|Wells Construction: Hand Dug and Hand Drilled (Peace Corps, 1980, 282 p.)|
|Section two: Hand dug wells|
Rural communities have frequently employed hand-dug wells to increase the supply of water available for individual use. Using simple construction techniques and suitable materials, hand-dug wells can provide reliable sources of water and offer the following advantages:
· Because the community can be involved in the actual construction, it is "their" well, which they are more likely to maintain.
· The equipment needed is light and simple and thus suitable for use in remote areas.
· The construction techniques are easily taught to unskilled workers, thus cutting supervision time.
· With the exceptions of cement and reinforcing rods, the necessary materials are usually locally available, making it one of the cheapest methods of wells construction in a rural community.
· A completed well provides a reservoir at the source which will accumulate and store water from aquifers that would otherwise be too weak to use.
On the other hand, hand-dug wells present certain limitations:
· 60 meters is usually the practical limit to the depth that can be reached, although most dug wells are less than 20 meters deep.
· Construction is slow.
· Extracting large quantities of water with motorized pumps is not feasible.
· Hard rock is very difficult to penetrate and often can only be accomplished by blasting, which is slow, hard work.
· Because it is difficult to penetrate very far into the aquifer, slight fluctuations in the water table often make hand-dug wells unpredictable and unreliable.
The hand-dug well is the only method of well construction where people actually go into the well to work educational campaign to demonstrate and explain what each part of the well is and how it works might help villagers understand and then want to work with and maintain it more. In so doing, proper maintenance necessary in keeping the well functioning might be better carried out, particularly in communities using a well or an improved water source for the first time.
For sanitation reasons a pump is desirable. If installed on a hand-dug well with a full cover, a pump will help reduce chances of contamination significantly. In rural areas where pump maintenance and repair can be a real problem, large diameter wells are often the best solution to water supply problems. Pumps can be installed while leaving an accessway through which water can be drawn by rope and bucket if the pump should break down. (See Pumps Appendix).
Compared to other well sinking methods, digging a well by hand takes a long time. An organized and experienced construction team consisting of five workers plus enough people to lower and raise loads in the well can dig and line 1 meter per day in relatively loose soil that does not cave in. However, the bottom section is likely to take 2 or 3 days per meter because of the difficulty in working while water continually enters the well. Depending on how you plan to develop the well, the top section can take anywhere from a day or two to several weeks. An experienced team sinking a 20 meter well and installing pulleys on the top structure could easily take 5 weeks, including occasional days off (this, of course, assumes no major delays). A new or inexperienced group would be expected to take twice that time.
Hand-dug wells should be dug during the dry season when the water table is likely to be at or near its lowest point. The well can be sunk deeper with less interference from water flowing into it. The greater depth should also ensure a year-round supply of water.
If the well cannot be dug during the dry season, plan to go back to it at the end of the dry season to deepen it.
B. Work Outline
Outlined below are the major steps involved in digging a well. The appropriate community leaders, health committee, public works committee, and others who are interested should be involved in all the planning decisions.
· Begin community education and awareness activities to enable the people to understand what is happening and how they can benefit.
· Choose a well site based on geological factors, user preference, sanitary conditions, and accessibility.
· Determine available expertise - people (including yourself) with well or general construction experience.
· Assess materials available - tools, cement, reinforcement rods (re-rods), sand, gravel, wood.
· Select methods of construction that are most suitable for the use and available materials, considering shape, size, depth, lining, bottom, and top.
· Plan and begin any training that will be necessary for workers.
· Before construction begins, put down in writing the workplan for the construction of the entire well.
· Gather all equipment and materials needed for construction of the well at the well site. Arrange these at the site so as to facilitate construction as much as possible.
· If concrete lining rings are to be used, begin constructing them in advance. Each ring must be cured for at least 4 days before it is put in place at the well head.
· Lay out the hole with provisions for checking diameter and plumb (see p. 53).
· Arrange for people and materials to get in and out of the well.
· Dig and line the middle section.
· Continue the digging and lining procedure until
(1) you reach water, or
(2) some obstruction causes you to
(a) change digging/lining procedure
(b) abandon this well and pick a new site.
· Dig and line the bottom section as far as possible into the aquifer. The method used to dig and line the bottom section will often be different from the digging and lining method used in the middle section. This may be necessary because you are not only concerned with digging, lining, and possible hole collapse (as in the middle section), but also with removing enough water from the well to permit work to continue.
· Install a simple sand and gravel filter or porous concrete plug across the bottom of hole.
· Extend the lining up above ground to form the head wall.
· Build and install the well cover.
· Install the pump in the cover on the well.
· Disinfect the well.
· Build the apron (platform) around the head wall to channel the run-off to one particular place.
· Build a drainage pit or other device for removal of standing water.*
· Build an animal trough.*
· Build a wash-basin platform.*
*These items are not always necessary but should be considered.
To design a well, it is necessary to decide what materials will be used and how they will be put together. This includes determining:
· the size and shape of the hole;
· which digging and lining methods will be followed;
· how much water needs to be available, and, therefore, how deep the bottom section should go into the aquifer;
· how the top section should be constructed to best protect the well from contamination, while allowing easy access to water by those who will use the well;
· the anticipated well depth.
This chapter discusses the decisions that must be made and presents options for consideration.
B. Well Shape
The shape of the well is what it would look like if you were looking straight down into it.
C. Well Size
The size of the well is a measure of how wide it is. Some holes are very large, and some are very small. The size will be largely determined by: (1) the way it is excavated, (2) the materials used to line it, and (3) the purpose of the well.
The size of the round hole is usually expressed by its diameter, a measurement from one edge of the hole through the midpoint of the well to the other side of the circle. (See Figs. 3-2 and 3-3.)
Although wells can be dug in any shape, almost all wells are round. The reason for this is that a round well produces the greatest amount of water for the least amount of work. Also, a round lining is the strongest that can be built for the smallest quantity of materials. Thus, while other well shapes have been used without problems, a round shape enables the builder to get the most from available time, money, and materials.
Square or rectangular wells are usually dug where materials to be used in lining the well necessitate such a shape. This is most often the case when flat wood board" are the only lining materials available. Wood, however, is not recommended for several reasons which will be discussed later. (See p. 74.)
FIG. 3-1. SQUARE WELL
Before the actual digging work begins, the exact diameter of the hole must be decided (see Figs. 3-2 and 3-3).
FIG. 3-2. ROUND WELL
FIG. 3-3. DIAMETER IS THE LONGEST MEASUREMENT ACROSS THE HOLE
Many factors could determine which diameter should be used.
· If there is a government-sponsored organization or agency which does wells construction using a standard diameter, you should consider using the same diameter. Doing so will make the eventual incorporation of the well into community and government planning and development much easier.
· If forms or pre-cast lining sections are available, you might consider using them. It would necessitate choosing an appropriate diameter for the particular equipment you have. However, if the former situation (mentioned above) also exists, it should in most cases receive priority.
Generally, the choice of diameter will be based on two considerations. The well should have (a) the smallest diameter which still provides (b) a comfortable working space for the number of people that will be working in the well at one time.
a. The smaller the diameter of the well, the less soil and rock will have to be dug and the less materials will be required to line the well. Remember, if you double the diameter of the well, you increase the amount of soil and rock that must be dug by four times. For example, as indicated in the table below, a 1.0-meter diameter well 20 meters deep requires removal of 15.7 cubic meters (m³) of material while a 2.0meter diameter well 20 meters deep will require the removal of 62.8 m³.
Diameter x Diameter x 0.7854 = Area.
Area x Depth = Volume
b. The workers will need enough space so that they are not hampered in their work. There must be enough space for them to use their tools and for the bucket which will remove excavated materials from the well. Without enough space, they will continually bump into each other and the wall. During stages of its construction, a well may have two or sometimes three different diameters (see Fig. 3-4).
(1) The hole is dug to the diameter decided upon.
(2) When a lining is installed, the diameter is further reduced along with the available working space.
(3) You may be installing the bottom section lining inside the existing lining. This will further reduce the diameter.
FIG. 3-4. THREE DIFFERENT WELL DIAMETERS USED DURING CONSTRUCTION
D. Ground Conditions and Lining
It is very difficult to anticipate what the final depth of a well will be before it is begun. However, if there are other wells in the area, it is possible to get an idea of the approximate depth of the water table. This can be a great help when gathering supplies needed for lining construction, because it will enable you to stockpile approximately enough materials to complete the well.
All wells, except those drilled through rock, can be expected to cave in with time unless a lining is installed to support the well. The lining thus helps to keep the well open. There are certain acts of nature, such as earthquakes or even gradual ground shifts, which will break even the strongest linings, but these cannot be planned for or anticipated. Occasionally slight ground shifts can put pressure on linings causing them to split and separate if not strongly built. Geologists can usually predict where such shifts are likely to occur. If no such information is available, it is recommended that you build the lining strongly enough to withstand normal earth stresses.
Depending on ground conditions, you may or may not be able to dig the complete hole and then line it. In very loose sandy soil, for example, the sand from the walls of the hole will frequently cave into the hole, seriously hampering efforts to deepen the hole. There are often relatively simple methods of dealing with such problems (see p. 58).
Designing the lining for the middle section is largely a matter of assessing the ground conditions and materials availability to determine the lining materials and method most appropriate for the situation.
1. Ground Conditions
· Very loose soil (example: dry sand) - the hole is as wide as the hole is deep because its sides continually collapse and cave in (Fig. 3-5a).
· Loose soil (example: damp sand) - a relatively shallow (1 to 5 meters) hole can be dug before its sides may cave in Fig. 3-5b.
· Firm soil (example: compacted clay and sand mix) a hole can be dug to the water table with minimal danger of collapse and cave in Fig. 3-5c.
FIG. 3 -5. GROUND CONDITIONS
Unless you have had substantial experience digging in the area and this particular type of soil, or have been trained in the identification of soils and their properties do not leave the hole unlined for more than 5 meters.
The only possible advantage to digging the entire hole first is that you can then be certain that water can be reached before you start using your often expensive materials to line the well. However, if there is any question about the safety of working in an unlined section of the well, it is not worth the gamble to leave it unlined.
