(1) A road leads through clayey terrain and becomes impassable in wet weather. Recommend two different approaches to make the road passable. Calculate for each solution how much material would have to be imported per kilometre of road and describe which operations have to be carried out to implement the solution.

(2) When the terrain mentioned in (1) consists of silt, can we use the same approaches as above? If not, can you suggest an alternative?

(3) When a certain fine grained soil was tested in the field, all field tests indicated that this soil consisted of a mixture of clay and silt. Laboratory tests proved that the soil contained 60 per cent silt, 30 per cent clay and 10 per cent sand. Can we mechanically stabilise this soil? If yes, which materials should be added? Can we chemically stabilise this soil? If yes, should we use cement or lime?

MODULE CHECKPOINT

Indicate True (T) or False (F) in the blank:

1. Organic soil mixed with the right amount of clay can be chemically stabilised with lime

....

2. A mixture of sand and clay can be suitable for a surface layer of a rural road with a traffic density of approximately 50 vehicles per day

....

3. Dry silt rapidly absorbs water

....

4. A sample of wet silt will shrink when it is dried

....

Answer briefly:

5. What is the meaning of the word “well graded” in soil mechanics?
...............................................................................................................
...............................................................................................................
...............................................................................................................

6. What would be an ideal mixture for a rural road surface layer soil? Comment on the sizes of the different soil fractions.
...............................................................................................................
...............................................................................................................
...............................................................................................................

7. Why is a road compacted and why is an optimum moisture content important?
...............................................................................................................
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Field Instructions

FIELD TESTS TO DETERMINE THE NATURE OF THE SOIL

Remember that:

- before starting to build a road you should find out which kinds of soil there are along the planned alignment. Take some samples of the different types of soils you find along the road;

- local people living along existing old tracks can often provide valuable information concerning:

(i) the behaviour of streams and rivers;
(ii) the behaviour of soil types (black cotton soil, clay, loose sand) in different types of weather;
(iii) the location of gravelling material;

- to determine the proportions of gravel, sand and fine particles in a soil sample, you can do the vibration test and the settling test. The cohesion test will tell you whether or not the soil contains silt or clays. Dry sand/gravel soils feel coarse and gritty. Dry clay feels hard and smooth. Dry silt feels floury. Wet clayey, silty soils feel sticky and will stain the fingers;

- the moulding test and the drying test will help you to distinguise clay from silt. Moist clay can be moulded into long threads and cracks and shrinks when it dries out. Moist silt forms, when moulded, only short threads. Dry silt will crumble and disintegrate into fine powder if it is lightly rubbed;

- organic material (topsoil) contains many fibres or small roots and smells distinctly earthy and of plants, especially when heated.

THE GRAVEL LAYER

Remember that:

- a mixture of roughly 35-65 per cent stones, 20-40 per cent sand and 10-25 per cent clay will produce a good surface layer. The stone proportion should preferably be on the high side (between 50 and 65 per cent);

- the stone fraction in the mixture should be well graded, i.e. of various sizes;

- clayey, silty and organic soils are generally unsuitable for surface layer;

- stony particles bigger than 3 cm should be crushed with a sledge-hammer or by roller;

- if stones (gravel) are not available, reasonable surface layers can be obtained by a mixture of sand and clay (maximum proportion of clay 25 per cent).

COMPACTION

Remember that:

- compaction is used to pack the soil particles closer together so that a denser and more stable soil results;

- compaction is best done when the soil contains a certain quantity of water (between 8 and 20 per cent);

- compaction can be done by non-vibrating rollers, vibrating rollers, mechanically operated tampers, hand-rammers or indirectly by natural settling;

- vibrating rollers are performing best on granular soils;

- rubber-tyred rollers can be used on most soil types;

- loaded vehicles can also be used quite effectively to compact, e.g. freshly dumped gravel;

- sheeps'-foot rollers are best suited to clayey soils;

- black cotton soil shrinks and swells rapidly when it dries out or becomes wet and is therefore unsuitable material for a road base. If the alignment passes through a black cotton soil area, a sand layer of minimum 15 cm thickness should be provided before the surface layer is placed. Another way of limiting the bad effects of the black cotton soil is to provide a gravel layer of an increased width and thickness.

Learning Element: LE-0 Module learning objectives

After you have learned this module you should be able to:

- describe the composition of concrete;
- describe the various steps in the manufacturing process;
- explain how concrete should be placed, compacted and cured;
- describe how the ingredients should be stored, mixed and handled;
- describe how concrete culvert rings are manufactured.

Learning Element: LE-1 Definition and characteristics of concrete

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- explain the nature and composition of concrete;
- describe the different factors influencing its final strength;
- explain why concrete is reinforced in certain circumstances

DEFINITION AND CHARACTERISTICS

Concrete is a mixture of aggregate, adhesive and water. The aggregate usually comprises a mixture of various sizes of gravel and sand. Cement is normally used as the adhesive, i.e. binding the aggregate together. When water is added to the adhesive, a chemical reaction takes place which causes the mix to harden.

Figure 1 shows the composition of concrete.


Fig 1

The density and strength of the finished concrete very much depends on the distribution of the particle sizes.

It is therefore very important to mix the right quantities of various sizes of gravel and sand.

The bigger-sized particles of the gravel form the skeleton of the concrete which gives the concrete its compression strength. The smaller-sized gravel particles and the sand fill the empty spaces between the bigger particles, while the water cement paste fills the smallest spaces, coats the particles of the aggregate and binds these together. By using different quantities of gravel, sand and cement, concrete of various strengths can be produced. The desired mix therefore depends on the purposes for which the concrete is to be used.

When, after the aggregate and the adhesive have been thoroughly mixed, water is added the concrete stays as a flexible plastic material for approximately one hour. It should therefore be handled quickly and poured into its final shape before the setting starts.

The compression strength of the concrete increases continually until approximately 95 per cent of the final strength has been reached after a period of 28 days. The increase in strength is most rapid during the first 24 hours of the hardening process.

The tensile strength of concrete is low. To improve the tensile strength reinforcement steel is embedded in the concrete in those places where tensile stress will occur. After being firmly attached to the hardened concrete the steel bars serve to take the tension (reinforced concrete). The climatic conditions greatly influence the final strength of the concrete. When the outside temperature is high, the water in the concrete will evaporate quickly which may give rise to cracking and loss in strength.

A high humidity is an advantage because this causes the setting and hardening process to take place more gradually. The ideal climatic conditions for setting and hardening are a constant temperature of 20°C and a relative humidity of 80-90 per cent.

Learning Element: LE-2 Cement (properties and storage)

LEARNING ELEMENT OBJECTIVES

After you have learned this element, you should be able to:

- understand the nature of cement and its function in the manufacture of concrete;
- understand the importance of careful handling and proper storage;
- describe the correct manner of storage.

CEMENT AND PROPERTIES

Cement is made from a mixture of lime-based material and clayey material. Cement and water react chemically. This chemical action causes the water-cement paste to harden and to become a solid mass. As discussed in LE-1, we make use of this property of cement to bind aggregate, sand and cement particles together to obtain concrete. The very fine cement particles also fill the smallest empty spaces, thereby giving the concrete its density and impermeability. It is understandable therefore that within certain limits, concrete becomes stronger when more cement is used.

However, cement is the most costly of the materials required to make concrete. This implies that in order to minimise the costs of the concrete, the amount of cement used will depend on the purpose for which the concrete is required.

Concrete used for culvert beds and foundations needs less cement than concrete used in reinforced bridgedecks. Table 1 shows the cement proportion (kg/m3) in various types of concrete.

Table 1: Cement required for 1 m3 of finished concrete

Type of concrete

Purpose

kg cement/m3 concrete

Lean concrete

Culvert beds

175

Mass concrete

Foundations, etc.

250

Dense concrete

Culvert pipes, light reinforced structures

300-350

Dense, plastic concrete

Heavy reinforced structures

350-450

Storage

It is important to remember that the cement bags must be handled carefully. If a bag is torn the moisture in the atmosphere will be sufficient to start the chemical action causing the cement to harden. Hardened pieces found when using the cement must be removed, because the strength of the concrete will be reduced if they remain inside the batch. The bags have to be stored in such a way that the chances of the bags getting damp are minimised. Figure 2 shows a correctly constructed cement store. The floor is at least 30 cm from the ground, firstly to avoid moisture penetration from below into the floor of the store and secondly to ensure a better ventilation.


Fig 2

Learning Element: LE-3 Aggregate

LEARNING ELEMENT OBJECTIVES

After you have learned this element, you should be able to:

- explain the function of the aggregate in concrete;
- describe various sources and types of aggregates;
- explain the precautions to be taken in the selection, use and storage of the aggregates.

The aggregate forms the “skeleton” of the concrete. Various sizes of aggregate are required to make a strong concrete. Stone grains up to 5 mm (usually sand) are used as “fine” aggregate, while stone grains bigger than 5 mm (gravel, crushed stone) constitute the “coarse” aggregate.

Aggregate can be obtained from various sources. Natural aggregates for direct use without any mechanical treatment can be extracted from riverbeds, gravel pits, sea and the dunes. These aggregates have round and polished particles.

Before using aggregates for concrete manufacture it is advisable to carry out tests to ensure that the particles are of sufficient strength and are not prone to erode or deteriorate. Sands from gneiss or mica schist and soft lime stones are examples of unsuitable aggregate material for concrete manufacture.

The porosity percentage of aggregate should be very small and the materials should be neither brittle nor soft. For large works it is common practice to produce aggregates by crushing hard rock, such as basalt, quartzite, granite, limestone and porphyry. Aggregate particles produced in this manner are usually sharp edged and angular.

In principle it is advantageous to use natural rather than crushed aggregate for concrete work as the round shape of natural aggregate increases the workability of the mix. Moreover, the particles have a smaller surface area so that less cement and water are needed to obtain the required strengths.

However, a well graded, well compacted concrete mix with angular aggregate particles also produces an excellent concrete, because the particles “lock” together to form a strong “skeleton”.

If the aggregate contains a large percentage of flat saucer-shaped particles it should be rejected as far more cement and water would be required to obtain acceptable results.

It is necessary to wash the aggregate if it contains impurities, as these negatively influence the strength of the finished concrete. Clay impurities, especially, cause a high level of shrinkage, and prevent the concrete-steel adhesion in reinforced concrete. Dust and fine crushing residues reduce the mechanical strength and chemical resistance. To keep the aggregate clean, they should be stored on clean hard ground (preferably a concrete floor) away from trees. When possible, aggregates should be stored under shelter to avoid changes of the moisture content.

Aggregates of different sizes should be stored separately. Too often, heaps of different-sized gravels are dumped next to each other with no separation in between. The heaps overflowing into each other cause the different sizes to get mixed. As a result it will be very difficult to obtain a mixture containing the right quantities of different-sized aggregates.

Learning Element: LE-4 The manufacture of concrete

LEARNING ELEMENT OBJECTIVES

After you have learned this element, you should be able to:

- explain how the density and final strength of the concrete is influenced by the proportions of the components;

- describe various mixtures for different types of concrete;

- say approximately how much concrete of various types can be produced with one bag of cement;

- describe how concrete is mixed by hand and by concrete mixer.

The final strength of the finished concrete depends on:

1. the proportions of the components (i.e. whether the correct quantities of gravel, sand, cement and water have been used);

2. the quality of the components;

3. the distribution of the grain sizes of the gravel and sand;

4. the way the components are mixed;

5. the way the mixture is transported, placed, compacted and cured.

Proportion of the components

When the mixture is prepared take care that:

1. the proportions of the aggregates used in the mixture are correct;
2. the right quantities of cement and water are added.1

1 The water should be fresh and clean. If the only available water contains a lot of silt, it could be stored in drums to allow the silt to settle on the bottom of the drum before use.

Sand and gravel contain particles of different sizes with empty spaces in between. Generally, you can assume that sand and gravel, in a dry condition, consist of 60 per cent solid matter and 40 per cent empty spaces.

To obtain a dense concrete the empty spaces need to be filled. The empty spaces between the sand grains will be filled by the cement while the sand-cement-water paste (mortar) will fill the empty spaces between the gravel particles.

An extra 10 per cent of mortar is necessary to “coat” the particles completely.

This means that for a certain quantity of gravel we would need 50 per cent wet mortar in order to fill the empty spaces and to coat the particles. In theory, therefore, a mixture of 1:2:4 (cement: sand:gravel) will produce a dense concrete. In practice, when a dense but plastic concrete is required, a mixture of 1:2:3 is often applied mainly because of the variation in the percentage of empty spaces. However, we have already seen (LE-2) that different mixes can be used depending on the purpose for which the concrete is used. Table 2 shows various mixtures producing different types of concrete (volumetric proportions).

Table 2:

Type of concrete

Mixture
(cement:sand:gravel)

Purpose

Lean concrete

1 : 4 : 8

Culvert beds, fills

Mass concrete

1 : 3 : 6

Non-reinforced structures

Dense concrete

1 : 2 : 4

Culvert pipes, light reinforced structures

Dense, plastic concrete

1 : 2 : 3
1 : 1.5 : 3

Heavy reinforced structures

YOU SHOULD REMEMBER THAT THE ABOVE TABLE PROVIDES RULES OF THUMB ONLY.

If a very high quality concrete is required, laboratory tests must be carried out in order to determine the optimum granulometric composition of each aggregate, i.e. what are the best proportions of particles of different sizes in a particular type of aggregate.

Remember that the volumes of cement and sand can vary with the degree they are compacted! In addition, the volume of sand varies with its moisture content. For example, the volume of one kilogramme of sand increases by 15-50 per cent if the water content is increased by 3-12 per cent. For these reasons, volumetric proportioning as shown in table 2 is never used when big quantities of high quality concrete must be produced. In this case, the materials are weighed and the moisture content is measured, so that the exact quantities of the different components (gravel, sand, cement and water) can be determined. For most concrete works carried out on rural roads, however, the above rules of thumb can be applied.

Water/cement ratio

The quantity of water divided by the quantity of cement gives the water/cement ratio. The water/cement ratio varies between 0.4 and 0.5 for practically all types of concrete. This gives the minimum amount of water necessary to react with the cement and to make the mixture workable. To ensure sufficient workability for hand-mixed and hand-placed concrete sometimes more water is necessary (water/cement ratio between 0.5 and 0.65) but great care should be taken not to use too much water. The wetter the mix, the weaker the finished concrete !

It is very important to remember that much more than 1 m3 of components is required to produce 1 m3 of concrete. This is easily understood when we realise that the cement fills the empty spaces between the sand particles and the mortar fills the empty spaces between the gravel particles. Table 3 shows the material required to produce one cubic metre of finished concrete.

Table 4 shows the approximate yield of concrete per bag of cement for the four different mixtures. The tables assume that one bag of cement weighs 50 kg and has a volume of 40 litres.

Table 3:

Concrete type

Material required to produce 1 m3 concrete


Cement (kg)

Sand (m3)

Gravel (m3)

1 : 4 : 8

3 bags (150 kg)

0.48

0.96

1 : 3 : 6

4 bags (200 kg)

0.47

0.93

1 : 2 : 4

5.5 bags (275 kg)

0.44

0.88

1 : 2 : 3

6.5 bags (325 kg)

0.51

0.77

Table 4:

Concrete type

Material required per bag of cement

Approx. yield per batch (m3)


Cement (kg)

Sand (m3)

Gravel (m3)


1 : 4 : 8

50

0.16

0.32

0.34

1 : 3 : 6

50

0.12

0.24

0.27

1 : 2 : 4

50

0.08

0.16

0.19

1 : 2 : 3

50

0.08

0.12

0.16

Gauge boxes can be used to measure the volumes of the components. Figure 3 shows a gauge box having a volume corresponding to the volume of 50 kg of cement (weight/volume = 1.25) i.e. a bag of cement has a volume of 40 litres.

Length 40 cm


With 25 cm

Inside measurements cm volume 40 litres

Height 40 cm



Fig 3

Mixing

The purpose of mixing is to obtain a homogeneous mixture of a maximum compactness and a suitable workability.

Hand mixing

When mixing is carried out by hand a suitable floor (metal sheets, boards, lean concrete) should be available to ensure that the mixture is not contaminated with soil. This floor should be sufficiently large to permit a continuous mixing process. Figure 4 shows a good way to mix by hand.


Fig 4

Steps

- The size of the batch should be 1/3 cubic metre maximum.

- Mix the aggregate. Place first a layer of aggregate, then a layer of sand and finally a layer of coarse aggregate. Mix thoroughly by turning the heap over several times. This can be done best by two men facing each other from opposite sides of the pile.

- Add cement and turn over until the batch is uniform in colour.

- Sprinkle the predetermined quantity of water gradually on top of the pile while the heap is turned over another three times.

Mechanical mixing

Mechanical mixing produces a more homogeneous and better mix. Never fill the concrete mixer completely.

A large number of different types of concrete mixers exist. Some commonly found types are:

- tilting drum mixers (figure 5);
- rotary drum (non-tilting) mixers (figure 6).


Fig 5


Fig 6


Schema

1. Hooper
2. Drum
3. Discharge chute

Tilting drum mixer

The drum rotates on an inclined axis when mixing and on a tilted axis for discharging. Three positions are used:

- charging position;
- mixing position;
- discharging position.

Rotary drum (non-tilting) mixers

The drum has large openings at both ends, one for feeding in the materials to be mixed and the other for discharging the concrete down a chute which is lowered when mixing is completed.

Loading concrete mixers

The following procedure is recommended for loading concrete mixers:

(1) place a part of the water and some coarse gravel into the mixer to clean the drum walls of any concrete left over from the previous mix;

(2) add the contents of the container which has been filled as shown in figure 7. Take care to never fill the concrete mixer completely. Mix dry for one minute;

(3) add the rest of the predetermined quantity of water and mix for another 1-2 minutes.


Fig 7

When only a gauge box is available to load the machine the mixer can also be charged as follows:

(1) charge a part of the water, enough to wet the drum;
(2) charge half the volume of coarse gravel;
(3) charge the sand and finer gravel;
(4) charge the cement;
(5) charge the remainder of the coarse gravel;
(6) mix dry for one minute;
(7) add the water and mix wet for another two minutes.

The mixing time varies with the proportions and the total quantity of the components, the capacity and rotation speed of the drum and the desired plasticity. Usually 1.5 to 3 minutes is sufficient to obtain a good mixture. Mixing more than 3 minutes does not improve the quality of the mixture.

Ensure that all the tools, platforms and mixers are thoroughly cleaned after the mixing has terminated. If the left-over concrete hardens, the equipment will not be usable the next time.

Learning Element: LE-5 Placing, compacting and curing of concrete

LEARNING ELEMENT OBJECTIVES

After you have learned this learning element, you should be able to:

- explain why transport, placing, consolidation and curing of concrete is important;

- describe the correct procedures in respect of transport, placing, consolidation and curing of the ready-made mixture.

TRANSPORT OF CONCRETE

The transport time of the ready-mixed concrete should be as short as possible. Ideally, the concrete should be poured within 15 minutes after the mixing has been terminated. When rotating drum trucks are used, a maximum of two hours can be permitted for transport.

When wheelbarrows are used for transport, take care to avoid long barrow runs over rough ground because the vibration will cause the segregation of the coarse particles from the fine. To avoid drying out, wet sacks can be used to cover the concrete in the wheelbarrow.

Also ensure that the concrete mixture is not dropped freely from heights greater than 1.5 metre. This causes segregation as the coarse particles drop more rapidly than the finer ones. For the same reason, the mixture should not be thrown far with the shovel, but should be taken as near as possible to where it has to be poured.

Clean and oil the forms or shutters before the concrete is placed!

Placing of concrete

The concrete should be placed in layers not higher than 30 cm when compacted by hand and in layers not higher than 60 cm when compacted by poker vibrator (figure 8).

Fig 8


Hand-rammed concrete


Vibrated concrete

Pouring in layers is correct whereas pouring in heaps causes segregation (figure 9).


Fig 9

The consolidation/compaction of concrete is necessary to improve the density, imperviousness and strength of the finished product as well as to improve the adhesion of the reinforcing bars to the concrete. The consolidation can be done by hand with hand-tampers or iron rods. A better method is to use the poker vibrator (figure 10). This is a steel tube, housing a rotating excentric mass driven by compressed air or a petrol engine.


Fig 10

The vibrator is immersed into the concrete (not further than 2/3 of its length) at distances of ca. 50 cm. When the water wells up to the surface it is slowly taken out. Do not vibrate longer than 15 seconds in one place and do not place the vibrator closer than 10 cm to the formwork. Too long vibration can cause the particles to segregate!

Curing of concrete

Curing of concrete is necessary to prevent surface evaporation of water during the setting and hardening stage. Curing means preventing evaporation by keeping the exposed surface of the concrete moist for a period of at least seven days.

This can be done by either:

(1) sprinkling or flooding the surface frequently;

(2) covering the surface with wet jute, paper bags, sand, sawdust, banana or palm leaves or similar materials.

Proper curing will ensure that cracks in the surface layers, caused by an insufficient binding due to the non-availability of water, will not occur. Freshly poured concrete should never be exposed to intensive sunlight.

Learning Element: LE-6 Manufacture of concrete culvert rings

LEARNING ELEMENT OBJECTIVES

After you have learned this element, you should be able to:

- describe how concrete culvert rings are manufactured;
- explain which factors have to be considered when a manufacturing site is selected.

Culverts form an essential part of the drainage system of the road. It is of the greatest importance that the quality of concrete culvert pipes is good.

Circular culvert pipes are normally manufactured making use of steel culvert moulds of different sizes (figure 11).

Fig 11


Steel moulds


Plan

The most common diameters for culverts are 60 and 90 cm. The length of a pipe can vary between 75 and 100 cm.

Manufacturing site

When the culvert pipes are to be manufactured locally, the manufacturing site should be chosen considering the following:

(1) the costs to transport the materials (gravel, sand, cement and clean water) to the site;

(2) the distance to the places to be supplied with the produced culvert rings;

(3) the availability of ample space for storing of materials and culverts.

Remember that ready-made culverts need at least seven days curing and should not be transported within that period.

The following (temporary) structures are required:

(1) cement store;

(2) bays for the aggregates (LE-3) which could be made of a lean concrete;

(3) a mixing platform covered with a simple roof. The platform should be levelled to prevent water from flowing off;

(4) a loading ramp to enable the workers to roll the pipes onto the lorry.

The moulds should be cleaned after each stripping and oiled before they are used again. A mix of two parts diesel with one part used oil should be applied with a brush to the inside of the mould, the hinges, wedges, etc.

Make sure that the inside mould is carefully centred to ensure a uniform thickness of the ring (figure 12).


Fig 12

The mixture for concrete culvert rings should be 1:2:4 (1:2:3 for sizes bigger than 90 cm).

Use aggregates of different sizes (e.g. 40 per cent aggregate 1/4” and 60 per cent aggregate 1/2”). Compact the concrete with a steel bar and by pounding the moulds lightly with rubber-headed hammers producing vibration (figure 13).


Fig 13

The moulds can be taken off the concrete rings after 24 hours if the rings themselves can be left undisturbed. Wait at least 24 hours more before you move the rings.

The rings should be transported standing in a 10-20 cm layer of sand (or similar shock-absorbing material).

Learning Element: LE-7 Module summary and checkpoint

LEARNING ELEMENT OBJECTIVES

After you have learned this element, you should be able to:

- describe how concrete is manufactured;

- recognise which factors contribute to the final strength of the finished concrete;

- describe which measures can be taken to produce a concrete of an optimum quality.

Concrete is a mixture of aggregate, adhesive and water. The chemical action of the adhesive and the water causes the mixture to harden. The hardening to approximately 95 per cent of the final strength takes 28 days. The increase in strength is most rapid during the first 24 hours. The final strength of the concrete depends on:

(a) the quality and quantity of the components (usually gravel, sand, cement and water) used in the mixture;

(b) the mixing, placing and curing procedures.

The aggregate should not contain more than 5 per cent impurities. Proper storage and - in some cases - washing before use is necessary. Cement should be kept dry as it hardens when it becomes moist. Water should be used sparingly. Usually the water/cement ratio,

This means that approximately 25 litres of water per bag of cement should be used. If the mixture is not workable, one of the following measures can be taken to improve the workability:

(1) add more cement so that the quantity of water may be increased;

(2) improve the granulometric composition of the aggregate by changing the quantities of certain sizes of aggregate.

Different types of concrete can be produced by varying the gravel/sand/cement ratios.

The concrete can be mixed by hand or mechanically. The mixing should be carried out and controlled carefully so that a homogeneous mixture results.

Transport and place the ready-made mixture within a period of one hour after the water has been added. The shorter the transport and placing time, the better the end result.

Compaction is necessary to obtain a maximum density. This compaction can be done by hand (iron rod, rammer) or mechanically (poker vibrator). The exposed surface of the concrete has to be protected against drying out by keeping it moist for at least seven days.

Steel moulds are normally used to manufacture concrete culvert rings of various sizes.

The manufacturing site for the production of concrete culvert rings has to be chosen with care. The costs of the transportation of the components as well as the ready-made rings have to be taken into account. The site has to be big enough for the storage and the handling of the components as well as the culvert rings.

The mould should be cleaned and oiled before a new ring is produced.

PROBLEMS

Solve the following problems:

1. Sand and gravel to be used for concrete production stored on open ground have become saturated with water. What does this mean in respect of the quantity of water to be used?

Calculate the quantity of water to be added to the following mixture: 125 kg (two and a half bags) cement, 200 litres sand and 400 litres gravel.

Data:

water/cement ratio 0.5;
the sand contains 10 per cent water;
the gravel contains 4 per cent water.

2. How much water should be added to the mixture described under 1 if the aggregates are completely dry and the water/cement ratio is 0.6?

3. After the predetermined quantity of water has been added to the gravel, sand, cement mixture and the components have been mixed thoroughly, the mixture is very stiff and difficult to handle. What can be done to improve the workability?

4. When the moulds are stripped off recently manufactured culvert rings, the surface of these rings show “pockets” of gravel (the concrete is not dense and smooth but the gravel grains are clearly visible and can be removed). What are the likely causes? If this occurs only at the bottom side of the culvert, what can you conclude?

MODULE CHECKPOINT

Indicate True (T) or False (F) in the blank:

1. The exposed surface of the concrete needs to be cured for a period of:

one hour

.....

twenty-four hours

.....

seven days

.....

twenty-eight days

.....

2. We want to produce concrete 1:2:4

For each bag of cement (50 kg) we have to add approximately:

- 80 litres of sand + 200 litres gravel

.....

- 100 litres of sand + 200 litres gravel

.....

- 100 litres of sand + 160 litres gravel

.....

- 80 litres of sand + 160 litres gravel

.....

3. Concrete 1 : 2 : 3 is stronger than concrete 1 : 3 : 6

.....

Answer briefly:

4. Which factors contribute to the final strength of the concrete?
.......................................................................................................................
.......................................................................................................................
.......................................................................................................................

5. Which is the correct method to mix gravel, sand, cement and water by hand? Add drawings to illustrate the mixing procedure.
.......................................................................................................................
.......................................................................................................................
.......................................................................................................................

6. Which is the correct method to charge a concrete mixer and what is the approximate mixing time?
.......................................................................................................................
.......................................................................................................................
.......................................................................................................................

7. How long do culvert rings have to harden before they can be transported and what measure can be taken to avoid breakage during transport?
.......................................................................................................................
.......................................................................................................................
.......................................................................................................................

8. Give one example of a purpose for which “lean” concrete (mixture 1:4:8) can be used.
.......................................................................................................................
.......................................................................................................................
.......................................................................................................................

9. Describe two methods of curing concrete.
.......................................................................................................................
.......................................................................................................................
.......................................................................................................................

Field Instructions

STRENGTH AND QUALITY OF CONCRETE

Remember that:

- the strength and quality of concrete depends mostly on:

(i) whether the correct quantities of gravel, sand, cement and water have been used;

(ii) whether the grain sizes of the gravel and sand are correct (a good distribution of different sizes);

(iii) whether the mixing, placing, compacting and curing is carried out correctly;

- twenty-eight days after placing, the concrete has reached 95 per cent of its final strength;

- reinforcement steel is embedded in the concrete to take the tensile stress in those places where tensile stress will occur.

CEMENT

Remember that:

- cement reacts chemically with water and should therefore not be exposed to humid conditions before it is used in manufacturing concrete. Store cement properly in a well-ventilated dry store. When cement bags are torn, remove (part of) the contents as necessary, sieve out the hardened pieces and put the remainder in a new bag or container;

- six to seven (50 kg) bags of cement are required to produce 1 m3 of dense concrete. Less cement is required to produce concrete for culvert beds and foundations.

AGGREGATE

Remember that:

- the aggregate forms the skeleton of the concrete and generally consists of sand and various sizes of stone (also called gravel);

- when the aggregate contains a lot of flat flaky particles, it is not suitable to be used in concrete;

- the aggregate should be stored on a hard, clean surface;

- the aggregate of different sizes should be stored separately so that they will not get mixed;

- dirty aggregate containing clay impurities, dust or fine crushing residues should be spouted clean before it is used for concrete manufacturing.

MANUFACTURE OF CONCRETE

Method

- Find out for which purpose the concrete is required. Concrete for culvert pipes requires a 1:2:4 mixture of cement, sand and gravel. Concrete for structures needs a mixture of 1 measure of cement, 2 measures of sand and 3 measures of gravel (1:2:3). For very high quality concrete laboratory tests are necessary to find out exactly how much gravel and sand of different sizes is required to produce the best mixture.

- Measure the correct quantities of the materials with a gauge box (recommended size 0.4 m × 0.25 m × 0.4 m). When the mixing is done by hand the maximum size of the batch should be 1/3 cubic metres.

- First mix the gravel and sand by turning over the heap several times.

- Add cement and turn over until the batch is uniform in colour.

- Sprinkle the right quantity of water (approximately 25 litres of water for each 50 kg bag of cement used) on top of the batch while the batch is turned over another three times.

- When a concrete mixer is used, determine first the total quantity of mixture it can contain. Concrete mixers should never be filled completely!

- Calculate the quantities of the components required for one mixture.

- Charge the mixer with some coarse gravel and a part of the water and turn it for a few seconds. This will clean the drum wall of concrete left over from the previous mixing.

- Fill the mixer with:

(i) half the volume of the coarse gravel;
(ii) the fine gravel;
(iii) the sand;
(iv) the cement; and
(v) the rest of the coarse gravel.

- Mix dry for one minute.

- Add the rest of the water and mix for two more minutes.

- Thoroughly clean all tools, equipment and mixing platforms before the concrete hardens!

Remember that much more than 1 m3 of material is required to produce 1 m3 of finished concrete!

PLACING, COMPACTING AND CURING OF CONCRETE

Remember that:

- the transport time of ready-mixed concrete should be short. Unless rotating-drum trucks are used, the concrete should be poured within 15 minutes after finishing the mixing;

- the concrete mixture should not be dropped freely from heights greater than 1.5 m. It should also not be thrown far with shovels.

Method

- Clean and oil the forms, moulds or shutters before placing the concrete.

- Place the concrete in layers not higher than 30 cm when compacted by hand and not higher than 60 cm when compacted by poker vibrator.

- Consolidate the concrete directly after it has been poured. Use hand tampers and iron rods or a poker vibrator.

- Vibrate until water starts appearing at the concrete surface (not longer than 15 seconds in one place).

- Smoothen the surface of the concrete with trowels, suitable pieces of board provided with a grip or straight edges.

- Keep the surface moist for seven days by:

(i) covering it with wet jute, paper, sand, sawdust or similar material, or
(ii) sprinkling it frequently with water.

MANUFACTURE OF CONCRETE CULVERT RINGS

Remember that:

- the culvert manufacturing site should be as close as possible to:

(i) a source of clean water;
(ii) the places where the materials (gravel, sand, cement) are obtained; and
(iii) the places where the culverts have to be laid;

- the site should be big enough to contain:

(i) a cement store;
(ii) bays for the gravel and sand;
(iii) a mixing platform covered with a simple roof;
(iv) a loading ramp;
(v) the ready-made culvert rings; and
(vi) a store for the tools and equipment;

- culvert rings should be cured for seven days following their manufacture.

Method

- Clean the moulds and oil them with a mix of 2 parts diesel and 1 part of used oil.

- Centre the inner mould so that the thickness of the culvert ring will be uniform.

- Prepare a batch of 1:2:4 or 1:2:3 concrete (see manufacture of concrete) according to instructions (preferably use gravel of different sizes).

- Pour the mixture into the mould using a suitable container. Compact the concrete with a steel bar and by pounding the moulds lightly with rubber-headed hammers.

- Leave the rings inside the moulds for at least 24 hours (wait another 24 hours before you move the rings).

- Take off the moulds, clean and oil them.

- Keep the rings moist for a period of seven days.

Tools and equipment

- Moulds of various sizes (at least double the quantity of the required daily production).
- Spanners and screwdrivers.
- Steel bars, rubber-headed hammers.
- Water-hose and water container.
- Containers for used oil and diesel, brushes for oiling.
- Shovels.
- Trowels, planks and straight-edges.
- Gauge boxes.
- Concrete mixer (optional).