2. Dig and line options
· Dig a short section and line
One source has suggested that for safety reasons, no more than 5 meters of a well should be dug and left unlined. More commonly, this cautious method is used in loose soil. This means of construction is also recommended in all soils when workers are inexperienced. Using this method, wells are dug in 0.5 to 5 meter sections, and then lined.
· Dig to water table and line
This method is commonly used in firm soil, especially where the water table is not very deep. It has the previously mentioned advantage of not using any expensive materials in a well until a good supply of water can be assured. However, this method should not be attempted by workers inexperienced with well work.
· Dig complete well and then line
This method is not recommended because of the danger of cave-ins beneath the water table which would undermine the entire well shaft. The only situation in which this method might be justified is where the middle section lining must rest on the bottom section lining for support, but there are many ways of avoiding that necessity.
E. Design: The Bottom Section
There are two basic methods for constructing the bottom section - sink lining and dig-and-line.
1. Sink lining into place. Advantages include:
· The method protects workers from cave-ins during sinking;
· Workers in the well can put all their effort into removing soil and water, presumably allowing greater well penetration into the aquifer.
A disadvantage is the possibility that workers may have difficulty in firmly attaching the rings together. (See lining rings, Fig. A.)
2. Dig and then line. Advantages include:
· The method requires less special ,reparation;
· The bottom section lining attaches directly to the lower part of the middle section lining, thus producing a stronger, continuous structure.
The disadvantages include:
· Workers probably cannot get as far into the aquifer as in the other method because of the necessity for workers to remove soil and water and place reinforcing rod and concrete at the same time;
· There is greater possibility of cave-ins because of a need to work beneath concrete that has not had time to completely set;
· The method may require special fast-setting cement so as not to be washed away by water entering the well.
The purpose of the bottom section is to allow as much water as possible into the well without permitting any of the fine soil particles from the surrounding aquifer to enter the well.
There are three commonly used methods of allowing water to enter the well. (See Fig. 3-6.)
· Through porous concrete lining - Lining rings sunk into the bottom section can be made of porous concrete which acts as a filter to prevent soil particles from entering the well.
· Through angled holes in the lining - Holes can be punched in a freshly poured concrete ring which, when cured, can be sunk into the bottom section. These holes are more effective at preventing soil entry if they are slanted up toward the middle of the well.
· Though the bottom The bottom of the well should always be constructed to allow water to come up through it. Often the bottom is simply left open and uncovered but it is preferable to prevent soil entry and the gradual filling up of the well.
FIG. 3-6. WATER ENTRY INTO WELL
F. Design: Top Section
The purpose of the top section is to provide safe and easy access to well water and to prevent as much contaminated surface materials as possible from entering the well.
The design of the top section is strongly influenced by two aspects of well usage: (1) access to water or how water is drawn from the well, and (2) preventing, as much as possible, surface contaminants from entering the water. These two functions are not always compatible. It is often necessary to compromise sanitation for the sake of water access and community acceptance. Obviously you want to do this as little as possible but not to the point of jeopardizing support from the local community or government.
A top section, in fact, is not absolutely necessary for the function of a well. However, the different design of the parts of the top section is intended to make the well safer, cleaner, and more convenient for users.
Here are the major components of the top section:
· Head wall;
· Drainage apron (platform);
· Animal trough;
· Wash basin.
1. Head wall
A head wall should be built on all wells which will not be fitted with a permanent cover and a pump as a simple inexpensive safety feature which will prevent people and animals from accidentally falling in.
This is simply a wall which extends above the surface of the ground far enough to prevent most accidental entry of people, particularly children, and animals. Its external dimension is dependent on how thick you want the head wall to be. A head wall that is unnecessarily thick will encourage people to stand on it to draw their water, creating an unsafe situation. The easiest and best way to construct the head wall is as an extension of the lining. In most cases it will be convenient to build the head wall as an extension of the lining above ground. You will already have the equipment and supplies on site with which to do this. The head wall should extend 80 to 100 cm above the ground surface or apron, if there is one (see Fig. 3-7).
FIG. 3-7. TOP SECTION
2. Drainage apron (platform) (See Fig. A.)
A drainage apron is most often a reinforced concrete slab 1 to 2 meters wide which surrounds a well and, because of its slight slope, channels surface water away from the well. Wire mesh reinforcing may be used if it is available.
By forcing water to flow away from the well, the apron serves two functions:
· It prevents contaminated surface water from following the outside of the lining and flowing back down into the well before it has had a chance to be sufficiently filtered by the earth.
· It prevents the formation of a mucky area immediately around the well which can be a breeding ground for disease and a source of contaminants to the well water.
A sloping platform (Fig. 3-8) will simply move the mucky area from direct contact with the head wall to the edge of the platform. It will still be an eyesore and health hazard, although not so much as if it were right next to the well.
By installing a shallow channel (Fig. 3-9) or very short wall (Fig. 3-lO) around the edge of the platform, the water can be funnelled off to one specific area away from the well where people and animals will not have to track through it to get to the well.
FIG. 3-8. SLOPING PLATFORM
FIG. 3-9. SHALLOW CHANNEL
FIG. 3-10. SHORT WALL
The apron should be strongly and carefully constructed as it will receive a lot of wear, and any cracks or chips which develop will decrease the effectiveness of the apron.
An apron can be built of stone with mortared joints if cement is in short supply. If for some reason it is not feasible to build an apron, dirt should be built up around the well so that water spilled will tend to run off away from the well rather than collect around it.
A cover can improve the sanitary quality of the water in the well by preventing the dust and dirt normally carried in the air from entering and contaminating the water. It also prevents people from dropping things into the well.
There are two basic variation of well coverstemporary (removable) and permanent (fixed in place).
· A temporary cover would be one that covers the well between the times it is being used, but must be removed to pull water from the well. For example, a temporary cover would be a wooden cover that rests on top of the well but must be removed to throw a bucket, tied to a rope, into the well. This is a limited step toward protecting the well water from surface contamination.
· A permanent cover is usually made of reinforced concrete. It can be poured in place on the well or pre-cast in one or more pieces and later set over the well. (See Fig. 9-9.) Pump mounting bolts and an access door can be cast into the concrete. Pre-casting the cover in one or two pieces may be easier because of the difficulty of building a form which is both strong enough to support the weight of the concrete over the open well and which can then be removed after the concrete has set.
4. Drainage pit
In some areas it may be necessary to construct a special drainage pit to allow spilled and run-off water to soak into the ground. This may be used where other measures cannot feasibly prevent the build-up of standing water. If such a pit is deemed necessary, make sure that it is at least 10 meters from the well. The pit can simply be a hole dug in the ground which is then filled with loose rock and gravel.
NOTE: Where the water table is less than 3 meters from the surface, a drainage pit should not be dug because of the danger of directly contaminating the water supply.
5. Animal trough
If an animal trough is necessary, it should be built far enough from the well so that neither the animals nor their dung will collect around the well and thus contaminate the water.
6. Wash basin
It may be useful to build a wash basin if clothes washing is done at the well. It is important to prevent wash water from pouring back into the well and thus contaminating it. The basin should, therefore, be watertight and built at an elevation below the mouth of the well. Where there is no place to build a basin below the level of the well, it can be located 10 meters from the well.
Supplies include the materials, tools, and equipment necessary for construction of the well.
The following supplies will be needed for construction of any hand dug well:
· digging fools - shovel, pick, mattock;
· equipment to lower and lift people and supplies in and out of the well;
· construction material and the tools needed to work with it.
Tools that may be used with the various materials include the following:
· reinforced concrete - cement, sand, gravel, forms, reinforcing-rod, tie wire, re-rod cutter, re-rod bender, wire cutters, pliers, buckets, 1 trowel (2 would be better), mixing hoes, aggregate gauge box;
· concrete - cement, sand, brick or rock, mixing area, buckets, trowel, mixing hoes, aggregate gauge box;
· wood - wood, saw, nails, hammer.
B. Tools and Equipment
· lowering equipment - tripod, headframe or pulley support with pulley;
· cable or rope - 12 mm steel or 25 mm hemp;
· buckets - 1 large, 2 small;
· square nosed shovel (mixing);
· round nosed shovel (digging);
· hard hat for each worker in well;
· spare shovel and hammer handles'
· plumb bob and plumb line; · level.
Most of the lining methods discussed here rely on the use of cement in one form or another, including concrete, mortar, and porous concrete. For the construction of permanent, sanitary hand-dug wells, cement compounds are the only materials in common use around the world. There is no other material so readily available worldwide that combines the strength, workability, and adaptability of cement compounds. Without cement it will be very difficult to build a permanent, sanitary water source. (See Cement Appendix, p. 221.)
a. Limited amount of cement available
Where there is a limited supply of cement available which will not permit you to pour linings or build masonry, you might consider building a loose rock or brick wall. Simply mortar the inside layer to prevent it from falling into the well. This should only be attempted in very stable ground not subject to collapse.
In some areas only the top 3 meters of the well needs to be lined. This reinforces the area of the well which is most likely to cave-in and uses a minimum of material, and it can be attempted only where a mortar or concrete lining is built in place in a well which is being sunk in firm soil.
Wells lined like this are rarely more than 10 meters deep.
b. Cement Substitutes
Cement substitutes are sometimes commercially available or can be manufactured locally. Lime, for example, can be mixed with sand and water to form a mortar-like paste which can be used in laying brick or rock walls. Like other cement substitutes, it does not have anywhere near the bonding strength of cement mortar.
2. Other Materials
If you cannot get cement in a reasonable length of time and at a reasonable cost, you should consider building the well with other materials such as wood or unmortared rock. These other materials will not last as long as cement but they will enable you to make water available where it is needed now.
· Unmortared brick or rock
Many wells have been built of unmortared rock and have lasted for hundreds of years. This, however, is only where highly skilled stone masons and suitable rock are both available.
Normally unmortared brick or rock is only a temporary solution which will collapse in time. Where the ground is very stable, you might consider such a lining, although the well may stand unharmed without a lining for a long time in this case.
This also is only a temporary solution. It will rot soon (in 1-3 years), tainting the water and allowing caveins.
D. Organization of supplies at the well site
In your planning consider:
· the space needed for each operation;
· the arrangement of supplies within each operation space;
· which operations must be performed at the same time so that their respective spaces do not conflict or cross;
· that a different lining method may be used for the bottom section which will require space for working and materials.
There are 3 different types of operations which must be performed during the sinking of a well:
· removing excavated soil and rock and dumping it;
· lowering and lifting people and supplies in and out of the well;
· preparation and placement of lining materials.
The purpose of organizing the supplies around the well is to provide comfortable working spaces for each of these operations where they will not conflict with each other but are still close enough to be coordinated and efficient. (See Fig. 4-1)
FIG. 4-1. LAYOUT OF MATERIALS AND SUPPLIES
1. Removing excavated soil and rock and dumping it.
A great deal of time will be spent performing this same basic operation over and over again. As a result workers will be tempted to do the least amount of work necessary to get by. It is important that the dumping area be:
· far enough away from the well to not subject the well to danger of caving in;
· not so far away as to slow well sinking because excavated material is not being dumped fast enough;
· located so that rain will not wash material back into the well;
· located in order that water pulled from the well for sinking the bottom section will run off and not collect near the well;
· large enough to hold all the material taken from the well.
2. Lowering and lifting _people and supplies in and out of the well.
This operation is critical for the whole well sinking process but is also so flexible in its placement that too often it is simply assumed that space can be found after everything is set up. Unnecessary delays and possibly dangerous situations can be avoided in later work by simply arranging the supplies so that nothing ever obstructs the lane the pullers must use to raise and lower supplies in the well. Also avoid placing supplies so that they must cross the pulling lane to be used.
3. The preparation and placement of lining materials.
Most hand dug wells need space to store and later mix sand, water, gravel, and cement to form concrete. The actual mixing area should be close to the well and next to the pulling lane to permit easy movement of concrete into the well. The materials necessary for concrete should be stored near the mixing area but far enough from the well to prevent danger of cave-ins.
Sand and gravel storage will take up a large amount of space, which should preferably be easily accessible from the mixing area and from the opposite side to allow for replenishment if necessary. Also, if you plan to use pre-cast Lining rings, you will have to allow space for their construction.
The importance a few minutes worth of planning can have is demonstrated in the following example.
Assume that we have a well with a 1.3m interior finish diameter, a 7.5cm thick reinforced concrete lining, and a depth of 20m - an average sized well. This well will require about fifty 50kg bags of cement for construction of the middle and bottom section. If only complete bags of cement are mixed at a time that will require:
· 50 trips to and from the cement storage area;
· 50 wheelbarrow trips of sand;
· 100 wheelbarrow trips of gravel;
· some multiple of 50 trips to and from the water supply;
· about 500 trips with concrete from the mixing area to the well, depending on the size of the concrete buckets.
Obviously the shorter all these trips can be, the less time and energy will be consumed by them. Remember too that as you take sand and gravel from the near side of the storage piles you have to walk farther and farther.
At some point most materials and equipment involved in hand dug wells construction will be lowered into or pulled out of the well. This raising/lowering operation is so basic to wells construction that it will be discussed here in some detail. (See Fig. 5-1)
The raising and lowering operation will eventually become routine because it must be performed so often. Before the real well sinking work is begun, you should try to plan that routine, by considering what equipment will be used and how the workers should be organized to use the equipment safely.
FIG. 5-1. RAISING AND LOWERING OPERATION
Most major accidents or injuries that happen during the construction of a well result from faulty raising and lowering procedures. Accidents usually occur because someone forgot, didn't understand, or wasn't ready to perform his/her part of the operation. Remember that people's lives depend on how carefully this operation is performed.
There are many tools, pieces of equipment and items of knowledge that can help in this operation by making it easier and safer. They include:
· tripod, headframe;
Everyone coming in contact with this raising/ lowering operation should be thoroughly familiar with it. It is a good idea to do a couple of practice runs so that everyone understands exactly what is involved. Switch people around to help them understand what is happening at different points in the operation. It is also useful to have well workers lowered in and out of the well to help resist any later tendencies for joking while pulling on the rope. That way, everyone understands the reality of being suspended on a rope with no way to help yourself if a problem occurs.
Certain safety features should be followed:
· All workers in the well should wear hard hats;
· People are most safely raised or lowered in the well on a bosun's chair made of a board, log or other appropriately strong seat firmly tied to a rope (see Fig. 5-2).
· Nothing should ever be left lying on the ground near the opening of the hole. In a working situation, people can easily trip and fall, knocking loose materials into the well.
· It is also necessary to be concerned with the safety of people other than the well workers. Passersby have a natural curiosity and wish to find out what is happening. Before construction is begun, it is very useful to establish some kind of perimeter, or even a fence, which is to be crossed only by those actually involved in construction. This may be difficult enough to enforce during working hours but becomes more important and more difficult at night and on nonworking days. It may then be necessary to use a guard to remind passersby that they are not supposed to be in the area. The guard can also make sure that tools and supplies are not taken from the site.
FIG. 5-2. TWO SUGGESTED DESIGNS FOR A BOSUN'S CHAIR
It is very helpful to have a set of signals to control raising and lowering. These should be voice commands as well as hand signals. Four simple signals will usually cover most of your needs (see Fig. 5-3).
FIG. 5-3. HAND SIGNALS
C. Lowering Supports, Tripod, Headframe
Some type of lowering support is necessary when digging, except in very shallow wells. It provides a much safer and easier way to lower tools and materials for use in the well and remove the soil and rock dug up in the hole. Such a structure usually has 1 or 2 pulleys which can be suspended over the center of the hole or offset. The offset arrangement is often easier to work with.
Choose the type of lowering support most suitable for the kind of work you expect to be doing and the materials you have available. (See Figs. 5-4, 5-5, and 5-6.)
FIG. 5-4. WOOD LOWERING SUPPORTS
FIG. 5-5. TRIPOD
FIG. 5-6. HEADFRAME
If they will not obstruct other operations,the lowering supports can be erected before you begin digging. Or, more often, erect them after you have dug a meter or two and passing buckets by hand in and out of the well begins to get difficult.
It is the operation of this unit which will largely determine the safety of the workers in the hole. Emphasize this major point to all workers and visitors at the well site. Also, observe these six points on safety:
1) Lowering and raising materials and people should always be done with enough people on the rope. It is dangerous to rely on one or two strong individuals. Using several people assures control of the load even when a hauler trips or is otherwise unable to continue supporting the load, when people are in the well, or when someone is on the rope being raised or lowered in the well.
2) Someone should always be posted at the well edge, watching the load being worked within the well. In case of a problem this person can alert the haulers or other workers. The same individual could also hook and unhook buckets, loads, and people from the rope and ease them into and out of the well.
3) When pulling large buckets full of soil, rock and water from the well, two people may be needed to pull the heavy bucket out of the well and place it on the ground next to the well for later dumping.
4) There should be an established set of signals which the person at the well head will use to direct those hauling on the rope. (See Fig. 5-3) These signals should be taken very seriously and are to be used only when necessary, but with no hesitation when they are necessary. Practice in using and understanding the signals is advisable.
5) Throughout the well sinking process be especially careful that nothing falls into the well. Even a small pebble unknowingly knocked into the well can cause serious injury to a worker at the bottom if it falls from a distance. Take preventive measures. Be careful how you work around the edge of the well.
6) Always leave a safety line hanging in the hole. This is a rope tied off at the ground surface which can be used in an emergency as an exit from the well.
D. Other Raising/Lowering Arrangements
While lowering supports have been found to be the easiest and safest form of entry and exit, many other methods have been used. In these other methods tools and materials are lowered by rope by people standing next to the hole. People may be raised and lowered in this way but it is usually easier for individuals to make their own way in and out of the hole by climbing up and down:
· an anchored stationary rope which may be knotted at regular intervals; a rope ladder; or
· a regular wooden ladder which may be constructed in stages for use in deeper wells and anchored to the walls of the hole.
E. Common Raising/Lowering Problems
· Lack of tools or materials from which to make the equipement necessary for the raising/lowering operation
This is rarely a real problem because some way can always be
found to raise and lower people
and supplies if you and the local people are committed to constructing the well.
· Equipment breakage
This is usually due to misuse by overstressing or old age.
Misuse can be prevented by careful
work habits and frequent inspection of the equipment.
Equipment will gradually wear out with time. While this cannot be prevented,it can be anticipated, so that old equipment is replaced before it becomes dangerous.
· Lack of sufficient people to perform raising/ lowering operation
This too is more of a perceived problem than a real one. If there are enough people in an area to warrens digging a new well,then there should be enough available to help with this aspect of the work.
If vehicles are available they can be more reliable pullers than people. Workers can also easily raise and lower themselves where necessary. (Figs. 5-7 and 5-8)
FIG. 5-7. WHEEL RIM CAPSTAN
FIG. 5-8. LOWERING/RAISING ROPE ATTACHED TO VEHICLE
F. Useful Equipment
The lives of you and your workers will depend on the ropes you use, so be very careful in selecting rope.
Make sure that the rope you use is strong enough for the loads you will impose on it. (See Rope Strength Appendix, p. 267.) It should be inspected regularly for flaws and traying. If possible, use new rope. As rope ages it loses up to half its strength. You should take this into consideration during selection and use.
Hemp rope is usually available and is very suitable for wells construction, although contact with cement will speed its natural aging and deterioration.
Nylon rope is often available and suitable, although it will stretch as a load is put on it.
Wire rope is excellent, combining strength and small size although it is really only suitable for use with a winch. Wire rope should not be pulled by hand without the use of gloves as it tends to fray, leaving ends sticking out which easily cut the skin.
A few basic knots, when used properly, can help make the work easier and safer.