Learning Element: LE-0 Module learning objectives

After you have learned this module, you should be able to:

- explain the importance of the location, choice and design of water-crossing structures;

- explain which factors have to be considered when a culvert is designed and when it is constructed;

- describe the function and construction of drifts and causeways;

- describe how a timber bridge is constructed and which factors have to be taken into account.

Learning Element: LE-1 Nature, definition and types of structures

LEARNING ELEMENT OBJECTIVES

After you have learned this element, you should be able to:

- explain why the location and design of water-crossing structures is important;

- describe which factors should be considered before a particular type of structure is chosen;

- describe four different types of structures and explain when, in principle, they are used.

Structures such as bridges, drifts and box culverts usually account for a high proportion of the total cost of a road. They are the weak links in a road system, because the damaging effects of floods or high rainfall are concentrated at the points where the water crosses the road. Their failure will not only lead to high replacement costs, but may also make the road useless for long periods.

For these reasons extreme care should be taken that suitable structures are constructed in the best place possible and that the right type and design of the structures is chosen.

The location of the structures should be considered at the time of the first survey when the preliminary alignment is chosen.

The choice of the type and design of water crossings for rural roads should be governed by:

(1) The nature of the river or stream

- Is water flowing throughout the year or is the stream/river dry and flooded periodically?

- What are the maximum and minimum quantities of water flowing through the cross-section at the point of crossing?

- What is the profile of the cross-section; narrow with high banks or wide and flat?

(2) The cost of construction and maintenance of alternative structures

- Is local material available?
- What are the transport costs for materials not locally available?
- Which skills are necessary?
- Is any sophisticated equipment necessary?

(3) The expected traffic density

- What will be the effects if the stream/river cannot be used for certain periods per year? What are the costs to the road users if transport is delayed?

- What is the quality and design of the rest of the road?

The following structures are widely used as rural road water crossings:

- Culverts. These can be subdivided in log culverts, concrete pipes, box culverts and corrugated-steel culverts of various shapes and sizes.

- Drifts or fords. These are structures which provide a firm place to cross a river or stream. When a river/stream contains water, it flows over the drift, so that the vehicle will have to pass through the water. This implies that usually for permanent streams other solutions are preferable. Because drifts are cheap and easy to construct, they are well suited to cross wide, normally dry rivers which are periodically flooded.


Fig 1

- Causeways, culvert drifts or submersible bridges. These are different names for structures which are designed and in such a way that the normal dry-weather flow of the river passes through culverts below the roadway. The occasional floods pass both through the culverts and over the road, which means that - similarly to the drifts described above - the road is not always passable.


Fig 2

- Bridges. These are structures which are constructed above the maximum flood level, so that the road is always passable.

Many different types of bridges exist:

- Single span girder bridges;

wood, steel, reinforced concrete

- Multi-span girder bridges;

wood, steel, reinforced concrete

- Masonry arch bridges;

wood, steel, reinforced concrete

- Bailey bridges;


- Floating bridges;


- Suspension bridges.



Fig 3

Bridges are usually more expensive than other types of structures described above. However, when we have a permanent narrow stream with steep rocky embankments the construction of a simple girder bridge may be an economical solution, because the approaches to the crossing do not have to be excavated as would be necessary in the case of a drift.

This exemplifies that it is very important:

(a) to locate the structure at the best available crossing place, and
(b) to examine which type of structure is technically and economically preferable.

Learning Element: LE-2 Culverts for streams and small rivers

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- explain which factors have to be considered before a big culvert is designed;
- give a rule of thumb to determine the approximate size of a culvert;
- describe important points to be taken into account when a culvert is constructed;
- describe the advantages and disadvantages of corrugated-steel pipes.

Concrete pipe culverts are discussed in module M-10, LE-5 Drainage.

The culverts discussed here are larger structures designed to accommodate larger flows.

All culverts require a good foundation whether they have a large or a small diameter. A foundation can be provided in different ways:

(1) by improving the existing subsoil (place gravel);

(2) by constructing a floating floor. This method is especially suitable in soft swampy areas and consists of placing several layers of round timber (diameter 5-10 cm) on top of each other, so that a floating timber floor results;

(3) by pouring a concrete floor (usually only applied when major structures are concerned).

Fig 4


Box culvert


Arch culvert - Scaffolding for the arch e.g. removable armco pipes

Box culverts are generally made of reinforced concrete, although the walls can also be made of masonry. The thickness of base, walls and roof and the reinforcement steel required depend on:

- the over-all dimensions of the culverts;
- the type of soil in the area where the culvert is constructed;
- the layer of soil covering the culvert.

Since these factors vary with the circumstances encountered, the design of these culverts should be done by qualified engineers. There are, however, a number of points which are generally applicable to all culverts:

- the foundation for pipe culverts should always be shaped to fit the pipe;

- the maximum area of the waterway needs to be determined in order to calculate the size of the culvert required. As a rule of thumb you can assume that the area of the culvert required is one-third of the area of waterway when the stream has reached its highest flood level, i.e. for every three square metres of waterway one square metre of culvert should be placed;

- an apron (figure 5) should be made to protect the bed from erosion at the outlet side of the culvert. Aprons can be made of different materials (reinforced concrete, gabions, masonry). A filter construction made of stones of various sizes, as shown in figure 2, is cheap and very effective.


Fig 5

- head/wing walls should be made at the inlet and outlet side of the culvert. They protect the road embankment from the river water and serve also to support the embankment, so that the soil does not slide into the river when a vehicle passes;

- it is very important to properly compact the soil around, between and on top of the culverts. This compaction should be done in layers not exceeding 15 centimetres.

- the gradient of culverts should normally be the same as the gradient of the stream bed. However, to prevent silting the gradient should not be less than 3 per cent. A gradient greater than 5 per cent is not recommended unless the circumstances are exceptional. In this case take protective measures to prevent erosion!

- when a natural stream crosses the road at an angle, it is often better to construct a skew crossing or to realign the road, so that a 90° crossing can be constructed. If an existing channel bed is altered, usually a lot of erosion problems can be expected.


Fig 6

Corrugated steel culverts

This culvert type consists of sections of metal which can be bolted with simple tools (figure 7).


Fig 7

The assembly and installation of these culverts is not described here, as the instructions can be easily obtained from the supplier. The advantages of these steel culverts are their strength, their flexibility and their easy assembly. However, steel is a costly material and in many countries the pipes are not locally manufactured and readily available.

Always make a cost comparison between alternative solutions. You will find that for rural roads the manufacture of structures with locally available materials and skills is often cheaper and preferable.

Learning Element: LE-3 Drifts

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- explain the function of a drift;

- explain when drifts can be applied and more particularly in which circumstances the different types are constructed;

- describe how drifts are constructed.

Drifts basically provide a firm surface over which vehicles can pass a waterway provided that the level of the water is low enough. From an economical and technical point of view drifts are usually the most appropriate solution when wide, normally dry rivers have to be crossed.

“Gabion” non-surfaced drifts

Two alternative designs are discussed.

Alternative 1 can be applied on roads with a low traffic density when a stream/river with a large fall and a stony/gravelly bed is crossed.

Alternative 2 is more suited to rivers with a loose sandy bed and a gentle fall.

Alternative 1

This drift consists in principle of a porous dam which retains the gravel carried by the water of the stream. The top of the dam is between 15 and 30 centimetres higher than the river bed at the downstream end.


Fig 8. Gabion non-surfaced drift

The dam is formed by wire baskets (gabions) filled with rocks. A common size of these gabions is 2 × 1 × 1 metres. The baskets are connected on the site with steel wire. The construction process is simple. A 1.20 metre-wide trench is dug along the downstream edge of the future road. The depth of this trench is determined by deducting the depth of the gabion from the future road level at the downstream side.

The empty gabions are connected so that a line of gabions of sufficient length is produced. This line of empty gabions is placed in the trench after which the centre gabion is filled with rocks. Using this centre gabion as an anchor, the line of gabions is pulled taut and straightened by a rope or chain attached to a truck or winch (figure 9).


Fig 9

Tension is kept on the line of gabions while they are filled to ensure that they settle back in a straight line. After the filling has been completed, the gabions are closed with wire and the trench is backfilled with suitable granular material. The compaction of this backfill is extremely important and should be carried out in layers not exceeding 15 centimetres thickness.

The road surface is formed by placing of a well-graded gravel. Fine material (sand, clay) transported by the stream will gradually fill the voids between the gravel particles leaving a satisfactory running surface. When the river bed consists of loose sand this design is not satisfactory, as the gabions will eventually be undermined. In this case Alternative 2 (figure 10A) where the top of the line of gabions is at river-bed level and apron and solid riding surface are provided, is better.

Alternative 2 (figure 10A)

When the river bed is sandy and the fall is gentle, a better design is to keep the top of the line of gabions at the river-bed level. To prevent scouring at the downstream end, an apron of well-graded stones should be provided. If at all possible a “reverse filter” construction should be applied with the bigger stones on top and the smaller stones at the bottom, so that a suitable changeover to the existing river-bed material is established (figure 2). To minimise the risk of the drift being undermined, the downstream foundation should be dug as deep as possible, preferably until bedrock is reached. Two lines of gabions on top of each other may be necessary, providing a 2 metre deep foundation wall. Hand packed stones are placed to form a solid crossing surface for the traffic.

Depending on the height of the river banks the slopes of the approaches may vary from 10 to maximum 20 per cent. Of course 10 per cent is preferable and steeper slopes should only be applied in exceptional circumstances. You should ensure that there is a gentle transition from the horizontal stretch to the sloping stretch, so that abrupt changeovers are avoided.

A different cross-section has also been successfully applied (figure 10B). In this case a “box” is constructed with two lines of gabions connected by a bottom layer of stones encased in wire mesh. The “box” is filled with gravel or crushed stone which provides a smooth riding surface. This structure is certainly more expensive than the one shown in figure 10A so that the increased costs can usually only be justified when a higher traffic density is expected.

Fig 10A


Figure


Longitudinal section


Cross section


Fig 10B

Paved drift (figure 11)

When the traffic density and the function of the road warrant it, a more sophisticated structure can be considered. Similarly to the unpaved drifts previously discussed, a retaining wall on the downstream side of the drift combined with an apron prevents the undermining of the structure. A head-wall on the upstream side will make the structure more stable firstly by lengthening the distance which the subsoil low has to traverse and, secondly, by providing a more solid foundation on the upstream side, so that a traffic load on this side is better distributed.

To ensure a smooth transition from the road to the drift surface it is good practice to improve the stretch of road adjacent to the drift approaches. This can be done by digging out the existing soil over a width of one metre and a depth of one metre, placing a layer of hand packed stones and backfilling with a gravelly material. The backfilling should be done in layers not exceeding 15 cm and these layers should be well compacted to obtain a hard and smooth transition.


Fig 11. Cross section

The retaining wall on the downstream side can be made of gabions as described before or constructed in masonry as shown in figure 11. Gabions are preferable, because they are more flexible and can therefore follow settlements of the subsoil more easily.

Longitudinal section

The concrete slab should be carried into the river banks, well above the maximum flood level recorded. The approaches should preferably not be steeper than 15 per cent and the connections between the horizontal stretch and the approaches should be rounded.


Fig 12

Learning Element: LE-4 Causeways

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- explain what a causeway is;
- describe which specific design points are particular to a causeway;
- comment on how to locate a causeway;
- describe how a typical small causeway is constructed.

In engineering textbooks a water crossing having a number of culverts to receive regular waterflows and which is submersed during floods is described with a great number of names; culverts drift, causeway, submersible bridge or vented drift. In this learning element the structure is described as a causeway.

A causeway can be defined as a structure which has a double function:

(a) it allows the normal dry weather flow of a river/stream to pass through the culverts below the roadway; and

(b) the occasional floods pass both through the culverts and over the roadway.


Fig 13A


Fig 13B

Because they have this dual function causeways present hydraulic problems which are peculiar to this type of structure and great care should be taken with their construction. Many causeways have failed because of a wrong location or a wrong design.

Design

If the culverts are concentrated in the centre of the causeway, the high speed water jets coming out of these culverts will cause heavy scour at the sides of the culverts (figure 13A).

This implies that in designing causeways the culverts should be distributed evenly throughout the length of the structure (figure 13B).

When the water flows over the causeway it moves as shown in figure 14. The forward roller shown on the right moves the sand forward. The back roller makes the sand surface on the left side steeper and steeper until it collapses. This effect causes the foundation to be exposed very quickly unless effective protective measures are taken.


Fig 14

A suitable “apron” of concrete or well-graded stones is therefore absolutely necessary to break the energy of the falling water.


Fig 15

To improve the hydraulic properties of the causeway it is recommended to have a one-way camber towards the downstream end of the causeway. The rounding of the upstream edge further contributes to a smooth discharge of the overflowing water.


Fig 16

As regards the culvert capacity, when suitable divergences (flared end sections) are provided at the end of the pipes this can be increased by as much as 50 per cent. Such sections are manufactured in steel or concrete and should be considered when more major causeways are constructed.

Fig 17


Elevation


Plan

When roadside markers are provided to show the edges of the causeway and to indicate the stream level, care should be taken that these offer as little resistance as possible to the flow. This means that such markers should be streamlined to the greatest possible extent.

Location

The location of a causeway is - as with all structures - extremely important. Efforts should be made to locate the most stable portion of the river bed, so that a better foundation can be provided to the structure.

Also, avoid locating a drift, bridge or causeway near or in a river/stream bend. During floods the water will tend to cling to the concave bend of the river and cause continuous erosion at that side. At the convex bank silting will take place.

When a lot of debris (tree trunks or branches, etc.) floats in the river there will be a danger that the culverts get blocked. If this happens, usually a transverse flow will occur along the upstream side of the culverts creating a deep trench which can undermine the structure. To avoid this, debris diverting or collecting structures, such as debris deflectors or racks, may be necessary.

Construction

Two trenches are dug at the up and downstream edges of the causeway. The trench at the downstream side should be at least 1.50 metres deep if an overflow of 50 cm over the roadway is expected (see figure 16).

Between these trenches a suitable bed for the culverts is prepared of a well-graded gravel material. The provision of a 7.5 cm concrete floor is recommended. This floor should be indented to accommodate the shape of the culvert, so that the culvert is evenly supported. After the culverts are laid the head walls are constructed up to the road level and the area between the head walls is backfilled with a well-graded stony/gravelly material. Lean concrete may be used also as a backfilling material, although the costs of the structure will be considerably higher in this case.


Fig 18

A 1:3:6 concrete surface layer of 10 cm thickness is laid at a 3 per cent cross-fall, the highest point being at the upstream side. If possible, the upstream side edge is rounded. The connections between the approaches and the horizontal stretch are rounded to provide a smooth riding surface. Finally, the aprons are constructed, as shown in figure 18. For this type of small causeway the minimum width of the apron at the downstream side should be two times the height of the causeway from the river bed.

Learning Element: LE-5 Bridges

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- identify different types of bridges;
- describe how the foundation and superstructures of small bridges should be constructed;
- describe the denomination and function of the various parts of the bridge.

Bridges are used to provide “all weather” crossings over streams and rivers. Since they are usually the most costly type of water-crossing, they should only be constructed on higher class “all weather” roads where the future traffic density is expected to justify the costs of the structure. The following types of bridges are commonly encountered on rural roads:

- single and multi-span bridges with wooden, steel or reinforced concrete girders;
- masonry arch bridges (figure 4);
- bailey bridges.

Less common are floating bridges and suspension bridges. All bridges need to be designed according to the length of the span to be crossed, the characteristics of the river and the expected traffic load. This learning element will discuss the basic principles of the construction of a girder bridge.


Fig 19

Figure 19 shows the nomenclature of the bridge components.

Foundation

Abutments and piers

The supports on each bank of the river for the bridge superstructure (the girders and the deck) are known as abutments. In case of wide rivers, the bridge superstructure will have some intermediate supports known as piers. The clear distance between two supports is called a span. Therefore, a bridge without piers will have only one span, a single-span bridge, while a bridge with piers will be called two-span, three-span or, in general, multi-span bridge.


Fig 20

The number of spans should be as small as possible, because, the greater the number of piers, the more the flow of water under the bridge will be obstructed. However, the bigger the span, the stronger the girders need to be. For spans bigger than 10 metres it will, therefore, be necessary to make cost comparisons.

To get an idea of the nature and thickness of the subsoil layers at the place of the foundation a few trial pits should be dug.

When large bridges have to be constructed the advice of the geology department on the bearing capacity of the foundation soil layers is necessary.

Although in minor structures the pressures on the foundation are usually not that high, a good foundation is still extremely important. The stability of the bridge depends on a solid foundation which cannot be undermined by scour. It is, therefore, essential to found the abutments and piers either on rock or into a layer of soil beneath the river bed. This layer should have a sufficient bearing capacity and should be resistant to erosion.