· bowline - This knot can be tied to form a loop of whatever size you need that will not slip or come loose. Its most common use is in rescue operations where people have to be hauled or lifted out of dangerous situations. (Fig. 5-9)
FIG. 5-9. BOWLINE
· square knot - This knot is used to join two ropes together. (Fig. 5-10)
FIG. 5-10. SQUARE KNOT
· half hitch - This is very commonly used to attach a rope around a solid object. Once tied, it is difficult to tighten the rope although the knot itself will only become tighter if the rope is pulled. (Fig. 5-11)
FIG. 5-11. HALF HITCHES
The use of two or three different kinds of buckets is often convenient for work in a well.
To avoid the possibility of tipping over while being lowered or raised in the well buckets should have two features (Fig. 5-12).
· handles attached to the bucket along its upper rim. Any weight in the bucket will tend to keep it upright.
· buckets should be deeper than wide, which concentrates the weight further down making the buckets even harder to tip over.
FIG. 5-12. BUCKET
Buckets, like ropes, should be checked regularly for defects. Particularly look for:
· a weak bottom;
· a worn handle/bucket junction;
· cracks or broken places;
· worn handle.
Discard rejected buckets, if possible, to avoid possible future confusion and unsafe use in the well.
Buckets may be used for different operations and their desired features will vary accordingly. Buckets used in excavation, such as removing soil and rock from the hole should have:
· handles that connect to the bucket rim;
· greater depth than width;
· reinforced bottoms;
· handle safety latches; and
· hooks in the middle of the handles.
Buckets used for cementing (lowering cement into the well) should have:
· handles connecting to the rim of the bucket;
· wide rims for pouring.
Buckets used for lowering and raising tools should have:
· handles connecting to the rim of the bucket;
· greater depth than width.
The use of a pulley will greatly facilitate well construction work. If unavailable, arrange a cross piece with a smooth surface over which you can pull the rope. It is far more preferable to pull the rope over a pulley than try to stand at the well edge and pull the rope straight up hand over hand. Ropes will wear much faster when pulled over even a relatively smooth surface than when a pulley is used and should therefore be checked frequently.
· Pulleys are often available as a unit with a hook. (Fig. 5-13)
· Pulleys can also be built with shaft ends. (Fig. 5-14)
· Pulleys may be mounted in hard wood blocks anchored to solid frame. (Fig. 5-15)
FIG. 5-13. PULLEY WITH MOUNTING HOOK
FIG. 5-14. PULLEY ON A SHAFT
FIG. 5-15. PULLEY MOUNTED IN WOODEN BLOCKS
5. Brake Post
The brake post is a log set in the ground 4 to 5 m from the well which, when the raising/lowering rope is wrapped around it several times, acts as a friction brake. This can help, especially when lowering heavy objects into the well, by allowing much easier control of the lowering speed. A person standing behind the brake post controls the lowering speed by the amount of tension he/she keeps on the rope as it feeds around the post and on to the well.
A brake post should be:
· located 4 to 5 m from the well straight out from the pulley (See Fig. 5-1.);
· about 25 cm in diameter, round, and smooth;
· set in concrete 1 m into the ground;
· set at an angle of about 60° from the well.
Hand dug wells are sunk working down inside the hole to loosen the sub-soil which is then lifted to ground level and dumped. How one actually digs and then lifts out the loose dirt and rock depends largely on personal preference, based on the available equipment and safety procedures one wishes to follow.
The tools you need depend on the soil conditions. Shovels, pick* mining bars, hoes, jack hammers, and hands have all been used. Anything that will loosen the dirt and rock so that you can then load it into containers and haul it to the surface will work.
NOTE: By shortening the normally long handles of shovels in particular, you can reduce the possibility of injuries and make them easier to use in the confined space at the bottom of a well.
Digging a hole that reaches water and becomes a well is slightly different from just digging a hole. Pay attention to the diameter of the hole, how smooth and even the walls are, and whether the hole is straight up and down.
Many people have found it convenient to follow these steps when using a sinking technique where the hole is first dug, and lined afterward.
· Dig down and remove a layer from 10 to 40cm thick that comes to within 5 or 10cm of the desired diameter (see Fig. 6-1b);
· Continue removing layers of soil until you have reached a depth of about a meter (see Fig. 6-1c);
FIG. 6-1. STEPS TO DIGGING THE HOLE
· Now go back and trim out this section to the desired diameter, making sure that the hole walls continue to be plumb (see Fig. 6-ld);
· Continue this process, which may be broken up by periods of lining the hole, until you reach the desired depth of the hole.
While digging a well, you are likely to encounter several different types of soils which range between very loose and very hard. It can be difficult to sink a hole into either extreme. Digging in loose, dry sand can present some serious problems because of its tendency to fall back in. Dry sand acts like a very thick liquid. Unless you can stop the sand from continually flowing back, your well will end up V-shaped but even wider at the top than the hole is deep. In such a situation you can sometimes stop the sand by digging 10 to 15 cm and then splashing a mixture of cement and water on the wall. This will dry in minutes to form a thin hard layer. If that fails, pour 200 liters of water into the hole before digging the next meter. This saturates the sand to make it more stable. If this fails, you should consider sinking the lining through the sand.
D. Hard Rock
Digging down and reaching a hard rock layer above an aquifer is one of the most discouraging things that can happen to a well digger, who must then either fry to continue sinking the well into and through the rock layer by whatever means possible, or abandon the hole and try again someplace else.
In order to proceed correctly you need to research and analyze existing information on water a:cessibi]ity and rock layers. Often there will be only minim documentation available. Wells project workers may have to reach conclusions on the basis of incomplete information. Here is a series of questions, the answers to which will assist well workers in developing such information:
· Has this or a similar rock layer been encountered before in other attempts to sink wells?
· Where were the wells attempted?
· At what depth was rock encountered?
· Is this a situation that local people know about?
· Is this a general long slope of the ground surface in a certain direction?
· What do you think happens to this rock layer in that direction?
· Is there another source of water that could be further developed to supply additional water?
· How important is it that these people have a well to supply them with water?
An object is plumb if it is straight up and down (perfectly vertical). Wells should be as plumb as possible for convenience and to avoid many other possible complications. This is particularly important in sinking pre-cast lining sections. Structurally, a well that is plumb is generally stronger than one that is not.
Plumb can be checked with simply a string with a weight tied to the bottom of it (see Fig. 6-2). Plumb-bobs can be purchased commercially and are relatively cheap. For convenience sake it may be useful to purchase one, especially if there is a sliding piece for the top which is exactly the same width as the plumb-bob itself so that you can easily get exact measures.
FIG. 6-2. PLUMB-BOB BEING USED TO CHECK PLUMB OH HOLE
It will be necessary to regularly check the diameter of the well for many reasons. The hole you dig should have the exact diameter that you have designed for it.
If the hole diameter is lest than planned, the precast lining may not fit or poured concrete walls, will be thin and weak. This can be easily fixed by trimming the hole to the desired diameter.
If, on the other hand, you have dug the hole too wide, it probably won't affect the final condition of the well, but you will have spent unnecessary extra time digging and it will take more time, and perhaps materials, to fill the well back in to the desired diameter. Excessive width is especially serious when pouring concrete walls in place in the well. The diameter of the forms is set and filling in behind them will require a great deal more concrete.
G. Methods of Checking the Diameter
1. Drop Bar
· Bend 6mm re-rod into a circle which conforms to desired circumference of the well.
· Continue bending the re-rod around a second time to make double bar and tie the two together.
· Check roundness by rotating a straight bar the same length as the diameter within the circle drop bar.
· Dig one meter and be careful not to displace the drop bar or excessively dig outward at the wall. After you have dug one meter, you are ready to use the dropbar.
· For right-handers, place your left hand lightly on the bar. Hold the trowel sideways under your left forearm. Using the upper back corner of the trowel, work towards your body and scrape away the dirt below the bar.
· Work along the circumference, scraping away just enough dirt to allow the bar to drop. Continue in this manner, working around the hole until the bar is at the bottom of the hole.(see Fig. 6-3.)
· If you constantly look at the bar and keep it level, the bore of the well will be straight and consistent.
· If you now plan to pour the lining in the well, dig a bit with the trowel until the bar is below the floor of the first dig and cover it with sand.
· Do not smooth the wall.
2. Other Methods
This next method can be used to check both the diameter and plumb and can also be used to center forms used in the well.
This method involves hanging a measuring bar from the center of a centering board which fits across the top of the well and is located by a stake on either side of the well. (See Fig. 6-4.)
· Dig anchoring holes for re-rod stakes, one on either side of the location of the well and about 30cm back from what will be the edge of the hole.
· Cement stakes in place and place tops of both stakes through holes already drilled in centering board before the cement sets, to exactly locate stakes.
· Locate a hook or hole in the center of the board
· Draw a circle of the desired hole size in the ground centered at the hook. This exactly locates the hole to be dug.
· As each meter is dug, hang the measuring bar from a line board over the hook. The bar should pivot freely, just touching the edge of the hole with both ends to indicated that the hole is both plumb and the proper size.
A variation of this would be to use a plumb-bob to locate the center and gauge the radius of the well from there. (See Fig. 6-5) Hang a plumb-bob from the hook to locate the center of the hole.
· With a re-rod measuring piece cut to the exact radius of the hole, check the diameter as shown in Fig. 6-5. The measuring piece should just fit between the plumbline and the wall.
A person could also stand in the hole with a rod which is the exact diameter as the hole in order to trim the hole down to the desired size. (See Fig. 6-6.)
The middle section of the well is the first part to be built. It involves digging and lining the hole from the ground surface to the water table.
B. Lining: Purposes
The lining is a circular wall made of a strong, permanent material placed or built adjacent to the walls of the hole.
The lining has three purposes:
· it retains the walls after completion; it keeps the hole from caving in;
· it acts as a seal to prevent polluted surface water from entering the well;
· it serves as foundation and support for the well top.
Wells should be lined if possible. Without linings, wells are subject to damaging cave-ins.
Only in hard rock formations are linings not necessary because rock is not liable to cave-in. Linings should be built of permanent materials which will not rot, decay or otherwise lose their strength in a relatively short period of time.
In some places wells are lined, meter by meter, only down to a depth of about 3 meters. From there down the walls are left unlined. Such a limited lining reinforces what is normally the most unstable part of the well. If caving in is not a problem, this amount of wall reinforcement may enable the development of a semi-permanent water source. It requires a minimum amount of materials but is not really suitable for development as a sanitary water source.
It would be tempting to think of this kind of well as an appropriate temporary solution which could later be easily up-graded to meet the need for a permanent sanitary water source. However, consider that although the partlylined well meets the current water needs, it may be difficult to motivate local inhabitants to donate time and money to later improve a source that already meets their felt needs.
There are two basic methods of lining a hand-dug well.
· Dig and line
The hole or a portion of the hole is first dug and then lined. This is a very flexible method and is therefore suitable for use in many different ground conditions.
· Sink lining
Pre-made lining sections are put in place and sunk by digging soil out from inside the bottom lining section. This method is used primarily in loose, dry sands where dig-and-line methods will not work because hole walls cave in too quickly.(Fig. 7-1)
1. Dig and Line
This method,which has been traditionally used for well sinking in all parts of the world,has many variations that make it suitable for many different construction materials and ground conditions.
In this method the hole is always dug first and then lined.
Commonly used variations of this method are:
· dig-a-meter, pour-a-meter (see Fig. 7-2);
· dig-and-line-in-short-sections(see Fig. 7-3);
· dig a hole to water and line;
· dig a hole as far into water as possible and line.
The advantages of a dig-and-line method are that:
· it enables you to dig as deep as you can before committing expensive materials to lining the hole;
· it is capable of penetrating anything except hard rock (hard rock can sometimes even be penetrated for short distances);
· the lining can be built in place in the well or built on the surface and lowered into the completed hole;
· the lining curbs (see p. 69) can be built in to increase lining stability.
Its disadvantages include:
· possible dangers from cave-ins if the hole is dug too far without lining;
· requiring more work in the well than "sink-lining."
2. Sink Lining (Caissoning)
This method is most suitable for use in loose fluid soils.
The lining is assembled at the surface before it is sunk. It can be made of pre-cast reinforced concrete rings which are set in place at the ground surface one by one as the others are sunk, or brick or circular rock walls which are built up at the surface layer by layer as the column sinks.
The lining is sunk by workers standing inside the lining column and digging out soil. As more supporting soil is removed the lining will gradually sink under its own weight.
The sinking proceeds by digging out soil until the top of the lining sinks almost to ground level and adding another lining section, digging and sinking that section to the ground level, and so on until the lining is sunk as far into the water as possible.
FIG. 1-2. DIG-A-METER, POUR-A-METER
FIG. 7-3. DIG-AND-LINE-IN-SHORT-SECTIONS
As a well sinking method to be used from the ground surface,it is useful for shallow wells and those wells with larger than normal diameters where the ground in free from boulders and other huge obstructions.
This method has these advantages:
· the equipment is simpler than that required for dig and line;
· a headframe is not essential;
· not as much re-rod is required;
· most construction work is carried out on the surface;
· advance preparation of lining can significantly speed the sinking process.
The disadvantages include the following:
· it is difficult to keep shaft vertical;
· boulders or even large stones in the ground can cause the column to tilt;
· the stability of the completed lining depends entirely on the friction between the lining and the hole wall and since this is irregular, stresses are set up which may cause slipping, jamming or opening construction joints;
· as the depth of the well increases, a tendency develops for sections to "hang", causing tension failure, or to buckle or crush under the weight of the column of rings;
· the aquifer may become contaminated by seepage from the surface down the outside of the lining ring because it is not sufficiently form-fitted to the hole wall.
D. The Curb
The curb is an elbowshaped cut made in the wall that is poured full of concrete and acts as an anchor to prevent the lining from sliding down through the ground. The bottom of the cut is flat, 40 cm into the wall, and the top is cut 40 cm higher than the bottom cut, and allows concrete to flow to all parts of the curb. The rerod should be attached to each vertical(as shown in Fig. 74). It will obviously take more concrete for a pour that includes a curb.
FIG. 7-4. LINING CURB
There is no commonly accepted standard regarding a recommended distance between curbs. In fact, some sources question whether they are even necessary where a concrete lining is poured in place. Other sources recommend curbs every 5 meters to 10 meters. It is recommended here that curbs be installed every 10 meters.
It is always a good idea to install a curb at or near the bottom of the middle section lining, just above the water table.
E. Cast-in-Place Versus Pre-Cast
The lining is built in place right up against the walls of the hole.
Sections of the lining are built above ground and later lowered or sunk into place in the well.
2. Decide Whether to Pre-Cast or Cast in Place
The decision to pre-cast or cast in-place the middle section lining usually depends on such factors as size of the project, lining materials, equipment available, ground conditions, and personal preference.
Generally, a lining that is built in place is better than one that is built ahead of time and then installed in the well. By building it in place, especially if using reinforced concrete or mortar, the lining conforms to the walls better, providing greater adhesion, stability, and sealing, as well as preventing the entry of contaminated surface water.
· generally preferable - strong, waterproof, permanent;
· e forms helpful but not required;
· bonds well with ex- cavation walls;
· pre-made concrete sections available.
· limited working space
· provides more time in the well and space to build a stronger lining;
· may not bond well with the hole wall;
· requires heavy lowering equipment.
To prevent tools and materials from being accidentally knocked into the well and the possible collapse of the looser surface soil into the well you may wish to install a lining in the top meter of the well as soon as it is dug. This can be either a temporary or permanent lining depending on what materials you have available and the lining method you plan to use.
When in place, this lining should reach at least 10 cm above the ground surface to prevent tools, materials, and other articles from falling into the well.
An inside lining mold can be used as a temporary lining. (See Fig. 7-5.) There must be enough room for people and supplies to move in and out of the hole. The mold should be carefully packed in place to prevent it from slipping down while the hole is being excavated beneath it. Remember that, once in place, this temporary lining cannot be moved until the construction is complete. It is, therefore, necessary to plan on making or purchasing this in addition to all lining molds needed for the construction of the permanent lining.
If you build a permanent lining, pour it in place possibly with an anchor above ground.
As you sink the well beyond this top support, taper the inside hole walls out to the desired hole diameter about 30-40cm below bottom of top support. This will help support it while digging continues below it.
FIG. 7-5 LINING MOLD AS TEMPORARY LINING
If you decide not to put a top support in you should make some effort to ensure that workers in the well are safe from ground surface collapse and accidental entry of anything into the well. For example, four boards may be laid in a square around the hole and used to accomplish this.
Such a construction will help to distribute weight near the edge of the hole and act as a constant reminder to well workers and others to be careful.
Many wells have been dug without the use of any measures to prevent accidents except to warn everyone about the consequences. Well workers will all be sensitive to the need for safety but local villagers often are not.
In some cases wells projects have established rules that no one, including village elders and children, can come within 2 meters of the well until it is completed. This may be hard to enforce but is nevertheless a very useful procedure.
NOTE: Hole diameter and lining thickness: As a general rule the lining thickness should be about 1/20 of the hole diameter but never less than 7 cm.
Finish well diameter = interior lining form diameter = 9/10 hole diameter (for wells w/hole diem. 2 1.40m)
F. Lining: Materials
The availability of material. largely determines how you line the well. Here are the most frequently used lining materials in order of preference.
This order is generally applicable to most locales although there may be minor changes due to limitations of geological conditions. Such limitations are almost always due to the fact that soil conditions will not permit you to dig down to the water table without first reinforcing the walls. The installation of temporary reinforcing is sometimes an appropriate solution, but that involves a large added expense for temporary lining which is beyond the scope of this manual.
· Reinforced concrete
This is probably the best material now commonly available for large-diameter well lining. When properly cured it is very strong. Depending on its ingredients it can be virtually waterproof or quite porous. Re-rod is available in most countries, although it is sometimes difficult to transport to the well site. Concrete can be precast into appropriately sized sections and then lowered into the hole or it can be poured and cured in place inside the well. Depending on what other equipment and materials are available for use with it, it is the most adaptable material for many of the situations which may be encountered in wells construction.
· Reinforced mortar
This material has all the same advantages and uses as reinforced concrete. The difference is that only sand is added to cement to make mortar while sand and gravel are added to cement to make concrete. Gravel acts as a stretcher to make the concrete go further so mortar is more expensive for the same volume of final product. Mortar is a more "workable" mixture and in some cases may be more suitable than concrete.
Concrete without re-rod is not as strong as reinforced concrete.
This has the same qualities as concrete but is more expensive.
Brick should be solid rather than hollow. It is best suited for digging to the water table and then lining the complete well. It can be used to line in short sections although it is difficult to support a lined section while digging beneath it. Mortared brick makes a strong lining but does not adhere very well to the hole wall.
Rock is very similar to brick in that it is best suited for lining a hole that has been completely dug. Mortared rock walls can be quite strong although because of their irregular shapes and strengths they will often contain weak spots and be subject to cracking.
Unmortared rock walls can also be very strong if built properly, but require proper materials and a degree of expertise which cannot be adequately covered in this manual.
FIG. 7-6. ROCK WALL
· Timber (wood)
The only advantages of wood are that it has been generally available, moderately strong and cheap. In many areas of the world wood is no longer generally available or cheap, and its disadvantages are such as to prevent any serious consideration of wood as a lining material for a permanent potable water source. It is liable to rot, taint the water and harbor insects. It is impossible to make the lining watertight and so prevent re-entry of contaminated surface water.
G. Possible Digging/Lining Methods for Each Material
1. Reinforced concrete cast in place:
· Dig to water and line;
· Dig-a-meter, pour-a-meter;
· Dig and line in short sections.
2. Reinforced concrete pre-cast:
This requires heavier capacity lowering equipment.
· Dig to water and line;
· Sink lining.
3. Reinforced mortar thrown in place:
· Dig to water and line;
· Dig-a-meter, pour-a-meter;
· Dig and line short sections.
4. Reinforced mortar pre-cast:
· Dig to water and line;
· Sink lining.
5. Unreinforced concrete or mortar cast in place:
This has very little tensile strength.