If such a layer cannot be found, the abutment/pier will either have to be supported by piles or by a 60-100 cm thick foundation layer of a well compacted granular material laid at least 1 metre below the maximum depth of scour. Remember that at a bridge, the speed of water usually increases, so that the scour is increased as well.


Fig 21

Most failures of bridges occur because the foundation was not dug deep enough. Abutments and piers can be constructed of concrete, masonry, gabions or timber.

When timber piles are used in combination with a concrete abutment/pier, the piles should be located under the lowest water level, because they will soon start rotting if they are exposed to air.

When timber abutments are combined with timber piles, all exposed beams should be protected by several layers of wood preservative (creosote or other).

After the piles have been cut off at the same level the tops have to be soaked in wood preservative before the cap or crossbeam is laid.


Fig 22

This cap (usually 25 × 30 cm) is fastened to the piles with long spikes hammered through a pre-drilled hole of a slightly smaller diameter as the spike. These spikes should be hammered at least 25 cm into the pile. Wing walls are constructed at both ends of the abutment to support the earth fill of the road and to provide extra protection against erosion. The angle with the line of the abutments is usually 45 per cent, but can be different if warranted by the circumstances.


Fig 23

When concrete or masonry abutments are constructed, it is recommended that seepage holes are provided. These openings allow the water left behind in the earth fill behind the abutments to drain off. As a result water cannot press on the inside of the abutment after excessive rainfall or floods have occurred. The backfilling behind the abutment with a gravelly material or rubble further improves this drainage (figure 23).

Superstructure

Bridges have to be designed to support different types of loads:

(1) The dead weight of the superstructure (the girders, transverse decking, running strips, etc.) and, where applicable, snow.

(2) The live load of the traffice (for rural roads usually not more than 10 tons).

(3) The impact of the live load (bouncing of traffic, effects of braking).

(4) Wind.

The size and number of girders to be used depend on:

- the span to be crossed;
- the design load(s);
- the material used (steel, pre-stressed or reinforced concrete, timber).

When timber is used care should be taken that it is a durable (resistant to insects and rotting) type. Also the bending strength should be high. The effective life of girders which are either under-designed or of a species of wood with insufficient bending strength is very short. Such girders will bend excessively under a load that will not break them. This results in considerable friction and wear at the points where they rest on the cap or the abutment. Rot begins at this point and the timber is soon useless. Excessive bending also causes the floor planks of the transverse decking to move up and down and to wear out very quickly. A 5 metre span, single lane bridge, designed to carry trucks of a maximum weight of 12 tons (8 ton front axle load and 4 ton rear axle load) should have 30 × 30 cm hardwood girders spaced not more than 60 cm apart. Round (36 cm) or rectangular (20 × 40 cm) girders can also be used.

These sizes are mentioned only to give an idea of the approximate requirements. All bridge designs should be checked by a qualified engineer. It is extremely important that timber beams are free of cracks and rot and that they are treated with a suitable wood preservative before they are used.

When timber “caps” are applied the girders should be fastened to the caps with spikes or toe nails. Before this is done it should be checked that the tops of girders are at the same level, so that the floor planks of the transverse decking will rest evenly on each girder. Adjusting the level of the girders can be done with hardwood wedges (or mortar when the girders rest on a masonry/concrete abutment).

Fig 24


Sectional elevation


Elevation

7.5 cm thick planks of a length equal to the width of the bridge are often used for the transverse decking. These planks are spiked to the girders. Between the planks a small space is left to allow for expansion due to swelling.

An alternative and better transverse decking can be provided by using 5 × 10 cm planks, laid on edge to make a deck of 10 cm thick. As each of these planks is laid, it is nailed to the one next to it, so that we have, in effect, a solid deck of wood 10 cm thick instead of a number of individual planks. This makes a very stiff floor and helps to keep down vibration and movement in the structure. These planks need only be long enough to be supported by two girders. Quite often short planks of this size can be purchased quite cheaply, so that for a similar or lesser price a transverse decking of 10 cm thickness can be provided instead of one 7.5 cm.

To protect the deck and give a smooth running surface for the cars, running strips or running planks are provided. Figure 25 shows running strips bolted to a 7.5 cm thick transverse decking. Running strips should be bolted, not spiked, to the decking, because the spikes would soon work themselves loose. Kerbs are bolted to the decking to prevent vehicles from sliding off into the water. They also serve to provide a stronger floor, since the floor planks are prevented from working themselves loose.


Fig 25

A suitable size for kerbs is 15 × 15 cm.

It is good practice to provide an extra protection at the places where high wear may be expected. These are especially the places where the transverse decking is laid on the abutment or caps. Roofing felt, rubber or a suitable plastic can provide this protection.

Learning Element: LE-6 Module summary and checkpoint

LEARNING ELEMENT OBJECTIVES

After you have learned this element, you should be able to:

- describe the most important factors to be considered before a water-crossing is designed and located;

- describe simple designs of culverts, drifts, causeways and single-span girder bridges;

- describe the essential design points of the above structures.

SUMMARY

Structures are expensive when their cost is compared to the cost of the rest of the road. For this reason, their design and location are extremely important and should be carefully considered and checked before it is decided to build a particular type of structure in a particular place.

Culverts, drifts, causeways and bridges are widely used as rural road water-crossings.

The foundations for all structures are extremely important and adequate protective measures should be taken against erosion and scour. Although such measures may seem costly and time-consuming, they will pay for themselves because a well-protected structure will require less maintenance and will not collapse because of erosion.

Backfills should preferably be made of well-graded gravel (sometimes rubble may be used) and should be well compacted.

Drifts provide a firm surface over which vehicles can pass a water-way, provided the level of the water is low enough. Several alternative designs exist which should be applied as described in LE-3. Aprons should always be provided at the downstream side of the drift.

When the function of the road and the traffic density warrant more sophisticated structures, paved drifts, causeways and bridges can be considered. Causeways are drifts which have a number of culverts to receive regular waterflows. During floods, the water passes through and over these structures.

Bridges provide “all weather” crossings over streams/rivers and can be of various types. Those most commonly encountered on rural roads are: single and multi-span girder bridges, masonry arch bridges and bailey bridges. Bridge foundations should be resting on rock or should be placed into a layer of erosion resistant soil beneath the river bed.

MODULE CHECKPOINT

Indicate True (T) or False (F)

1. The location of structures should be considered at the time that the preliminary alignment is chosen

......

2. The drift surface should be continued into the river banks up to the maximum flood level.

......

3. The culverts in a causeway should be concentrated in the centre of the structure.

......

4. Aprons are required for all causeways and drifts

......

5. It is not good to construct a structure in or just after a river/stream bend.

......

Answer briefly:

6. Which factors need to be considered before type and location of water-crossings are decided upon?
...................................................................................................
...................................................................................................
...................................................................................................

7. Which different methods of providing a foundation for a culvert pipe crossing do you know?
...................................................................................................
...................................................................................................
...................................................................................................

8. Which are the most important design points to be applied when constructing a drift?
...................................................................................................
...................................................................................................
...................................................................................................

9. How should the culverts in a causeway be placed?
...................................................................................................
...................................................................................................
...................................................................................................

10. What is the function of:

(a) seepage holes?
(b) kerbs?
(c) girders?
(d) running strips?
...................................................................................................
...................................................................................................
...................................................................................................

11. How do you protect the downstream side of culverts/drifts/causeways from erosion?
...................................................................................................
...................................................................................................
...................................................................................................

Field Instructions

STRUCTURES

Structures are costly and difficult to replace. Their failure will make a road useless for long periods. Therefore:

- spend enough time in determining the best place for the crossing;
- ensure that the right type of structure is chosen;
- ensure that the structure is designed to carry the expected traffic loads and to resist scour;
- ensure that the construction is carried out according to the design.

Before you choose a particular type of structure, assemble the following data:

- what are the maximum and minimum quantities of water flowing through the cross-section at the point of crossing?

- how often and for how long a period is the stream flooded?

- what is the situation at various points of crossing? Can natural supports (rocky embankments, stony sections of the river bed) be utilised? What type of soil is found at the various alternative places of crossing?

- which local materials are available (lumber, rocks)?

- what will be the function and design of the road leading to the crossing?

- which skills are required and available to construct a particular structure?

Depending on the circumstances, you may decide to construct culverts (concrete pipes, box culverts, corrugated steel culverts), a drift (non-surfaced, surfaced), a causeway or a bridge (timber, masonry, concrete, steel).

LARGE-SIZED CULVERTS

Remember that:

- when the natural soil is soft or unstable a foundation has to be provided to support the culvert. Such a foundation can be flexible (floating timber in wet areas, a layer of gravel in dryer conditions) or rigid (lean concrete);

- box culverts need to be designed according to the required size, the type of soil in the area and the layer of soil covering the culvert. Assemble the data mentioned in FIELD INSTRUCTIONS - STRUCTURES so that a wall-founded decision on the type of structure can be taken;

- the minimum area of culvert space required is one-third of the area of the waterway at times of flood. If the cross-section of the waterway at a time of flooding is 30 m2 an opening of at least 10 m2 should be provided;

- box culverts should be protected from scour by headwalls, wingwalls and aprons;

- aprons can be made of different materials (reinforced concrete, gabions, masonry) but a filter construction made of various sizes of stone (the largest on top) is both cheap and effective;

- the gradient of the culvert should be between 3 and 5 per cent. If a gradient steeper than 5 per cent is necessary, aprons are essential!;

- whenever possible the opening of the culvert should be in line with the water flow in the existing stream bed;

- you should ask for instruction booklets and appropriate tools when you are requested to assemble corrugated steel pipes.

DRIFTS

Remember that:

- drifts are best applied in streams which are usually dry or have a very small permanent water flow;

- drifts and causeways should not be located near or in a river bend;

- different designs are used for streams with stony/gravelly beds than for streams with beds of loose sand;

- it is best to construct the drift within the dry seasons and, as near as possible, towards the end of the dry season when the water-table is lowest.

Construction

- Rechannel the water flow so that you obtain a dry working space.

- Set out the centre-line of the drift with survey pegs showing the future level of the drift surface.

- Set out reference pegs at the down-stream and the up-stream sides of the drift. These pegs should be placed at a fixed distance (say two metres) from the edges of the future drift so that they will not be lost during construction. The drift surface should preferably have a crossfall of approximately 3 per cent towards the down-stream side. This means that if your reference pegs are 10 metres apart the one on the down-stream side should be 30 cm lower than the one on the up-stream side. In case of doubt, always follow the gradient of the existing river bed. A string over the tops of the survey and reference pegs shows the level of the drift surface.

- The longitudinal section should be horizontal with two sloping approaches joining the drift to the road. These approaches should have a gradient of maximum 20 per cent (10 per cent is better) and should carry on well above the highest flood level. The transitions from the horizontal section to the approaches should be gently curved so that a smooth-riding surface is provided to the passing vehicles.

- Place multi-purpose pegs showing where the foundation trenches will be excavated. These trenches should be wide enough to allow sufficient working space to the masons or carpenters.

- Dig these trenches as deep as possible, preferably until you have reached a solid layer (rock, hard soil). When you encounter water very soon (high water-table) drain this water as well as possible (buckets, pump, mitre drain) and continue digging until further progress is impossible. In such conditions, a gabion foundation wall is best. If gabions are not available, fill the trench with rocks up to the water level and continue with masonry until you have reached the correct levels as shown by the strings.

- After completing the foundation wall(s) excavate the area in between and place the hand-packed stone.

- Finish with a layer of concrete if specified in the design.

- Excavate as necessary for the apron on the down-stream side of the drift, going from one metre depth at the drift edge sloping upwards to the river bed. The width of the apron should be minimum two metres but preferably more.

- First place small-sized stones, then larger-sized and put the largest at river bed level.

Note: Always study the design carefully. Depending on the design of the drift, sometimes a different work method will be necessary.

- After the work on the drift itself has been completed, improve (especially in sandy soils) the transitions from the road surface to the drift approaches.

- Dig trenches of one metre wide and one metre deep immediately next to the drift approaches, across the width of the road.

- Place a layer of hand-packed stones at the bottom of the trench and backfill the trench with layers of gravel. Compact each layer (maximum thickness 15 cm) thoroughly with hand-rammers.

CAUSEWAYS

Remember that:

- the culverts should be distributed evenly throughout the length of the structure;

- an apron of reinforced concrete or hand-packed stones is essential to break the energy of the over-flowing water;

- causeways should have a one-way camber of 3 per cent towards the down-stream edge and their up-stream edge should be rounded;

- road-side markers should be streamlined to limit their resistance to the over-flowing water;

- drifts and causeways should not be located near or in a river bend but in a stable portion of the river bed so that a good foundation can be constructed;

- the culvert bed should be a 15 cm gravel layer, preferably topped by a 7.5 cm concrete floor;

- the minimum width of the apron should be two times the height of the causeway from the river bed. This width should never be less than two metres;

- the depth of the foundation trench at the down-stream side of the causeway should be at least the height of the expected overflow of water over the structure.

Construction

- See FIELD INSTRUCTIONS - DRIFTS.

- Place reference pegs, survey pegs and multi-purpose pegs. Excavate the foundation trenches.

- Prepare the bed for the culverts. A layer of gravel (minimum 15 cm) preferably topped with a layer of 7.5 cm of concrete.

- Lay the culverts on the correct positions and at the correct gradient (3.5 per cent).

- Construct the headwalls up to road level.

- Backfill the area between the head walls with a suitable material and compact this material thoroughly.

- Pour a layer of concrete (10 cm) to provide a smooth surface for the traffic. The transitions between the horizontal stretch and the approaches should be gentle.

- Excavate the trench for the apron and place the stones (the largest at river bed level near the down-stream edge of the causeway).

BRIDGES

Remember that:

- bridges are expensive and their construction requires various types of skills. Engineering advice and an approved design should always be provided before the construction of a bridge is started;

- the foundation of the bridge is extremely important. The abutments (and piers, if these are built) should be founded on a solid layer of soil (or rock) which cannot be undermined by scour;

- timber used for bridges should be well treated with wood preservative;

- wing walls provide protection against erosion and support the road embankment;

- seepage holes are provided in the abutments to allow water drainage from the road embankment behind the abutment;

- running strips are often provided to protect timber bridge decks and to give a smooth riding surface;

- kerbs are provided to prevent vehicles from sliding off into the water;

- places where high wear can be expected should be extra protected with roofing felt, rubber or plastic.

Construction

- Set out the centre-line of the bridge and the positions of the abutments (and piers).

- Re-channel the water flow so that you have a dry working space. Construct first one abutment, then the piers (if any) and then the other abutment.

- Excavate the foundation as deep as possible. Try to reach a hard, solid soil layer or bed-rock. In soft soil, when ground water is encountered, continue the excavation until no longer possible. Then provide a 1.50 m wide layer of stones/rubble up to the ground water level. (Remember that generally this type of foundation will be sufficient only when small bridges are concerned. Bridges with larger spans, supporting greater loads may require piling to provide sufficient support to the foundations.)

- Provide a base of concrete (at least 10 cm thick) on top of the (levelled) layer of stones. This concrete layer provides a dry and strong base for the masonry abutment.

- Construct the abutment (or pier) to the required height. Do not forget the seepage holes in the abutments!

- Lay the girders. Place the tops of the girders at the same level using hardwood wedges or mortar. Place suitably sized pieces of strong plastic or rubber between the timber and masonry at those places where high wear may be expected.

- Spike the transverse decking planks to the girders. Leave a small space between these plants to allow for swelling.

- Bolt the planks for the running strips to the transverse decking. Depending on the width of these planks, two or three should be laid next to each other so that the running strips are wide enough to be used by vehicles of different sizes.

- Bolt the kerbs to the decking.

Learning Element: LE-0 Module learning objectives

After you have learned this module you should be able to:

- describe the function and composition of the gravel layer;

- explain how a choice of whether or not to gravel should be made and which alternatives to gravelling a particular alignment can be considered;

- describe how quarries should be selected;

- describe the organisational structure of the gravelling team and the site organisation of the gravelling operation;

- prepare a workplan for gravelling;

- use gravelling administration and report forms;

- list equipment, tools and materials used for gravelling.

Learning Element: LE-1 Nature and definition of gravelling

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- describe the function of the gravel layer;

- describe what gravel is and what should ideally be the proportions of its components;

- mention and describe one alternative way to surface a road with local materials.

NATURE AND DEFINITION

A road is gravelled to provide a surface layer which is passable in dry and wet weather and does not deform under the expected traffic loads. A gravel layer should therefore be of sufficient strength and thickness. The strength comes from the stone particles which should preferably be of various sizes. These stone particles lock together and form a strong skeleton which spreads the traffic load to the natural soil. Figure 1 shows that the pressure of the wheel is concentrated at the point of contact with the soil and decreases gradually. For this reason the best soil (gravel or bitumen) needs to be on top.


Fig 1

The stone particles should be kept together by a “binder”, a clayey material. This binder keeps the stone particles in place and restrains the formation of corrugations. However, in wet weather the presence of a clayey material is a disadvantage because it softens when water is added so that ruts are easily formed and the surface layer becomes slippery.

This means that the ideal composition of a gravel layer for dry weather conditions is different from the one for wet weather conditions. Therefore a compromise has to be made so that the surface layer provides a good riding surface during dry, as well as wet, weather conditions.

Very roughly, it can be assumed that a mixture of 35 to 65 per cent stones, 20 to 40 per cent sand and 10 to 25 per cent clay will provide a good surface layer. The higher proportions of clay are admissible in dry areas. The wetter the area the more important it is that the stone/sand proportion of the mixture is high and “well graded”.

In some parts of the world, a very effective surface layer is constructed from large (15 to 25 cm) stones which are carefully placed with the largest face downwards and then wedged with smaller stones and sand. However, extreme care should be taken if this method is used. Because this surface layer is permeable rainwater can penetrate into the underlying layer which may soften and cause failures. This method is only suitable in areas where (a) the natural soil does not contain a large proportion of clay and (b) large-size stones are readily available. Furthermore, local expertise with this method must exist and wages must be fairly low as a large number of man/days are required to complete a kilometre of surfacing.

The great advantage of this type of surface layer is its strength and durability if properly laid on a suitable (sandy) base.

Learning Element: LE-2 When to gravel and witch methods to use

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- explain which factors should be considered when a policy decision of whether or not to gravel a road is taken;

- describe what other alternatives there are if gravelling a certain alignment would be impossible or too costly;

- discuss different methods of gravelling and their advantages/disadvantages;

- describe which factors determine the choice of gravelling methods.

When to gravel

The basic functions of a gravel layer are (i) to ensure that the road is passable at all times under all types of weather conditions and (ii) to prevent deformation of the road under normal traffic loads. It follows, therefore, that natural soils which already possess these required properties do not need an additional gravel layer. Unfortunately for the road builder, such soils do not occur frequently in the places where they are needed so that gravelling is usually required.

However, the necessary excavation, transport, spreading and compaction make gravelling very expensive. The gravelling operation can add between 10 to 100 per cent to the total construction costs of the earth road, depending on a number of factors such as the type of road, thickness of the gravel layer, average hauling distance from quarry to site, organisation and planning, etc.

Given the fact that transport is expensive and gravel is not always readily available in many countries, it is understandable that at present not all roads are gravelled. It is a fact, however, that gravel roads are easier and cheaper to maintain and last longer than earth roads.

Usually, therefore, a policy decision on whether or not to gravel a road or a number of roads will have to be made based on the following criteria:

(i) what is the purpose of the road, i.e. how essential is it that the road remains open throughout the year?

(ii) what is the expected traffic density?

(iii) what is the composition of the natural soil?

(iv) what would be the cost for gravelling, especially in relation to the average hauling distance of suitable materials?

When, in order to keep a road open during normal weather conditions, it is clearly necessary to surface a road and gravel is not readily available the following options will have to be considered:

(a) follow a different alignment;

(b) do not construct the road;

(c) construct the road to bitumen standard;

(d) accept the high cost of gravelling with long hauling distances;

(e) construct the road to earth standards and accept that it will have to be closed for longer periods;

(f) only gravel those places which require gravelling most (spot improvement) so that the road will only be closed for short periods when the weather is exceptionally bad;

(g) crush rock either by hand or by (mobile) crusher when suitable rocky outcrops are available along or near the road.

The high costs of options (c), (d) and (g) can usually only be justified when the benefits of the roads and/or the future traffic volumes are expected to be high. Before the road is constructed the following data should therefore be assembled in the field, so that a well-founded decision concerning the above can be made:

(i) to what extent does the road require gravelling for technical reasons (i.e. which sections will not be passable even with light rains)?

(ii) how far from the road is the nearest source of suitable gravel material?

Methods of gravelling

In principle, the following methods can be used to gravel a road:

(i) use a mix of labour and equipment;

(ii) use labour for all activities except hauling over distances longer than 100 metres.

Furthermore, different types of hauling equipment can be used depending on such factors as average hauling distance, hardness of gravel material, method of loading, etc.

For example, tractors towing tipping trailers can be a very economical way of transport when the hauling distance does not exceed 8 km. The trailers are more suited to manual loading than lorries which are higher. Several trailers can be used for one tractor, so that one is loaded while the other transports material to the site. On the other hand, lorries are better suited to longer haul distances.

Another example is a quarry with rock which needs to be crushed to suitable particle sizes. The excavation can - depending on the hardness of the rock - be done with a bulldozer, blasting or hand tools such as pickaxes and crowbars. The crushing can be done either with a (mobile) rock crusher or by hand. Spreading can be done either by motor grader or with hand tools. With very short haul distances (less than 100 metres) it is even possible to carry out all activities with labour. The hauling can then economically be done with wheelbarrows.

In some parts of the world, where local expertise and means are available, animal-drawn carts may be economical to use for hauling of material (rock, soil).

Compaction can be done by the hauling equipment, small vibrating pedestrian rollers, self-propelled rollers or by direct methods (rain, traffic, etc.).

If you, therefore, want to apply the optimum technology in a particular situation, it is necessary to carefully consider all possible alternatives before you decide to use a particular approach.

In any case, it is very useful to pose yourself the following questions:

- what is the average distance from the quarry to the site?

- is labour available and willing to work on construction?

- which alternative methods of hauling can in principle be used? (determined by hauling distance and availability.)

- which methods of excavation and preparation of gravel material are possible? (Depending on type and hardness of material.)

- which methods of loading, spreading and compaction are possible?

Try to make realistic cost estimates for each alternative before you choose the methodology to be applied.

In many cases it may prove worth while to experiment with different methods on a small scale to find out what productivity and cost levels can be achieved.

Learning Element: LE-3 Selection of quarry

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- do some simple tests to find out whether a material is suitable to be used for a surface layer;
- mention some ways to find quarries of good gravelling material;
- describe and explain the aspects to be considered when selecting a quarry.

Materials which will form a good surface layer

Previous learning elements have discussed the function and the preferred composition of the gravel layer in detail. Naturally much depends on which materials are available in the area where the road is located. Suitable surface layers have been made of materials ranging from coral to very hard crushed stone. However, great care should be taken when a particular material is selected. While coral and most lime stones have the tendency to harden when they are exposed to air, water and traffic compaction, several types of rock will decompose to form clay after exposure to the atmosphere and traffic.

If you have had no experience with a particular material and the stone fraction of such a material is easy to crush with a small hammer, have a sample of the material tested to prove its suitability!

The following simple tests give an indication of the suitability of a particular material.

A. Take a small sample, moisten it and mold it into a ball. The presence of sand and fine stones can be found by the gritty feel of the sample. When, after drying, the ball retains its shape, it can be assumed that a sufficient amount of binder (clayey material) is present.

B. After flattening the wet sample, it should be difficult to penetrate with a pencil, thereby indicating the presence of a sufficient proportion of fine and coarse aggregates which interlock together. When the proportion of binder is too high, the wet sample can be easily penetrated and will leave the hands muddy and sticky.

How to find good quarries

The location of a quarry containing good gravelling material is often very difficult. In some cases it is cheaper to crush rock instead of hauling gravel over a long distance. When you prospect for suitable sources, collect samples of suitable rock and/or other material of which reasonable quantities are available and estimate (i) the hauling distance to the road to be gravelled, (ii) the required length of access road to the quarry and the type of terrain in which this access road will have to be constructed.

A number of sources of information of where good gravel can be found are:

- the ministry or government department responsible for the construction and/or maintenance of roads in a particular area. In some cases surveys have been made, indicating possible gravel deposits. Valuable information can often be obtained from government supervisory staff such as road inspectors who have been involved in gravelling work in the area. Even information about old exhausted quarries can be very useful as similar material can often be found in the surrounding areas;

- the local people. A good strategy is to leave samples of suitable materials with the local administration with the request to ask the local people to look for similar materials. If possible, a reasonable reward should be promised to the person who has located the quarry which will eventually be used;

- the exposed faces of road ditches or cuts in the area. Driving along the road and examining stony outcrops does not take a lot of time and can provide extremely valuable information.

Selection of quarry

The following aspects need to be considered:

- what is the quality of the material?
- what are the costs to excavate and transport the material?

These costs depend on a number of factors:

(i) is the land owned? Will compensation have to be paid? If so, there are usually established procedures and standard prices. You can normally obtain this data from the local road department;

(ii) what is the average hauling distance?

(iii) how much of an overburden will have to be removed to reach the good material?

(iv) how long an access road will have to be constructed and in which type of terrain?

(v) how much good material is available? Try to estimate the thickness and area of the deposit by digging holes in a systematic manner;

- Finally, it is important to find out whether or not the quarry is located in low-lying terrain. If so, this may well cause the quarry to become unworkable when it rains.

Learning Element: LE-4 Organisational structure

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- describe the organisational structure of the gravelling team;

- describe the functions and roles of the various people in the gravelling organisation;

- describe the general rules of the gravelling site.

At different levels, a large number of people are involved with gravelling. At headquarters, people are responsible for making policy decisions on which roads should be gravelled and what resources (labour, transport, money) will be available to carry out works in different regions. In the regions engineers or senior supervisors will be responsible for the general organisation of the works and the mobilisation of these resources. At site level the supervisor will be responsible for the organisation of the work and the administration of labour, tools, equipment and materials. At all these levels the engineering/supervisory staff are assisted by various types of administrative and operating personnel. At headquarters, we usually find executive officers and senior accountants; in the engineer's office - pay-clerks, accounting personnel and mechanics; and at site level - storekeepers, plant operators, drivers, carpenters, labourers, headmen and watchmen.

All these people have a specific role to play and have a place in the organisation. What these functions are and how they relate to each other can be drawn in an organisation chart (figure 2).

Fig 2.

The responsibilities of the persons performing the different functions in the organisation chart are:

For gravelling, a number of general rules have to be followed. Everyone on site should be aware of these rules, which are made to ensure that the gravelling can be carried out as planned.

Routine maintenance and servicing of equipment

Repair of equipment

Storing fuel and oil

Issue and return of tools and materials

Supply of drinking water

Payment of wages

Learning Element: LE-5 Organisation of the work

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- describe the preparatory activities to be carried out before the start of the gravelling;

- explain how the quarry should be laid out;

- describe how the workers and equipment should be organised;

- describe various methods of organising the hauling and dumping of gravel and discuss their respective advantages/disadvantages.

Before the gravelling can start, a number of preparatory activities have to be carried out:

(a) a suitable camp site has to be located. In principle, this site should be as close as possible to the quarry to keep the time used for transport between camp and quarry to the essential minimum. If, on the other hand, a fresh water source is nearby, you should carefully consider the advantages and disadvantages of choosing a particular location before you make your final choice;

(b) a good access road to the quarry has to be constructed. This road should be wide enough to allow two lorries/tractors + trailers/donkey carts (whatever means of hauling is used) coming from opposite directions to pass easily. In areas where it rains frequently you may consider providing a gravel layer of a minimum width of 3 metres to ensure that the access road stays passable even during rains;

(c) the camp and stores have to be constructed. Ensure that at least one (or, if many people will be staying in the camp, more as required) latrine is prepared and that a place for taking a shower and washing is available.

(d) trees and bushes must be cleared from the quarry site and the topsoil (over burden) covering the good gravel material must be removed and stockpiled. This topsoil should be respread after the quarry has been exhausted;

(e) when the topsoil has been removed, the loading bays (places where the transport will be loaded) should be prepared. It is recommended to stockpile, in advance, a volume of gravel sufficient for two days' gravelling. This allows a good control of the excavation gang which, ideally, should stay two days ahead of the loaders;

(f) turning bays should be prepared along the road. The distance in between two turning bays should be a maximum of 100 metres, so that no time and fuel is wasted during the gravelling operation;

(g) finally, the road to be gravelled should be repaired as necessary. Pot-holes should be filled and the camber should be restored where required. A length of road equivalent to one day's production should be prepared before the first load is transported.

It is recommended to limit the number of workers carrying out these preparatory activities to about 25. It is usually possible to select suitable gang leaders from this first group of workers. You car, do this by carefully observing individuals who demonstrate a positive attitude towards the work as well as man-management and leadership abilities.

LAYOUT OF THE QUARRY

The quarry should be operated in such a way that the lorries/tractors + trailers/donkey carts can enter and leave without being in each other's way. Unnecessary waiting time is very expensive an a should be avoided!

Figures 3 and 4 show two plans for a quarry layout. Figure 3 illustrates that a wider access road and more turning space is required when a quarry has only one access. The ring-road solution shown on figure 4 is very good as it allows the vehicles to enter and leave the quarry and to manoeuvre with a minimum of interference. There is a circular traffic flow and the access road to and from the quarry can be constructed to a width sufficient for one vehicle only.


Fig 3. Large turning place required


Fig 4. Ring road allows turning on a relatively small area

To facilitate the loading and to increase the daily production, you should ensure that at all times the vehicles to be loaded are situated at the lowest level possible.

In level terrain this can be done as shown on figure 5a (stages 1, 2 and 3). In a hillside quarry the material can best be excavated in steps (figure 5b).

This rule is extremely important as loading from above is far easier and loading productivity will increase considerably if the vehicles are placed in a low position.

Fig 5a. Quarry in level area


Stage 1


Stage 2


Stage 3


Fig 5b. Hill side quarry

A great advantage of using trailers for gravelling is that more than one trailer can be used for each pulling vehicle. This means that, while one trailer is transported to site, one or two others can be loaded. The pulling vehicle will therefore never have to wait as is the case when lorries are used. Also, when loading is done by hand, well-designed trailers are easier to load than lorries because of their low loading height.

When trailers are used, organise the work in such a way that full trailers are waiting by the time the tractors arrive in the quarry. The main objective is to keep the expensive hauling equipment (the tractors) running throughout the day.

Organisation of the workers

The quarry: excavation, stockpiling and loading

The best way to manage and control large groups of labourers is to form several groups performing well-defined tasks.

As far as the quarry is concerned, this means that a number of sub-groups should work on excavation, while others work on loading.

When the gravel is relatively easy to excavate, one way of organising the work is as follows: The workers doing excavation work in one section of the quarry stockpile heaps of loose material to be loaded the following day. The volume per day to be excavated and stockpiled by each gang should be set out before the work starts. This volume should be sufficient for next day's loading.

The loading groups are then given a daily task to fill a specific number of hauling vehicles. Each group should work with its own vehicle until the daily number of trips have been made.

The following day the groups move to each other's sections: the loading of the vehicles can now be done in that section where the excavation groups have stockpiled the loose material the previous day. The excavation and stockpiling should be carried out in the section where the vehicles were loaded the day before.

Learning element 6 “Preparation of Workplan” gives some details on average productivity (output per man/day) which may be expected from workers doing excavation, loading and spreading.

When the gravel is very hard to excavate, it may be easier and more productive to combine the excavation and loading into one task. A larger group of workers is then responsible for the excavation and loading of a specified number of vehicles. The advantage of this system is that the workers can have turns among themselves on the more difficult excavation work and that the whole group has the responsibility for loading the required quantity of gravel.

Hauling, dumping and spreading

Always aim to organise the dumping in such a way that waiting time of the vehicles is minimised. Make sure that enough turning places are available along the road and that the distance between them does not exceed 100 metres.

There are two ways to organise the dumping; towards or away from the quarry. There are a number of advantages/disadvantages for each of these methods: gravelling towards the quarry can be organised in such a way that the vehicles have only very short waiting times even if four or five loaded vehicles arrive at the same time. The vehicles turn in between the quarry and the dumping place and reverse until they have reached the point where the gravel should be off-loaded. After dumping, they drive back to the quarry and the next vehicle dumps the next load.

However, the use of this method means that the vehicles always drive over (sections of) earth road, which may cause these sections to be damaged especially in rainy periods. If it rains, it may even become impossible to continue gravelling, as the earth road will become muddy and slippery. Also (if tractors/trailers are used), reversing can pose considerable problems - in particular, uphill reversing requires very skilled drivers.

Gravelling away from the quarry does not have such problems as the vehicles always drive over already gravelled sections. This method has the additional advantage that the hauling vehicles frequently pass over the freshly-dumped gravel and compact the layer thoroughly. The drivers should be instructed to drive in such a way that the whole width is compacted and no ruts are formed.

However, gravelling away from the quarry also has its disadvantages.

(1) If a number of loaded vehicles arrive at the same time, waiting time will be considerable as the gravel dumped by the first vehicle will have to be spread before the next vehicle can pass over it.