· Dig to water and line;
· Dig-a-meter, pour-a-meter;
· Dig and line in short sections.
6. Unreinforced concrete or mortar pre-cast:
Its low tensile strength makes lowering it into the well difficult,
· Dig to water and line;
· Sink lining.
7. Brick or rock:
These materials also have relatively low tensile strength:
· Sink lining (on cutting ring);
· Dig to water and line;
· Dig and line in short sections with anchoring curbs.
8. Wood (timber or split bamboo):
· Employ verticals with horizontal supports all anchored to walls;
· Use any sinking method but try to avoid the use of wood in the first place, for reasons discussed above.
NOTE: Efficient use of cement
Cement is probably the most expensive material of all so it should be used as efficiently as possible.
Of all cement compounds, reinforced concrete requires the least amount of cement per volume of building material (either mortar or concrete) produced.
On one wells construction project where 50 cm rock walls were built, 15 cm reirforced concrete was tried instead.
The results were a 1/3 reduction in cement used, walls were more waterproof, and the construction time was significantly reduced. The initial cost was greater because of the need for forming materials, but forms can be re-used. By prorating the cost of forms over a number of wells and figuring the reduced labor costs the total cost of each well went down about 25%.
This chapter first presents a step-by-step description of the use of reinforced concrete in the construction of the middle section by three methods: 1) dig-a-meter, pour-ameter; 2) dig-and-line-in-short-sections; and 3) dig-to-thewater table-and-line. It then describes the use of the same material in building lining rings and concludes with a discussion of the use of alternative materials, mortar and mortared brick or stone.
B. Sinking Depth and Lining
How deep you sink the hole before you begin lining it is a question you must answer for yourself,given the local conditions. Major concerns are the ground conditions, the lining material to be used and personal preference.
· ground conditions
In loose soil, dig to whatever depth can be conveniently dug and lined without the hole walls caving in; 1/2 meter is the usual minimum.
In firm soil, the maximum recommended depth without lining is 5 meters. This is usually a safe depth and may prevent or at least limit the damage resulting from caveins.
Many experienced well diggers will simply dig as far as they can until either 1) walls show signs of loosening and possible cave-in, or 2) they reach the water table. This is, however, not recommended for beginning well diggers who have little or no experience estimating the strength of various ground formations.
NOTE: In some locales where the soil is firm all the way down to the water table, entire wells have been dug before any lining was constructed. Where necessary and where ground conditions permit, wells can still be dug that way.
C. Dig-a-Meter, Pour-a-Meter
One meter is the height of the circular forms, sometimes called molds, which will be placed in the well and around which concrete will be poured. Circular lining forms do not have to be one meter high and where they are not the general procedure for this digging/lining method can still be followed. Another name for this method could be dig one mold height and then line one mold height.
· Outline of Work
1. Dig the hole to the specified depth.
- Check the diameter and plumb.(See pp. 59-62.)
2. Assemble the re-rod cage in place.(See Fig. 8-1.)
- 25 to 30 cm of each vertical re-rod should extend into the soil below the bottom of this pour.
- Evenly space the horizontal re-rods along the height of the pour and tie them to the verticals.
FIG. 8-1. REINFORCED CONCRETE LINING BUILT IN PLACE (a)
FIG. 8-1. REINFORCED CONCRETE LINING BUILT IN PLACE (b)
a. The hole has been dug to the required depth and rerod verticals stuck into the ground. The rerod horizontals are then tied to the verticals.
b. The inside lining mold has been centered and leveled. Concrete can now be poured around the rerod between the lining mold and the side of the hole. Also see Fig. 83.
3. Lower and assemble the mold.
- Level and center the mold. This is very important for the first section.
4. Pour concrete behind the mold (see Fig. 8-lb) and on all sides of mold to evenly distribute and not displace the mold. Gently tap around mold with a hammer to settle the concrete and prevent honeycombing.
5. Leave the mold of first pour in place
- Dig beneath it to a depth of a mold plus 10 cm (815 cm) (If 1 m mold, dig 1.1 m; if 1/2 m mold, dig 60 cm)
- Check diameter and plumb.
6. Assemble the re-rod in place.
- Tie the top of the vertical piece to the bottoms of the pieces extending out below the previous pour.
7. Lower and assemble the mold (another one, or the drop mold used in the first section)
- Level and center the mold.
- There will be a 10 cm gap between the top of the mold and the bottom of the previous pour through which to pour the concrete.
8. Mix and pour the concrete behind the mold.
9. Continue as in second meter.
After the mold is placed for the third pour, plaster the joint between the first and second pours with mortar for a smooth continued surface.
A more specific work plan for each of these steps is given in the next section.
· Dig-a-Meter, Pour-a-Meter: Work Plan
1. Dig hole to desired depth
- Check diameter and plumb. (See pp. 59-62.)
- Determine depth of hole, as follows.
When you dig the hole for the first one meter section to be poured,the actual depth of the hole will depend on 1) how much of a headwall you wish to include as part of the first pour, and 2) how you can best leave 25 cm to 30 cm of each vertical re-rod beneath the pour in such a way that they will not be embedded in the concrete. This will later be connected to the next lower pour.
Here are various headwall options for your consideration:
- No headwall
The depth of the hole is the height of the mold so that the pour will be flush with the ground surface. (See Fig. 8-2.)
FIG. 8-2. NO HEADWALL
- 10 cm headwall
The depth will be 10 cm less than the height of the mold.
- 50m headwall
You will need an optional 50 cm exterior headwall mold. The depth will be 50 cm less than the height of the mold. For a 50 cm headwall the mold should be 1m high. Assemble the normal 1m re-rod cage in place. The cage will extend almost 50 cm above ground. Place, level and center the interior and exterior molds. The tops of both molds should be approximately even with the top of the re-rod slightly (3 cm) below that. The width of the headwall is determined in the same way as the hole diameter.
2. Assemble re-rod cage in place.
The total height of the vertical re-rods should be (mold height) + 20(re-rod diameter)+ 10 cm (for space to pour concrete between pours) + 10 cm (for one 5 cm hook on each end of the rod).
For example: When using a one meter mold and 8 mm re-rod verticals will be 1 meter (mold height) plus 0.16m (20x8mm re-rod diameter) also .10m (for concrete pouring gap) plus .10m (2-5 cm hooks). The verticals will be 1.36 meters long.
- Normally use about 15 vertical pieces although you may need up to 30 pieces in very loose unstable soil.
- Bend over a 5 cm hook on one end of each vertical re-rod.
- Space re-rods evenly around the hole. Push the unbent end of each re-rod about 30 cm into the ground or until the top of the hooked end is about 3 cm below the top of this poured section of lining.
- These re-rods should all be pushed into the ground where they will be approximately in the middle of the lining thickness. (See Fig. 8-3)
FIG. 8-3. PLACEMENT OF RE-ROD AND FORM IN HOLE
- If the ground is too hard to push the re-rod straight down, then dig down far enough to provide space for the bottom ends and fill the hole back in to the desired depth. However, this may waste time and effort that could be used more economically by digging the hole deeper before you line it. (See an alternative sinking technique on p. 86.)
- You will normally need 3 horizontal reinforcing rods per meter, although in loose soil 4 horizontal re-rods are better. These will be circles of re-rod with a diameter slightly less than the hole, they are usually placed on the outside of the verticals.
- To compute the length of re-rod which will need to be bent into the proper size circle for use as a horizontal reinforcing rod, used the following formula. The complete lengths of the re-rod will be the distance around the circle at the point where the re-rod will be placed, plus the length of the re-rod overlap, plus the length of the hook on each end of the re-rod.
[( hole diameter) -(lining thickness)] x 3.1416 + 20(re-rod diameter) + 2(5cm end hooks = length of horizontal re-rod
Example: For a well with a hole diameter of 1.5 meters with a lining thickness of 7.5cm (.075m) using 6mm (.006m) re-rod, the computation would look like this.
[(1.5) - (.075)] x 3.1416 + 20 (.006) + 2 (.05)
[1.425] x 3.1416 + 0.12 + 0.10
4.48 + 0.12 + 0.10 = 4.7 meters.
- Horizontals should be evenly spaced in each meter; for example, if there are three horizontals, one is placed in the middle of each meter; the other two are placed 37cm away on either side. (See Fig. 8-3) Be sure to include the 10cm or 15cm gap between successive poured sections if applicable in the computation of the length over which the horizontal re-rods should be evenly spaced. Tie all intersections of horizontal and verticals; make sure the re-rod is no closer than 3cm to the hole wall. Put pieces of stone or dried concrete between the rerod and the hole wall where it is necessary to ensure spacing.
NOTE: Instead of using a certain number of horizontal pieces, you can also use a continuous spiral re-rod which circles the hole 3 or 4 times per meter depending on how strong the lining must be.
3. Lower and assemble the mold.
- Lower the mold sections into the well and assemble them in place.
- It is also possible to assemble the mold above the ground and lower it into place.
- Center the mold in the hole making sure that it is level and that the distance between the mold and the hole wall is the same all the way around the hole. (See Fig. 8-4.)
FIG. 8-4. MOLD LEVELED AND CENTERED
- It is very important that the mold be correctly centered and leveled for this first section because all of the other pours must line up and be attached to it. If the first poured section is not aligned properly all subsequent sections, if followed from the first one, could magnify that error and cause the well shaft to curve or angle out of plumb as it is sunk.
- Especially with the first meter it is often a good idea to check the level and centering again, just before the concrete is poured, to make sure that it has not been disturbed.
4. Pour concrete behind mold.
- Concrete is mixed on the surface (See Cement Appendix, p. 221.) and lowered in buckets down to workers in the well who will pour the concrete behind the mold.
- When pouring concrete behind the mold, do so from alternating opposite sides of the mold. If you continually pour the concrete into one spot or unknowingly concentrate on one side of the concrete, it will soon build up enough weight that it will move the mold off center or out of line. Once that happens it is virtually impossible to move the mold back to its intended position. (See Fig. 8-5.)