(2) Unless a ring-road can be constructed (which is only possible in exceptional circumstances), drivers will tend to return to the quarry passing over and damaging the shoulders and ditches of the road.

(3) Gravelling away from the quarry means that the greatest number of workers will be required in the beginning stages of the work and that the first workers will have to be laid off very soon. (The longer the hauling distance, the less workers are needed for loading, excavating and spreading - see LE-6, planning.)

It is often best therefore to organise the gravelling in such a way that, to the extent possible, the advantages of both methods are exploited. LE-6 describes how a plan is made for the gravelling of a road - starting from the middle, working towards the quarry, returning to the middle and working away from the quarry to the end of the road.

Of course, even when the gravelling direction in general is towards a quarry, the actual dumping can be done away from the quarry (e.g. start at chainage point 3+000 and gravel up to point 3+500, then return to point 2+500 and gravel to point 3+000, etc.). In this way, the loaded hauling vehicles compact the gravel layer but also the employment and layoff of workers can be well organised - as described in LE-6.

At the dump site the width and length of the area to be gravelled by each load should be clearly indicated. Remember that the compacted thickness of a gravel layer is only about 80 per cent of the non-compacted thickness. In other words, in order to get a compacted thickness of 10 cm, a non-compacted layer of approximately 12.5 cm will have to be provided.

Thus a 3 m3 load of loose gravel is sufficient to provide 10 cm compacted gravel layer for a surface of 24 m2 (). If, for example, a road width of 4 m is to be gravelled, you should instruct your gang leader to set out rectangles with a width of 4 metres and a length of 6 metres. Ensure that the road surface is repaired and shaped correctly before any gravel is dumped.

The vehicle driver should be instructed to dump the entire load within one rectangle. To facilitate the work of the spreaders, the vehicle should slowly move forward while dumping the load so that the gravel is distributed equally along the length of the rectangle.

The labourers then spread the gravel with special spreading rakes or, if these are not available, with hoes or forked hoes. Oversized pieces (bigger than 5 cm) should be crushed with sledge-hammers or removed - to be used for drainage protection works (scour checks, lining of eroded ditches, etc.).

The camber of the finished layer should be equal to that of the earth road. For gravel roads, a minimum camber of 5 per cent is recommended.

Since the different activities are not equally arduous (spreading is easier than excavating or loading) and tasks do not finish at the same time (loading and spreading tend to finish later than excavation), you can consider to rotate the workers so that everybody gets his turn at each of the activities. This rotation can, for example, be organised on a weekly basis after each worker has done one activity during five or six working days. If there are older persons or women among the workers it is best to utilise these workers for spreading. In this case, only the labourers working on loading and excavation would change jobs.

Learning Element: LE-6 Preparation of workplan

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- list the inputs required for labour-based gravelling;
- make a workplan for labour-based gravelling;
- use one method of monitoring the gravelling operation.

A workplan for the gravelling should give information on inputs, quarry location and hauling distance and expected outputs.

Inputs

The inputs required for labour-based gravelling are:

- supervisory personnel (senior supervisor and gang leaders);

- skilled operators/drivers for the hauling equipment;

- other skilled personnel (mechanic, carpenter, storekeeper);

- labourers for preparatory and main gravelling activities;

- hauling equipment (which can be lorries/tractor-drawn, animal-drawn or wheelbarrows, depending on average hauling distance and availability);

- spare parts and fuel (if mechanical power is used);

- hand tools;

- setting out and controlling aids (camberboards, pegs, strings, tape measures);

- site camp (housing, sanitary facilities, stores).

The quantity of inputs required depends to a great extent on two factors:

(1) the availability of hauling equipment (how many lorries, tractors/trailers or animal-drawn carts can be made available for the project and how many on average can be expected to be operational throughout the project?).

(2) the average hauling distance.

Table 1 shows how many trips per day can be expected from a tractor having two trailers, one being hauled, while the other is loaded. It also indicates the optimal number of tractors for each hauling distance and the number of workers required. The output of the workers is estimated to be 3 m3 per man/day for excavation, 6 m3 (loose) per man/day for loading, and 12 m3 per man/day for spreading. The quantity of material transported per trip is assumed to be 3 m3. The average speed of the tractor/trailer combination is estimated to be 15 km/hour.

Table 1:

Hauling distance (one way)

No. of trips per tractor per day (6 hr. availability)

Optimum no. of tractors

Number of workers (exc. loading, spreading)

Total




Exc

Load

Spread


0-1

18

2

36

18

9

63

1-2

12

3

36

18

9

63

2-3

9

4

36

18

9

63

3-4

7

4

28

14

7

49

4-5

6

4

24

12

6

42

5-6

5

4

20

10

5

35

6-7

4

4

16

8

4

28

7-8

3

4

12

6

3

21

This table can be used for planning purposes but the figures given should be tested in the field and adapted to the local conditions. For example, the output per man/day may be higher if the worker works on piece rate. When the hauling route is in very good condition, a higher average speed may be possible. With other types of transport different speeds will be obtained, etc... By experimenting and studying the works for a period of two/three days, it should be possible to arrive at realistic figures for a particular situation.

After you have estimated the type and number of pieces of hauling equipment and the number of trips you expect these to make per day, you can estimate the number of workers required.

The workplan

The length of road section gravelled per trip can be calculated by dividing the quantity of material transported by (i) the gravelled width of the road, and (ii) the thickness of the loose gravel required.

For example, when it is specified that a road should have a gravelled width of 4 metres and the thickness of the layer of (loose) gravel should be 0.15 metre, one load of 3 cubic metres will gravel a length of 3/0.6 = 5 metres.

Figure 6 shows the location of the quarry. Since this information is of vital importance, this sketch should always be attached to the workplan. It is good practice to make the final sketch on the reverse side of the workplan.


Fig 6

Table 2 is an example of a prepared workplan based on table 1. Note that table 1 is only valid for the assumptions made on page LE-6/2. (Excavation 3 m3/man/day, loading 6 m3/man/day, spreading 12 m3/man/day, tractor speed 15 km/hour, quantity of material transported per trip 3 m3.)

The actual inputs and outputs should be recorded carefully on daily/weekly and monthly report forms. This data can later serve as a basis for even more accurate planning.

Table 2

WORKPLAN GRAVELLING

Project:

Date:

1

2

3

4

5

6

7

8

Hauling distance (HD) (km)

Length within HD interval (m)

Trips per tractor per day (Table 1)

Av. No. of tractors working

Planned daily trips (total)

Metres gravelled per day

No. of workers (table 1)

No. of working days within HD interval

2-3

500

9

4

36

180

63

2.8

3-4

1 000

7

4

28

140

49

7.2

4-5

1 000

6

4

24

120

42

8.4

5-6

1 000

5

4

20

100

35

10.0

6-7

1 000

4

4

16

80

28

12.5

7-8

1 000

3

4

12

60

21

16.7

Total

5 500



Total planned working days 57.6





Total planned man/days 1 933

As you can see, this workplan is theoretical. It is not possible to achieve the optimum number of man/days calculated because the numbers of working days used in the calculations are fractions. Also, it is not practical to employ 63 workers on day 1 and dismiss 14 of these workers at the end of day 3.

A good way to achieve a minimum number of man/days and to avoid problems with employment and lay-off is the following:

- carry out the build-up and the lay-off of the labour force in a limited number of steps;

- start gravelling in the middle of the road with a hauling distance in the middle of the range and work towards the quarry;

- when the quarry, or the nearest point to it on the road, is reached, return to the middle of the road and continue towards the far end.

In the example of table 2 and figure 6, this means:

- start gravelling at chainage point 3+500 (6 km from the quarry) with a labour force of 35. Theoretically the gravelling from chainage point 3+500 to chainage point 2+500 takes 10 working days (table 2). Therefore:

- employ 7 more labourers from working day 11.

- the gravelling from chainage point 2+500 to chainage point 1+500 takes 8.4 working days. Therefore:

- employ 7 more labourers from working day 20.

- the gravelling from chainage point 1+500 to chainage point 0+500 takes 7.2 working days. Therefore:

- employ 14 more labourers from working day 27.

- the gravelling from chainage point 0+500 to chainage point 0+000 takes 2.8 working days. Therefore:

- lay off 35 labourers from working day 30.

- the gravelling from chainage point 3+500 to chainage point 4+500 takes 12.5 working days. Therefore:

- lay off 7 labourers from working day 43.

- the gravelling from chainage point 4+500 to chainage point 5+500 takes 16.7 working days. Therefore:

- lay off 27 labourers from working day 60.

Table 3

You now use:

Workers


Working days

Man/days

35

×

10

350

42

×

9

378

49

×

7

343

63

×

3

189

28

×

13

364

21

×

17

357


59

1 981

You should aim to gradually lay off your labourers towards the end of the project, as otherwise the productivity tends to go down. By arranging the employment in this way, you are also able to select and keep the best workers for the longest period of time.

Of course the camp, the quarry and access road to the quarry need to be prepared before the tractors and trailers arrive. As mentioned in LE-5, this should be done with a maximum of 25 labourers. The number of man/days required depends on the quantity of work to be done and the final workplan should include an estimate based on a realistic quantity survey.

You can now make the final workplan. It is good practice to make this plan visible on a bar chart. Figure 7 shows how to do this.


Fig 7

It is immediately clear from this figure that:

(a) Gravelling is carried out from chainage 3+500 to 0+000 and subsequently from 3+500 to 5+500.

(b) Gravelling between, for example, chainage 4+500 and 5+500 is planned to be carried out in 17 working days using an average of 21 labourers and 4 tractor/trailer combinations.

Figure 8 shows how the weekly report data can be visualised to allow a comparison of the actual and the planned progress to be made.

At the end of each week the actual length gravelled is plotted and all relevant data such as actual number of tractors/trailers, actual labour force and number of working days are noted as shown.

At the end of week 6, the actual progress was 3 km of gravelling. Twenty-eight working days and 1,138 man/days were used to produce this result. The gravelling is two working days behind schedule and 67 man/days more than planned were used.

It can be seen that this is mainly due to the fact that in week 3 only 3 tractors were working. The employment of new workers was consequently delayed until week 4 and only 375 metres of road were gravelled.


Fig 8

In week 5, 6 days were worked and 840 metres were gravelled. In week 6, only 3 working days were available (public holiday, rain) and one tractor had broken down. In this week only 340 metres were gravelled.

This planning and monitoring system is particularly suited to the gravelling operation as it not only shows how much is done and which resources are used, but also why the original targets are or are not achieved.

Learning Element: LE-7 Administration and monitoring

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- use the various gravelling report forms;
- describe why these forms are necessary and what they are recording.

Gravelling is a complicated and expensive operation, involving a considerable number of skilled personnel (operators, drivers, mechanics) as well as various types of equipment.

To keep track of the results and the inputs required to achieve these results, a good administrative and reporting system is essential. This system should monitor the progress, the use of tools and materials, the number of man/days of skilled and unskilled personnel, the usage of fuel and lubricants, the number of vehicle-kilometres or hours, etc. Several forms have to be regularly completed for this purpose.

Daily/weekly report

This report describes how many and which type of people are working on the different activities, the number and type of vehicles working, the number of loads transported, the length of road gravelled and other relevant information. It should be filled on a daily basis and summarised at the end of the week.

Weekly report

At the end of each week the information of the daily report is added/summarised and additional information on the fuel, oil, tools and equipment situation is recorded.

Stores ledger

In this ledger all materials, hand tools, oil, fuel, etc. are recorded. Every time an item is received, issued or returned the storekeeper must make an entry in this book. It should always be possible to:

(1) physically check the balance of usable and unusable items and compare it with the balance as recorded in the ledger; and

(2) find out when and to whom tools/materials/fuel have been issued.

Monthly report

This report summarises the daily/weekly reports and the information of the work tickets. It should be completed by a senior supervisor in the presence of the supervisor in charge of gravelling. All information (progress, kilometrage of vehicles, fuel consumption, store balance, man/days) should be physically checked by the senior supervisor before the report is submitted to the site engineer.

Work tickets

On these forms travel with project vehicles is authorised. Details of vehicle - services, fuel, oil and lubricant consumption - should be recorded on these forms as and when they occur.

Normally only one person should authorise travel with project vehicles and each driver/operator should have his own vehicle. The names of these persons and their signatures should be shown on the work ticket so that unauthorised use of the vehicle can be controlled. The information recorded on the work ticket is summarised on monthly report at the end of each month.

Muster rolls

It is recommended to keep separate muster rolls for the regular (monthly employed) personnel and the unskilled (daily employed) labour. It is good practice to keep duplicates of each muster roll, so that a record is always available on site even at those times when the original muster rolls are required at headquarters for preparing and checking of payrolls.

Sample of daily/weekly report and instructions for use

Sample of monthly report and instructions for use

Sample of stores ledger and instructions for use

Sample of work ticket and instructions for use

Sample of muster roll and instructions for use

Learning Element: LE-8 Equipment, tools and materials

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- list the types of equipment, tools, materials and consumable stores available for the gravelling;

- describe the design and quality requirements of the most important hand tools for gravelling;

- describe how fuel should be handled and recorded.

For the gravelling operation you have at your disposal the equipment, tools, materials and consumable stores listed on pp. LE-8/2 and LE-8/3.

Remember to keep from the beginning good records of the numbers and the condition of all items allocated to you. Check and record the mileage/kilometrage or hours indicated on the various machines/vehicles and ensure that all meters are working. If not, report this immediately to avoid problems in the future. Fuel consumption should be checked regularly and compared to meter readings to avoid misuse of fuel. The supervisor in charge of equipment should know by experience approximately what the average fuel consumption for each vehicle should be. When it is evident that this consumption is either too high or too low, he should find out what the cause is (leaking tanks, faulty speedometers, misuse of fuel, etc.).

List of equipment

Description

Licence number

Number on site







List of tools and spare parts

Description

Number in store





List of materials

Description

Number on site





List of consumable items

Description

Number on site





The following remarks can be made concerning each type of equipment/vehicle:

For the gravelling equipment/vehicles the following service schedule should be adhered to:

When a piece of equipment/vehicle breaks down the repair should be carried out as follows:

Tools

Excavating, loading and spreading of gravel are very “tool-wearing” activities. For this reason it is extremely important that tools for gravelling are well designed and have a good quality. Pickaxes should be of the heavy type (weight of head between 3 and 3.6 kg) and fitted with a handle with an elliptical cross-section.

Shovels should be of the heavy duty, round-mouth type. The blade thickness should be a minimum of 1.75 mm.

The rakes for spreading should be of the heavy duty type. Garden rakes are definitely not suitable for spreading work. Special rakes have been designed for gravelling (see Guide to Tools and Equipment, page 1.53) and it may be possible to manufacture these locally.

Sledge-hammers should have a double-faced head and weigh between 3 and 4.5 kg. They are used to crush over-sized material on the spreading site. Sledge-hammer handles should ideally be between 75 and 90 cm long, with an elliptical cross-section and a raised safety grip to minimise the danger of the tool slipping out of the hands of the worker.

Consumable items

Of these, fuel is certainly the most costly and most frequently used. Proper handling, storage and recording of fuel consumption is therefore of utmost importance. When fuel is kept in drums, these should be handled gently to avoid leakages and damage of the drums. Also, take care to leave the drum standing for a few hours after it has been filled before you use the diesel petrol to fill vehicles. There is always a danger that the fuel has mixed with condensed water in the drum. After leaving the drum for a while the lighter fuel will be on top of, and separated from, any water there may be in the container.

When issuing fuel, keep separate records for each vehicle and note (i) the date of issue, (ii) the quantity and type of fuel issued and (iii) the registration number of the vehicle. The driver should sign for receipt of the fuel. Usually the same information should be recorded on the work ticket of the vehicle concerned.

Learning Element: LE-9 Module summary checkpoint

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should:

- know what the purpose of gravelling is;
- know how to plan, organise, implement and monitor the gravelling operation;
- know how gravel quarries are selected.

SUMMARY

The functions of a gravel layer are: (i) to provide an all-weather surface to a road and (ii) to prevent the formation of ruts and pot-holes by the passing traffic.

The decision of whether or not to gravel a road depends on the function of the road, the expected traffic density, the nature of the natural soil and the cost of gravelling.

Different methods of gravelling should be considered before gravelling is started. Depending on the hauling distance, different methods of hauling may be economical. Depending on labour availability, cost and motivation, some or most of the gravelling activities can be carried out with labour-intensive methods.

There are several simple field tests to determine whether a particular material is suitable to be used for gravelling. In case of doubt, have a sample tested in a soils laboratory.

Quarries can be found with the assistance of government departments responsible for road construction/maintenance, the local population, the local administration, or by a personal survey of exposed faces of road ditches or cuts in the area.