- Once the concrete is poured, gently tap around the inside of the mold with a hammer. This slightly vibrates the concrete so it settles into any voids that may have been left while pouring. After the concrete has set in an hour or two, you can begin working on the second pour.
FIG. 8-5. POURING CONCRETE EVENLY IN PLACE AROUND MOLD.
5. Dig hole beneath the first poured section
The depth of this dig should be the mold height plus 10 cm for a gap between the bottom of the first poured section and the top of the second pour through which you will pour concrete. (See Fig. 8-6.)
FIG. 8-6. PROPER HOLE DEPTH BENEATH AN ALREADY POURED LINING SECTION
6. Assemble reinforcing rod in place
Bend a 5 cm hook on the bottom ends of the re-rod extending down from the previously poured section.
Place the horizontal pieces in the well; pull the horizontal pieces up into place and tie all intersections with horizontals.
Assemble the same number of vertical pieces as in the first pour.
Bend a 5 cm hook on one end; stick the unhooked end into the ground 25-30 cm directly beneath the verticals of the previously poured section and inside the horizontals lying on the ground,
Connect the verticals to the previous verticals with sufficient overlap, twenty times the diameter of the rerod, and a 5 cm hook at either end of the overlap.
Place the horizontals as in the first pour. Make sure the re-rod is no closer than 3 cm to the hole wall.
7. Lower mold into place
If you have only one mold, remove and lower it from the previously poured section.
This way it is possible to dig and line about 1 meter each day in soil that is easily dug. A single mold should be cleaned and oiled at least once every 3 meters,
During the one day that each poured concrete section sits while the following section is dug, it will set sufficiently to enable the mold to be carefully removed. (See Cement Appendix, p. 221.)
If you have more than one mold you may leave them in place until needed for constructing subsequent sections.
Center and level (plumb) the mold.
8. Mix and pour concrete behind mold
Don't forget to alternate sides for pouring each successive bucket to allow the concrete to settle evenly.
9. Successive pours
- Continue as in second pour;
- Fill gaps between pours with mortar when the molds have been removed from both sections.
This lining method involves digging the hole to some convenient depth and then lining it. Once the lining for that section is complete and is anchored in place, continue digging the hole beneath the already lined sections. These sections are dug and lined until the water table is reached. The depth of each section can be whatever you feel comfortable with both from the point of view of safety and work convenience.
1. Dig hole to certain depth
Dig a hole until you reach a depth where you feel it might be unsafe to continue, or until you have sunk the hole a maximum of 5 meters. Depending on whether you want a headwall and, if so, on the height you want it to be, you may want to slightly change the hole depth so that the top of the mold will be at the desired top of the headwall after a number of poured lining sections are completed.
2. Assemble re-rod cage in place
- Where possible, use single long pieces of re-rod for verticals. These can be anchored to the hole wall with short pieces of re-rod bent into hooks and pounded into the hole wall around the verticals.
- Put horizontals in place, beginning at the bottom and working up.
- Tie each horizontal in place temporarily with enough ties to hold it in place. This allows for any adjustments that might be necessary later.
- Before the mold is put in place for each pour, securely tie all of the intersections of horizontals and verticals.
3. Lower and assemble mold
- Level and center the mold in the bottom of the hole.
4. Pour concrete behind mold
- Leave the top surface rough for a good bond to the next pour.
5. Lower and assemble another mold
- Set it directly on top of previous mold;
- Pour concrete behind mold;
- Repeat this step until you reach the ground surface or bottom of the previous set of pours.
6. Repeat steps 1 through 5
- Repeat these steps until water is reached. The bottom of the lining should be constructed with a curb just above the water table.
NOTE: If you are digging and lining beneath a previous lift be careful to leave enough room between the top of this lift and the bottom of the previous lift through which YOU can pour concrete (See Fig. 8-6.)
This method is recommended only where the water table is within a few meters of the earth's surface and the ground is firm. It is included here to demonstrate that the same operations must always be performed although, depending on the depth, they may be done in a slightly different order:
1. The hole is dug down to just slightly above the water table,
2. Dig out a curb at the bottom of the hole.
3. Assemble the re-rod in place.
4. Set the form in place. Remember to carefully center and level the form.
5. Mix and pour the concrete into the form.
6. Continue lowering molds on top of previous molds and pouring concrete behind them until you reach the ground surface.
F. Pre-Cast Reinforced Concrete Rings
Pre-cast reinforced concrete rings are often used to line wells. They are poured and cured above ground to be lowered into the well later. There are two basic methods for installing rings:
· lowering and stacking them in an already dug well: (See Fig. 8-7.)
FIG. 8-7. LOWERING PRE-CAST CONCRETE RINGS
· setting them in position at the ground surface and sinking them into place. (See Fig. 8-10.)
Rings should be made so that they:
· can be easily stacked or attached one on top of another;
· can form a watertight joint where they meet;
· are strong enough to support weight of a long column of rings;
· will not rot, corrode, rust or otherwise lose any of the above qualities;
· will not react with water to make the water less desirable for consumption.
This method is more useful in a large project where rings can be centrally manufactured and then transported to the well site for use.
Making your own rings will require the use of inner, outer, top and bottom forms. The inner, outer and top forms can be carefully removed after the concrete has just started to set, usually in an hour or two, making it possible to Make several rings each day if enough bottom forms are available. Where a number of forms are available,it is better to leave them in place to allow the concrete to cure better.
You can 1) make your own reinforced concrete rings or 2) sometimes obtain them on the local market as culvert pipe or a similar item. Both are suitable.
Determining how two rings fit together is a major concern when lining a well with concrete rings. Frequently flat edges have simply been mortared and stacked on flat edges. (See Fig. 8-8a.) This provides very little resistance if an unequal sideways force is exerted. A tapered or flared fit is better. (See Fig. 88b and c.) These will resist sideways forces better and should be arranged so that water flowing down the outside of the lining will tend to flow away from and not into the well. Where possible, the two rings should be firmly attached together. This is most often done with nuts and bolts through steel fittings cast into the rings. This type of connection is especially useful when the lining will be sunk into place but may not be necessary when stacking rings in an already open hole.
FIG. 8-8. CONCRETE LINING RING JOINTS
As with most other concrete casting techniques, the concrete in the rings can either be made watertight or porous. Porous concrete rings can be especially useful when sinking the lining into the bottom section of the well.
1. Stacked in open hole: Work Plan
- Dig curb slightly above water level;
- Reinforced the curb and pour it around a form in such a way that a lining ring will sit on top of it. (See Fig. 8-9.)
2. Lower ring into place on top of curb.
- The major problem with lining a well this way is that there is very little adhesion between the lining and surrounding soil.
3. Continue lowering rings and filling in around them until you reach the ground surface.
NOTE: It may be useful to install curbs or some other type of anchoring to help hoed the column in place.
FIG. 8-9. LINING RINGS LOWERED INTO PLACE ON ANCHORING CURB
2. Sunk from ground surface: Work Plan
This involves stacking rings as necessary on a cutting ring while the whole column is sunk by digging out soil from inside and underneath the bottom of the column. Because of its complete protection from caving this method is used in very loose soils that would be difficult to keep the column going out of plumb, especially in loose soils. The problem is compounded by the fact that once the column begins sinking askew it is very difficult and sometimes impossible to get it straight again.
A cutting ring is a special ring that provides a cutting edge to help sink the column and also helps to funnel soil directly underneath it into the middle of the hole for removal. (See Fig. 8-lOa.)
1. Set 1 or 2 lining rings on top of the cutting ring. The more rings you can stack, the more weight the column will have and the easier it will be to sink. However, the rings are very heavy and it will usually be very difficult to lift the rings any higher than 1 meter with locally made equipment.
2. Workers will stand inside the ring and dig to remove soil and permit column to sink. (See Fig. 8-lOb.)
3. Digging should be done especially carefully to try to remove soil evenly from around the bottom of the hole so that the column will sink plumb.
4. Add more rings when the existing column sinks down to where another ring can easily be added (usually about ground level or just above). (See Fig. 8-lOc.)
5. Continue sinking the column and adding rings as far as possible into the water table. (See Chapter 9, p. 99.)
NOTE: Because of the strong possibility of rings sinking askew and the uneven forces that will often have to be applied to the column in order to get it straight again, it is a good idea to have the rings firmly attached to one another.
FIG. 8-10. SINKING LINING RINGS
Often in loose soil, mostly sands, the rings sink askew, which causes problems, especially when the thickness of the loose layer is about 4 m or mole. In less thick layers (about 2 m) the best way to sink the column relatively plumb is to continue digging as fast as possible until firmer layers are struck. Even if the rings are standing askew, they may be put in an upright position again as follows.
If the rings are askew, as in Fig. 8-lla, work inside the column of rings to remove the soil from beneath the lowest edge of the bottom ring as shown. The hole you dig should extend underneath and beyond the outside edge of the ring. When this is completed, carefully dig the soil away from beneath the opposite side of the column but, on this side, try not to dig any further out beneath the ring than absolutely necessary. (See Fig. 8-llb.) As the column sinks from this point, it should gradually ease over toward the side that was first dug out. (See fig. 8-llc.)
FIG. 8-11. STRAIGHTENING A COLUMN THAT HAS BEEN SUNK OUT OF PLUMB
G. Mortar (plaster)
1. Cast in place
Reinforced mortar is used much like reinforced concrete. The major advantage of this method is that no forms are required because the mortar is thrown onto the wall in place.
This variation evolved in Senegal. It was difficult to transport heavy steel molds to the well sites,so the masons would "throw" the concrete onto the walls. But concrete is not very workable so the masons began leaving out the gravel and using just mortar.
Walls are normally plastered meter-by-meter as the well is dug. This is a variation of the "dig-ameter-poura-meter" method used with reinforced concrete.
This process is as follows:
1. Dig the hole about 1.1 m deep.
2. Check the hole diameter and plumb.
3. Cut and shape as much re-rod as possible beforehand because the next steps must be done quickly.
4. Splash the mixture (1:4, cement: water) on all surfaces to be plastered. If the surface dries too quickly, wet it again immediately before plastering.