When selecting a quarry the quality of the material, as well as the costs of excavation and transport, should be considered.

Before the gravelling operation can start, preparatory activities have to be carried out: the location and construction of a site camp, the construction of an access road to the quarry, the preparation of the quarry, the preparation of turning bays along the road and the necessary reshaping and repair of the earth road.

The quarry work, the hauling and the spreading should be organised in such a way that waiting time of vehicles and people is minimised and loading and spreading can be done with a minimum of effort.

A proper workplan for gravelling is extremely important. The plan should give information on inputs (workers, machines) productivities, outputs and timing of the work.

A good administrative and reporting system is essential to keep track of the inputs (workers, tools, materials, fuel), outputs (kilometres gravelled with a particular hauling distance) and productivities (number of loads transported per vehicle per day, number of man/days spent on each activity, etc.).

Reporting is done on daily, weekly and monthly report forms.

For gravelling, four categories of items are required: equipment, tools, materials and consumable stores. It is important to record the condition and quantity of each category from the beginning of the works.

Hand tools for gravelling should be of the heavy duty type.

MODULE CHECKPOINT

Indicate True (T) or False (F):

1. Gravelling is always necessary when a road carries more than 20 vehicles per day

.......

2. Hauling gravel over long distances can be justified if the future traffic volumes and the benefits of an all-weather road are expected to be high

.......

3. Spreading gravel with a motor grader is always cheaper and better than spreading by manual labour

.......

4. Depending on the hauling distance, different ways of hauling may be economical to use

.......

5. Vehicles to be loaded by manual labour should always be positioned as low as possible

.......

Answer briefly:

6. Which preparatory activities need to be carried out before the excavation and hauling of the gravelling can start and roughly how much time do you estimate to be required for each of these preparatory activities when a labour force of 20 workers is employed? N.B. Assume quantities of work to be done whenever required.
.................................................................................................
.................................................................................................
.................................................................................................

7. How would you go about locating a gravel quarry?
.................................................................................................
.................................................................................................
.................................................................................................

8. Which factors need to be considered when, from three different quarries, one has to be selected?
.................................................................................................
.................................................................................................
.................................................................................................

9. List briefly the procedures to be followed for the routine maintenance, servicing and repair of equipment.
.................................................................................................
.................................................................................................
.................................................................................................

10. How do you organise the work in the quarry?
.................................................................................................
.................................................................................................
.................................................................................................

11. Which information is given in the gravelling workplan?
.................................................................................................
.................................................................................................
.................................................................................................

Field Instructions*

* Project-specific, to be completed by the instructor.

SELECTION OF QUARRY - GRAVELLING

Remember that:

- information on the location of gravel can usually be obtained from:

(i) the ministry or government department responsible for the construction and maintenance of roads;

(ii) the local people (leave samples of the type of material you want them to find); and

(iii) driving in the surroundings and examining exposed faces of road ditches or cuts;

- the choice between several quarries should depend on the quality of the material and the cost of its excavation and transport to the site;

- you should find out who owns the land and whether compensation will have to be paid. If this is the case, inquire about the regulations in respect of payment of compensation;

- quarries in a depression may be difficult to drain when it rains. Organise the excavation in such a way that the rain water will not form big pools.

A simple test to provide an indication of the suitability of a particular material:

- take a sample of the material and sprinkle some water over it;

- mold it into a ball by pressing it hard. You can feel the presence of sand and stones (gritty) and the volume will not become a great deal smaller (the stones and sand form a skeleton);

- leave the sample aside for drying. When, after drying, it does not fall apart, you can assume that a sufficient amount of binder (clayey material) is present;

- take another sample; moisten it and make a flat, thick pancake. Take a pencil and try to penetrate the sample in different places. If the pencil penetrates easily without resistance and the wet sample leaves your hands muddy and sticky, this indicates that the proportion of fines (clay, silt) is likely to be too high and the material is not suitable;

- take a few of the bigger stony particles, moisten them and press hard between finger and thumb. If you manage to crumble the particles or if they break very easily it is likely that they either consist of hard clay or decomposed rock and therefore are not suitable to form the stone fraction of the gravel layer.

Note: Several types of lime stone or coral have been successfully used in gravel layers. If you are in doubt, consult your engineer or try to get advice from a soils laboratory.

COMPOSITION OF THE GRAVEL LAYER

Remember that:

- a good gravel layer should have stony particles which provide the necessary strength and a clay fraction which binds the layer together. In wetter areas the clay fraction should be smaller, in dryer areas more clay can be permitted. The clay fraction should be between 10 and 25 per cent of the total volume of the layer, the rest of the layer should consist of stones of various sizes and sand.

ORGANISATIONAL STRUCTURE - GRAVELLING

GENERAL RULES - GRAVELLING

Remember that:

Routine maintenance and servicing of equipment

Repair of equipment

Storing of fuel and oil

Issue and return of tools and materials

Supply of drinking water

Payment of wages

ORGANISATION OF THE WORK - GRAVELLING

Remember that:

Preparatory work

- A large number of preparatory activities have to be carried out before the gravelling can start. These activities have to be carried out when the equipment is still working elsewhere. Ensure that the following preparatory activities have been carried out before the hauling equipment is moved to the site:

(i) the location of a suitable site for the camp and the construction of accommodation, stores and latrines;

(ii) the construction of an access road to the quarry;

(iii) the clearing of the quarry site and the removal of the overburden on top of the gravel;

(iv) the preparation of loading bays in the quarry and turning bays along the road;

(v) the stockpiling of a two-day supply of gravel; and

(vi) the repair and re-shaping of a sufficiently long section of road.

The quarry

- The quarry should be laid out in such a way that the hauling equipment can enter and leave without being in each other's way.

- Vehicles to be loaded should be placed as low as possible.

- When tractors with several trailers are used, full trailers should be waiting when the tractors return with empty trailers to the quarry.

- The workers should be working in different groups on excavation, loading and spreading. The excavating and loading groups in the quarry should each work with their own piece of hauling equipment (lorry, tractor/trailer, donkey cart, or whatever is used).

- Tasks should be given to groups of workers to excavate a certain volume and to load and spread a certain number of vehicle loads.

- Sometimes it is easier to combine excavation and loading into a single activity. A larger group of workers is then required. If possible, however, it is preferable to separate excavation and loading, as mentioned above, because it will then be possible to stockpile a volume of gravel in advance. This will make it easier for you to plan, programme and control the daily number of trips of gravel to be transported to the site.

Hauling, dumping and spreading

- The hauling, dumping and spreading should be organised in such a way that:

(i) waiting time of the hauling equipment is minimum;

(ii) the gravel is compacted on the same day as it was dumped and spread;

(iii) the employment and dismissal of the casual workers can be done in steps. Find out for each situation how these objectives can best be achieved. You can gravel towards or away from the quarry and you can start gravelling at different chainage points of the road (see FIELD INSTRUCTIONS - PLANNING AND MONITORING - GRAVELLING).

- The road surface should be repaired and shaped correctly before any gravel is dumped.

- Pegs should be used to mark the area to be gravelled by each load.

- The layer thickness of compacted gravel is only about 80 per cent of the layer thickness of the not-compacted gravel.

- Gravel pieces bigger than 5 cm should be crushed with sledge-hammers or (in the case of suitably sized hard stones) be used for drainage protection works.

PLANNING AND MONITORING - GRAVELLING

Remember that:

- a workplan for gravelling should give information on inputs (supervision, labour, equipment and materials), expected outputs (number of loads transported, number of metres gravelled per day), hauling distance(s) and location(s) of quarry(ies);

- in order to make a workplan you have to estimate productivities (number of m3 one man can excavate/load/spread per day). Determine these productivities by monitoring the work performance of daily-paid workers for a period or obtain guidelines from your headquarters;

- you also need to know the hauling distances, the quantity of material transported by one vehicle and the average speed of these vehicles. Examples of data for gravelling with tractor/trailers are given in M-14, LE-6;

- when you have a certain number of vehicles available for the gravelling, assume when you are planning that at least 25 per cent will not be operating (services, breakdowns, other work, etc.);

- you should plan in such a way that the employment and dismissal of workers can be done in steps. One way to do this is to start gravelling at the middle hauling distance, to gravel towards the quarry, to return to the start point and to gravel away from the quarry until the job is finished (see example M-14, LE-6);

- at the end of each week the information of the actual inputs and outputs should be taken from the weekly report and shown on the workplan. If there are big differences between the expected outputs (workplan) and actual outputs (weekly report), find out why and report the reasons to your supervisor. (Wrong assumptions on productivities, more breakdowns than expected, shortages of fuel, shortages of labour, etc.)

ADMINISTRATION AND REPORTING - GRAVELLING

Remember that:

Daily report

Weekly report

Monthly report

Stores ledger

Work tickets

Muster rolls

EQUIPMENT, TOOLS AND MATERIALS - GRAVELLING

Remember that:

- you should, from the beginning, keep good records of the numbers, types and condition of the equipment, tools and materials issued to you;

- you should note meter readings of fuel-consuming vehicles on a regular basis and compare these readings with the actual fuel consumption of these vehicles;

- hand-tools for gravelling (pickaxes, sledge-hammers, mattocks, shovels, crowbars, rakes) should be of the heavy duty type, well designed and of a good quality;

- fuel should be properly stored and handled and separate records should be kept for each vehicle.

Service schedule for the gravelling equipment/vehicles*

Repair of gravelling equipment/vehicles*

* Project-specific, to be completed by the instructor.

Learning Element: LE-0 Module learning objectives

After you have learned this module you should be able to:

- explain why maintenance is necessary and what the purpose of maintenance is;

- describe routine maintenance activities;

- discuss some factors influencing the choice of routine maintenance methods;

- describe when, how and for which purposes equipment should be used;

- describe different systems to carry out routine maintenance and mention some advantages/disadvantages of those systems;

- describe which tools are necessary and how they are administered.

Learning Element: LE-1 Nature and definition of maintenance

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- explain why maintenance is necessary;

- describe a number of common failures occurring because maintenance is insufficient;

- describe three different types of maintenance.

However well roads are constructed, without regular maintenance they are doomed to deteriorate and eventually disappear. At first, there will be only slight damage when, because of a flattened camber the surface drainage will become inadequate. After rainfall, water will be left behind in the lower places of the road surface layer, softening and weakening such places. The passing traffic will then eventually and inevitably form pot-holes (figure 1).

Fig 1


Before


After

Another characteristic is the formation of ruts, caused by the tendency of most drivers to follow tracks previously made. This tendency is reinforced when grass and other vegetation on the shoulders of the road formation are not kept short.

Finally, corrugations are very often formed, especially on granular soils. Their rapidity of formation depends mostly on the amount and type of traffic and the soil type.

The second stage of deterioration is more serious. A flattened camber combined with silted ditches give the road an entirely different cross-section (figure 2).


Fig 2

As you can see, there is nowhere the water can go, which means that it will either be stagnant in the middle of the road if the road runs through level terrain or move downwards in the longitudinal direction if the road runs through sloping terrain. Where traffic has formed ruts, these ruts will act as channels which will be deepened and widened after every rainfall.

The third stage of deterioration occurs when the cross-drainage (culverts) become blocked. When the road formation is still higher than the surrounding area, the road will act as a dam, which in principle is still trafficable although the saturation with water will have a weakening effect. However, as soon as the water starts crossing the road, very serious damage will occur in a very short time. When a road is well-designed and constructed, most of these problems can be avoided by regular maintenance.

This type of maintenance is called routine maintenance.

When special problems occur such as landslides, rockfalls, culvert damage or road wash-aways, a special task force is required to reconstruct the road. In many cases, such problems are caused by design or construction mistakes. It is very important therefore to examine why certain failures have occurred.

Is the capacity of the culvert/structure sufficient?

Do headwalls or retaining walls require strengthening?

Has a water course shifted its route?

Are embankment slopes or cut faces adequately protected against erosion?

This type of maintenance is called special maintenance. The work involved varies from case to case and, as explained above, it is essential to remember that detailed examinations of why the failures occur are necessary.

Even when the routine maintenance is carried out with great care, after a number of years natural wear and tear will necessitate the upgrading of a road. For earth roads this upgrading would mean a thorough reshaping and where necessary raising the level of the crown of the road above the surrounding environment. For gravel roads, it covers the replacement of the gravel surface. All aspects of (re-) gravelling are described in detail in module M-14. For bitumen roads it usually consists of the provision of a surface dressing. These activities are called periodic maintenance.

Learning Element: LE-2 Routine maintenance activities

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- list the routine maintenance activities;

- explain why these activities are necessary;

- explain when, instead of maintenance, realignment or reconstruction are required.

ACTIVITIES

The activities to be carried out under routine maintenance are:

- the filling of pot-holes and/or ruts with material similar to material used for the road surface layer;

- the compaction of this material;

- the maintaining of the correct camber of the road by retrieving loose material which has been transported to the edges of the road and respreading and compaction of this material;

- the removal of corrugations;

- the cutting of vegetation growing on the verges of the road. The verges of the road include the shoulders of the road formation and the stretches of ground sloping down from the shoulders to the side drains;

- the repair of erosion channels which have been formed on the running surface, the shoulders or the ditch slopes;

- the clearing of waste material such as debris, vegetation and silt from the ditches, catchwater drains and run-off drains;

- the maintaining of the original cross-sections of ditches, catchwater drains and run-off drains;

- the clearing of silt and debris from culverts, drifts and other structures to allow a free flow of water.

It is important to remember that whenever routine maintenance becomes excessive (something which will have to be judged from case to case) reconstruction or realignment may be in order. If, for example, a culvert gets completely silted up after only a few rains, it is quite likely that either it is placed too low or not laid in the correct slope downwards. In both cases, the water cannot flow freely so that the silt can settle. The re-positioning of this culvert will, in the long term, certainly prove to be cheaper than the continuous removing of silt.

Always ensure that the workers responsible for carrying out certain activities have the right type and quality of tools to do the job. For example, a long-handled shovel should be provided for the cleaning of culvert pipes. Also, maintenance workers should have the possibility to repair/sharpen their tools when necessary. It is good practice for example to carry a number of maintenance tools and materials (pliers, bolts, nuts for wheelbarrows, a saw and some hardwood wedges to repair handles, etc.) during inspection tours. Workers can sign for such tools and keep them for a limited period, returning them during the next inspection tour.

Learning Element: LE-3 Choice of methods for routine maintenance

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- discuss some factors which influence the choice of routine maintenance methods for gravel and earth roads.

MAINTENANCE BY MEN, MACHINES OR A COMBINATION OF THE TWO?

It is possible to carry out all the above activities by manual methods provided that suitable tools are supplied. In fact, many of these activities can only be carried out by manual methods (culvert cleaning, clearing of ditches with obstructions such as scour checks, culverts, overhanging trees, clearing of catchment drains, filling of pot-holes). In those cases, however, where a smooth riding surface is of prime importance, the use of a motor-grader (in dry weather in combination with water bowser and roller) may be indispensable. We are thinking of heavily trafficked gravel roads with a high design speed where corrugations will form rapidly. On the other hand, the great majority of rural roads do not carry more than 30 vehicles per day. On such roads, an entirely smooth riding surface is not required, since speeds greater than 30-40 km/hour are neither essential nor desirable. The main requirement of this type of road is an all-year-round passability so that access into the rural areas is guaranteed.

It is also very important to consider how the road was constructed in the first place and what state it is in, before a maintenance method is chosen. If, for example, the road has not been constructed to very exact grades and cross falls, a motor grader could do serious damage to the gravel layer by shifting the gravel from the higher to the lower spots. This could not only result in an uneven thickness of the gravel layer, but in some places even in a total exposure of the earth foundation (figure 3).

Fig 3


Longitudinal sections


Plans

Finally, a factor to be considered when the gravelled width of the running surface is not great (say, 4 metres) is that a motor grader will often move inferior material from the shoulders onto the gravel, thereby reducing the effectiveness of the gravel layer.

Summarising, before a maintenance method is adopted for a particular road, it is important to consider:

(i) what is the purpose and function of the road?

(ii) how many vehicles per day are expected to use it?

(iii) is the road designed to be used at high speeds, i.e. is a smooth riding surface essential?

(iv) how was the road constructed and gravelled? - to exact grades and cross falls or with a gravel layer following an earth formation which was not strictly to level at the time of gravelling?

(v) is a motor grader likely to damage the surface layer by disturbing the uniformity of thickness of this layer (see (iv) or, when the gravelled width is limited, by placing inferior material on top of this layer?

(vi) where is the road located? Are there any possibilities to engage local labour to do the routine maintenance?

LE-4 discusses maintenance with equipment and LE-5 maintenance by labour. If a routine maintenance system by labour is considered, factors like labour availability, willingness and motivation become extremely important. The first two may be influenced by offering part-time employment while motivation can be improved by providing the right type of incentives. Which type of incentives can be successfully applied depends on the country situation (several types of contract systems have been used in different countries).

Learning Element: LE-4 Routine maintenance with equipment

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- describe for which purposes a motor grader is used and how the operations should be carried out;

- explain for which purposes tractors with drawn equipment can be useful.

ROUTINE MAINTENANCE OF GRAVEL ROADS USING EQUIPMENT

Running surface

In those cases where the type of road and the traffic density justify the use of a motor grader the machine is used to:

(i) remove corrugations;

(ii) retrieve the gravel moved by the traffic to the edge of the road and respread this material over the road surface;

(iii) correct the road profile in order to maintain the original cross falls.

It is important to ensure that the corrugations are removed to their full depth to prevent the remaining low spots from becoming filled with uncompacted material which will easily be washed away during following rains. Also, if the corrugations are not fully removed, they will reform very rapidly.

The removal of corrugations is best done with the cutting edge well in advance of the rest of the blade. In spreading loose gravel, the grader should start its first run with the blade well over the shoulder with the cutting edge behind the rest of the blade. When more than very light grading has been done, it will be necessary to compact the loose material which is left distributed over the road surface. Since the layer of loose gravel will usually not have a thickness greater than 5 centimetres, three passes of either an 8-10 ton smooth-wheel roller or a 2-ton vibrating roller should provide adequate compaction.

It is often advantageous to carry out major maintenance work after rains when the gravel or soil is suitably moist so that adequate compaction is more easily achieved. As a rule of thumb it can be assumed that a re-grading should be carried out after the passage of approximately 1,500 vehicles.

Even when a motor grader is used there are many tasks which can only be undertaken by labour. Examples of such activities are clearing of culverts, filling and compacting of pot-holes, repair of masonry work and cutting of grass. Different ways in which this labour can be organised are described in LE-5 “Routine maintenance with labour only”.

Where tractors are available it is often worth while to experiment with locally made “drags”, such as old tractor tyres, heavy brushes, worn grader blades or steel joists. Such implements can be very useful to spread loose, dry material (especially when the angle at which such implements are drawn can be varied) and dispersing corrugations before they become compacted. The frequency with which such dragging should be carried out depends on the type of drag, the traffic density and the soil type. As a rough estimate, it can be assumed that roads carrying around 100 vehicles per day ideally require dragging twice per week with heavy drags (steel joists). Roads carrying around 50 vehicles per day should be dragged approximately once every ten days. When the daily traffic is less than 50 vehicles per day, less frequent dragging will be required. By inspecting the roads regularly you should be able to determine the necessary frequency of dragging and to establish a programme accordingly. The lighter the drag the more frequently the dragging should be carried out. Very light drags (discarded tyres) are only suitable for roads not carrying more than 20 vehicles per day.

It is often advantageous to have grass growing on the shoulders and ditch slopes, as the roots will hold the soil together and decrease or even totally prevent erosion by surface water. Where tractors are used for dragging purposes, it is also possible to combine these with towed grass-cutting machines of which a large variety is available. Such machines are, of course, most effective when the shoulders and slopes have been constructed to an even cross fall and when the grass is reasonably short. If grass and vegetation have grown high, obstacles (stones, etc.) may be hidden and could damage the machine. Also the capacity of most machines would probably be insufficient in such cases, so that manual cutting would be preferable.


Fig 4. Standard wooden drag


Fig 5

Learning Element: LE-5 Routine maintenance with labour only

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should be able to:

- describe different systems to carry out routine maintenance and mention some advantages/disadvantages of those systems;

- describe which hand tools are used for routine maintenance.

As we have noted, labour can, in principle, handle all activities described in LE-2. The only activity for which equipment (whether motor grader or tractor drawn equipment) is more suitable is the removal of corrugations. Since these corrugations occur mostly on heavily trafficked, high-speed gravel roads, this automatically implies that such roads are less suited to the “labour-only maintenance systems”. However, even when equipment is used for the maintenance of the running surface, many routine maintenance activities still have to be carried out by labour.

On those roads where corrugations do not form rapidly or where a smooth riding surface is not essential (LE-3), the running surface can be perfectly well maintained by labour, so that in these cases a “labour only” routine maintenance system can be applied.

Several systems are possible:

A. Routine maintenance by gangs of labour employed on a permanent basis by a ministry or maintenance organisation. Such gangs are usually based in maintenance camps and have transport at their disposal. Although mostly such gangs are used in combination with equipment to carry out those activities which cannot be done by equipment, in principle they could carry out all routine maintenance activities. However, several factors make this system less effective. Firstly, it is quite common that breakdowns or lack of funds cause the available transport to be grounded at the camp, so that routine maintenance activities can only be carried out in the immediate surroundings of the camp. Secondly, the labourers are often not, motivated to have a high output, unless the system allows incentives (such as free time or extra money) to be used. Thirdly, a lot of time is spent on transporting the labour to and from the site of work. With camps situated 150 km apart, as much as four working hours may be lost with transport when work has to be carried out at a point of 75 km from the camp.

The administration and organisation of this system are relatively easy as the gangs are (i) permanently employed, (ii) based on one site. This facilitates issues like payment and supervision and is probably the reason why permanently employed labour gangs combined with equipment are the most commonly occurring maintenance system.

B. Routine maintenance on a contract basis. In principle, this system does not have any of the disadvantages mentioned above because the work is carried out by people living along the roads to be maintained. If, in the recent past, the road has been constructed using labour-based methods, it is best to contract workers who have been involved in the construction activities, not only because they have some essential know-how of the required activities but also because it is often known which individuals are most reliable and hardworking. Each individual (or group of people) is given a contract to keep a stretch (section) of road in good order. What is meant by “in good order” is specified in the contract which describes in detail the activities to be carried out by the contractor (LE-2). This contract should also indicate:

(i) when the section will be inspected;
(ii) which tools will be provided;
(iii) the amount to be paid to the contractor at the end of a specified period (usually one month);
(iv) the date of payment;
(v) the actions to be taken when the conditions of the contract are not fulfilled.

Point (v) means that, if the work is not carried out as specified in the contract, payment is withheld until the next payment date. When, by that time, the work is done satisfactorily, the contractor is paid fully for the two contract periods. If at the end of the second contract period, the contractor has still failed to produce the required results, the contract is terminated (depending on the proceedings adopted) and a different contractor (or group) is engaged. In this case, no payment at all is made to the contractor but the money which has been withheld is used to pay for the maintenance work required to bring the road back to its original condition.

Length of section

Since the contractors live in the area and would often be small-scale farmers, the length of each section to be maintained by one contractor should be determined in such a way that this man has sufficient time to attend to his other activities. In other words, the maintenance activity should be an additional source of income not taking more than half of his available working time. The length of a road section to be maintained varies with:

- the number and size of the vehicles using the road;

- the soil type (does it erode quickly?);

- the type of vegetation on the shoulders and ditch slopes (how fast does it grow i.e. how frequently will it have to be cut?);

- the gradients of the road section (steep sections will erode quicker and require more maintenance);

- the type of cross-section (are there big fills or big cuts with exposed faces?);

- the number of drainage structures such as culverts, fords and bridges.

Experience with labour-based maintenance projects indicates that a section of road with a length of between one and two kilometres (depending on the factors described above) can be maintained in good order by one individual who is occupied on a half-time basis. Remember that this experience was with rural roads of a formation width of 5.5 metres and with a traffic density of less than 20 vehicles per day!

Organisation

Since this system is a contract system where only results are important, it is not necessary to check how many days or hours are worked nor whether the contractor carries out the work himself or is assisted by others. If the road is in “good order” payments are made whatever the methods used.

This system can only work well if inspection tours are made on a regular basis (say once every two weeks) and payment is made promptly on the dates specified in the contract document (of course, only when the work is properly carried out). If this is not done, the contractor will soon feel discouraged, lose all motivation and neglect his maintenance work. This is easy to understand if you place yourself in his position!

A maintenance inspector, if employed full time on this activity and having a 4-wheel drive vehicle plus driver at his disposal, can cover approximately 150 kilometres of road. He should then be able to visit every contractor twice per month, once for paying and once for a thorough inspection of the works. Additional supervision can also be provided by a gang leader/headman who could be the best contractor on the road and is paid extra to supervise the other contractors. In this case, a bicycle should be provided. After a while it may be possible to reduce the visits of the maintenance inspector to once a month, so that he can cover more kilometres.

Tools

Whatever system is used, it is essential to provide the maintenance workers with good quality hand tools and equipment adapted to the various activities they have to carry out. Hand tools and light equipment suitable for routine maintenance are:

Tools and light equipment

Estimated life (months)
(maintenance use only)

hoe*

9

grass slasher*

4

bush knife*

9

shovel*

12

long-handled shovel (culvert cleaning)

12

rake*

12

pickaxe

36

sharpening file*

12

hand-rammer*

36

wheelbarrow*

24

Each maintenance worker should have the tools marked with an asterisk. Whether the unmarked tools are required depends on the situation. For example, if gravel deposits are located in the vicinity of the road, a pickaxe could be issued to allow the worker to excavate gravel. If a road section has culverts with a narrow diameter the worker should have a long-handled shovel.

When the tools are issued, the worker should sign for their receipt. It is good practice to clearly mark the tools so that they cannot be replaced by other inferior or older ones. It is essential to have regular (at least once per month) tools inspections to check the number and condition of the tools. Such monitoring will help you to anticipate the need for replacement of tools and will enable you to adjust the estimated life figures given above. The average lifetime of a tool is likely to vary from area to area! Simple standard forms developed for this purpose will provide necessary data relevant to other areas in the country. If such forms are used, ensure that they are filled in on a regular basis!

If the tools inspection is done on pay-day, the cost of “lost” tools can be deducted from the salary or contract sum of the worker. Whenever inspection rounds or payment rounds are made a number of new tools/light equipment should be carried to be exchanged for worn or broken ones handed in by the workers.

Gravel

At the time that the road is gravelled, it is recommended to leave heaps of gravel along the road to be used for repairing pot-holes or ruts. Piles of 3 m3 at intervals of 100 metres would be sufficient for this purpose. Take care that, firstly, the heap of gravel is dumped in a safe place (e.g. not in the inside curve where it may obstruct the sight) and, secondly, it does not obstruct existing foot paths. Of course, it should be easily accessible to the maintenance workers.

It is extremely important to provide these stockpiles of gravel as otherwise pot-holes and ruts will be filled with inferior material and the surface layer will soon be spoilt.

Inspection of the works

Before payment is made to the contractor his work should be inspected. This inspection should be fair and equal to all contractors. An inspection report should be completed at least once per month and should contain the following information: name and type of road; name of contractor; section for which contractor is responsible; date of inspection; condition of surface, ditches, culverts, structures; first payment withheld; instructions given to contractor; second payment withheld; dismissal and replacement.

Inspection report form

Learning Element: LE-6 Module summary and checkpoint

LEARNING ELEMENT OBJECTIVES

After you have learned this element you should:

- know the purpose of routine maintenance, special maintenance and periodic maintenance;

- know which options exist to carry out routine maintenance;

- know the advantages/disadvantages of each option;

- be able to describe which tools and equipment can be used for maintenance.

SUMMARY

Routine maintenance is essential to prevent a fast deterioration of the road and to avoid unnecessary damage under normal weather and traffic conditions.

When major failures occur (landslides, culvert damage, wash-outs), examine why these have happened before repairs are started. Higher costs are well justified when the cause of the failure can be found and rectified.

Periodic maintenance is necessary after a number of years to bring the road back to its original state. This type of maintenance normally involves re-shaping and re-surfacing.

Depending on the type and function of the road and its average daily traffic, different methods to carry out routine maintenance can be considered. The use of manual labour for most of the routine maintenance activities can often be economical and very effective. If labour-based maintenance methods are considered the workers should be motivated to continue the work effectively even in the long run. This means that incentive systems (rewards for effective work, penalties for poor work) should be introduced. Which systems can be applied (time off, extra pay, etc.) depends on the country situation and the local customs.

Often a combination of (various types of) equipment and labour can be used. Tractors with various types of drags can be quite effective in removing corrugations. Labour can be organised in different ways (gangs of labour living in road camps, village contracts, contracts to individuals).

When a road is being gravelled, stockpiles of gravel should be dumped at regular intervals along the road to be used for filling pot-holes and ruts.

MODULE CHECKPOINT

Indicate True (T) or False (F):

1. Corrugations are formed after every rain

....

2. Routine maintenance is done to keep the road as much as possible in its original (after construction) condition

....

3. When a culvert is blocked/broken or not functioning well it is sufficient to repair it into its original state

....

4. The removal of corrugations needs to be done every two months for gravel and earth roads

....

5. When grass and vegetation have grown high, grass cutters have to be accompanied by one or two workers

....

Answer briefly:

6. What are the activities to be carried out under routine maintenance?
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7. What is periodic maintenance?
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8. What is the purpose of grading a road?
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9. Which systems are possible to organise routine maintenance? What are the disadvantages of each system?
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10. Which type of tools should be provided for routine maintenance?
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Field Instructions

ROUTINE MAINTENANCE ACTIVITIES

The activities to be carried out under routine maintenance are:

- the filling of pot-holes and/or ruts with similar material as the material used for the road surface layer and the compaction of this material;

- the maintaining of the correct camber of the road by retrieving loose material which has been transported to the edges of the road and the respreading and compaction of this material;

- the removal of corrugations;

- the cutting of vegetation growing on the road;

- the repair of erosion channels;

- the clearing of waste material such as debris, vegetation and silt from the ditches, catchwater drains and run-off drains;

- the maintaining of the original cross-sections of ditches, catchwater drains and run-off drains;

- the clearing of silt and debris from culverts, drifts and other structures to allow a free flow of water.

Remember that:

- when the road is gravelled heaps of gravel should be provided at regular intervals along the road for routine maintenance purposes.

ROUTINE MAINTENANCE - TOOLS AND EQUIPMENT

The following tools and maintenance equipment should be available in the store for routine maintenance:

estimated life
(months, maintenance only)

- hoe

9

- grass slasher

4

- bush knife

9

- shovel

12

- long-handled shovel

12

- rake

12

- pickaxe

36

- hand-rammer

36

- wheelbarrow

24

- sharpening file, pliers, bolts and nuts for wheelbarrows, grinding wheels, whetstones, claw-hammers, hardwood wedges, 50 per cent spare handles for each type of tool described above.

Remember that:

- when tools are issued the worker should sign for receipt;

- tools should be clearly marked;

- inspections should be held at regular intervals at the site stores to check the number and the condition of the tools.

MAINTENANCE - GENERAL

Remember that:

- there are three types of maintenance: routine maintenance (the upkeep of the original shape and conditions of the road), special maintenance (the repair of specific failures) and periodic maintenance (the upgrading of a road to its original condition after a number of years);

- when special maintenance has to be carried out, you should always determine why the failure has occurred and reconstruct the road in such a way that another similar failure will be avoided;

- routine maintenance workers have to carry out various types of activities (see FIELD INSTRUCTIONS - ROUTINE MAINTENANCE ACTIVITIES). They should be given proper tools to carry out these activities as well as the means to maintain these tools;

- different methods of routine maintenance (men, machines) can be appropriate for different types of roads. Always examine and discuss the possibility to use alternative methods of routine maintenance with your engineer or headquarters;

- corrugations should be removed to their full depth and the loose material should be compacted. It is preferable to carry out this work after rains when the soil contains a certain amount of moisture. In some cases tractor-drawn drags may be sufficient to remove corrugations. The frequency of dragging depends on the weight of the drag, the traffic density and the type of soil being dragged;

- grass growing on the shoulders of the road and the ditch slopes holds the soil together and will check the erosion by surface water. The grass should be kept reasonably short;

- “labour-only” maintenance systems are best suited to those roads where corrugations do not form rapidly or where a perfectly smooth riding surface is not essential. “Labour-only” maintenance systems can be supported and complemented by “dragging” programmes or occasional gradings. Care should be taken that motor graders only are used on those roads which have been constructed to exact grades and cross-falls. If this has not been the case, the grader could easily do more harm than good!

- if a “maintenance by contract” system is used, it is best to use individuals who have been involved with the construction of the road concerned. The contract should describe clearly the responsibilities of the contractor, dates of payment and inspection, tools to be provided and conditions of the work. Regular inspections and payments are essential with this maintenance system. The length of road section to be maintained by one contractor varies with the traffic density, topography, the type of road concerned, the number of drainage structures, the frequency of rain and the type of vegetation on the road. Experience has indicated that this length varies generally between 1 and 2 km, assuming the contractor spends some 100 man/hours per month on the road.

MAINTENANCE INSPECTION REPORT

(Completed sample + instructions on how the forms should be completed).