5. Apply a 3cm thick layer of mortar with a trowel.
6. Place the re-rod.
7. Apply second 3cm coat of mortar while the first is still wet.
8. A third, thinner layer may be applied to make the walls as smooth as possible.
Mortar can also be used to pre-cast rings even where forms are not available.
1. Dig a round hole with the desired outside diameter of the ring. (See Fig. 8-12a.)
- The hole's depth will determine the height of the ring.
2. The hole is then plastered and reinforced just as it would be in the well. (See Fig. 8-12b and c.)
3. After it has cured for several days it can be dug up and later installed in the well. (See Fig. 8-12d.)
- This type of ring should be allowed to harden for at least one week before it is placed in the well.
FIG. 8-12. CASTING CONCRETE RINGS IN EARTH FORMS
H. Mortared brick or stone (masonry)
Rock has been used to construct linings in wells for centuries. Construction of a well using rock usually involves digging the hole as far as possible into the water table before the lining is built. Once the foundation under water is established, the walls are usually built up in a double layer about 50 cm. thick. Rock walls are very weak in tension so they can be easily cracked if any uneven stresses are put on them.
This type of well can provide acceptable quality well water if precautions are taken to prevent contamination from surface water seepage through the lining. A 5 cm. thick layer of concrete or mortar around the top 3 m. of the lining can prevent the seepage of contaminated surface water back into the well. In fact, the surface water seepage may eventually reach the water table and find its way back into the well, but if this seepage has been filtered through at least 3 m of soil, the potentially harmful contaminants will have been removed. (See Fig. 813.)
Brick or stone masonry can be used in most of the different digging and lining methods but, because of its tendency to crack under stress, it is not normally used except to build the lining from the bottom of the well up, in one continuous operation.
A brick or rock wall can be built up on a cutting ring and sunk into the ground (See Fig. 8-14.) but any caving around the outside could crack or topple the wall. If such a column were to sink askew it could be very difficult to get it upright again without exerting some kind of sideways force on the column which could crack or destroy the column.
FIG. 8-13. ROCK LINED WELL 3m OF CONCRETE OR MORTAR SEAL IS TO PREVENT CONTAMINATION
FIG. 8-14. BRICK LINING SUCK ON A CUTTING RING
It is possible to use reinforcing rod to help hold a brick lining together. This is sometimes used in sinking the bottom section of a well. It is not normally used for construction of the middle section because in most cases a reinforced concrete or mortar wall could also be built with the same materials, and provide a stronger lining. (See Fig. 815.)
It is also possible to use masonry to line the wall in sections. When a lining is needed after having dug down a number of meters, a curb can be installed and the lining built up on the curb. The curb serves as a base on which the lining can be built up and as a solid anchoring piece which can be supported to allow digging to continue beneath and through it As a safety precaution, the curb should be supported by long pieces of wood, or whatever else is available, which can be wedged between the bottom of the hole and the curb. (See Fig. 8-and the curb. (See Fig. 8-16.)
FIG. 8-15. REINFORCED BRICK LINING
FIG. 8-16. SECTION OF BRICK LINING
This chapter provides detailed information to assist wells workers to construct the bottom section of the well. Describing the unique features of that section, it also raises such special concerns as the choice of sinking/ lining methods, selection of materials, keeping water out of the well to allow diggers to dig deeper, and facilitating water entry.
The bottom section is that portion of a well below the water table. Its purpose is to allow as much water as possible to enter the well while blocking fine soil particles from the surrounding aquifer. This is accomplished by digging a hole and extending an appropriate lining as far beneath the water table into the aquifer as possible.
As the bottom section is being sunk, using whatever method, water will have to be removed to allow continued sinking. This can be done by simply filling the buckets normally used to remove excavated material and emptying them at the ground surface, or by using a pump. Pumps are generally easier to use because they allow water to be removed from the well faster; a well can be dug deeper than if buckets were used. This will be especially important if the bottom section is being constructed when the water table is above its lowest level and the well is being finished at any other time than the end of the dry season. (See Fig. 91.)
The construction of the bottom section consists or the same two operations,digging and lining,as the middle section. However, because water is entering the well while the digging is proceeding and the lining is being installed, the whole process becomes more complicated.
These two situations will limit your ability to dig deeper:
· water flowing into the well
As you begin to get into the water layer there will probably be little water entering the well. As you go further down,water will enter the well faster until 1) it is coming in so fast that you can no longer remove enough of it to be able to continue digging or 2) you reach the bottom of the water bearing layer. If you reach the bottom of the water bearing layer you will have to evaluate whether the supply of water is sufficient for the needs of the local users. Again there are two possibilities: a) the water flows very slowly,or b) this is an isolated small pocket of water that is limited and will be quickly used up.
FIG. 9-1. USING A PUMP TO REMOVE WATER FROM BOTTOM SECTION WHILE SINKING
In a), the well will always have water in it but there may be very little and people's water consumption may be limited. It might be possible to dig other wells which could each supply a limited amount of water but which combined with the rest would allow for an increase in water consumption.
In b), the decision will have to be made whether to try to continue sinking this well or start another well in a different location.
· walls collapsing as you dig
This is likely to be a problem in all wells, but it will be much more serious in wells which reach aquifers composed of loose, usually sandy soil. The deeper the hole is sunk into the aquifer, the more serious the problem will become. The collapsing of the walls is caused mainly by the flow of water coming into the well. The water, as it flows into the well, exerts pressure on the aquifer particles around which it must pass. The faster the water enters the well, the more pressure it exerts and therefore the more likely the walls are to collapse. By sinking the bottom section lining ring into place, the possible hindrance and danger of collapsing hole walls can be prevented. (See Fig. 9-2.)
FIG. 9-2. HOLE WALLS COLLAPSING IN BOTTOM SECTION
B. Sinking/Lining Methods
1. Sink Lining
To sink the lining into the bottom section the rings must be made just small enough to fit inside the middle section lining. When using reinforced concrete a different set of forms from those used for the middle section lining will be needed. A well can probably be sunk a little deeper using this method rather than others,because it allows workers to spend all their time removing soil and water from the bottom of the hole,thus deepening it.
This method is limited to sites where ground conditions permit digging below the water table. This can only be done where aquifers are made up of relatively consolidated material and is not recommended where the aquifer consists of loose materials that tend to cave-in.
NOTE: Wells have been sunk into loose, unconsolidated aquifers by using temporary lining to reinforce the walls and prevent cave-ins. In most cases today time and money that would before have been spent on temporary lining can now be more usefully and safely spent working with permanent cement linings installed as the hole is dug either by sinking the lining or digging and lining in short sections.
It may be possible in slightly caving formations to dig and line several short 1/2 m sections before reaching a point where water enters the hole fast enough to slowly erode the earth walls or wash away wet concrete or mortar from inside a form. At this point the fining can no longer be built in complete rings but will have to be continued by completing each 1/2 meter ring in several sections one at a time. The use of quick drying cement will significantly aid this work.
Some have used this technique because it requires less preparation than sinking a lining which requires pre-casting lining rings. It requires less equipment and may produce a structurally superior well,although the structural continuity achieved here may not be of significant advantage in the overall functioning of a well. On the other hand, for shallow wells that reach a limited supply of water, expenditure in time and money on a more elaborate well structure probably will not be worthwhile. Also, the actual work required is somewhat complicated, requiring well diggers to perform various tasks simultaneously and with great competence.
C. Water Entry
The bottom section should tee constructed so that the water comes in through the bottom, and perhaps lining, depending on the bottom lining procedure and the ground conditions. (See Fig. 9-3.)
FIG. 9-3. WATER ENTRY INTO BOTTOM SECTION
Water entry through:
bottom, and perhaps lining
bottom, and perhaps lining
It is best to construct the bottom section so that water will come through the lining as well as the bottom. The more area you have open for water to enter the faster the well will recharge and the less strain there will be on those areas where water comes through. However, an impermeable lining is recommended where the aquifer is likely to be very fine sand that will clog or come through even porous concrete.
1. Water Entry through Lining
There are two methods used predominantly for constructing lining sections that will permit water to flow through. In both cases it is better to construct the lining sections on the surface where their construction can be more easily controlled before they are sunk into the bottom.
a. Leave holes in lining sections
· Forms should have holes through which you can stick re-rod to make the holes in the concrete,
· Holes should be sloped up and to the middle to prevent aquifer particles from entering the well. The holes could be perfectly horizontal where aquifer particles are larger than the holes.
· Holes can also be poked in the bottom of the well, although this has rarely been attempted.
2) Brick or Pock
Dig to the bottom of the well and stack unmortared brick or rock. The open spaces between the stacked brick or rock allow water to enter.
Many successful wells have traditionally been built this way but it can be difficult to do, is sometimes dangerous, and cannot usually penetrate an aquifer as far as other lining methods.
b. Use porous concrete (cement and gravel with little or no sand)
· Use porous concrete to make lining rings. Simply substitute porous concrete for regular concrete when casting lining sections. Some have suggested using regular concrete for 10 cm of top and bottom edges to strengthen these. Porous concrete is not as strong as regular.
· Use porous concrete to make blocks which can be set on a cutting ring and sunk or built up from the bottom.
2. Water Entry through the Bottom
The bottom of the well should normally permit water to come up through it. However, in unconsolidated (loose) aquifers the water will tend to carry particles of the aquifer along with it up into the well to completely fill and clog it.
Where the well is sunk into a relatively consolidated (firm) aquifer, water coming in through the bottom tends to act similarly but the aquifer particles are usually too big and heavy to be moved by water. In both cases it is advisable to put a filter or plug across the bottom of the hole.
When the aquifer is composed of very fine sand, this sand tends to act as a liquid and follow the water up through the bottom filter. The filter, if properly constructed, will prevent the sand from doing this but must, at the same time, be able to withstand the pressure of the sand and water. Rather than coming up through the filter the sand and water can possibly exert sufficient pressure to push the whole filter layer up into the well and clog it. This is admittedly an extreme case but one that must be carefully guarded against.
D. Construction of the Bottom Section
1. Sink Lining Method
· Lining Rings:
· are manufactured on the ground surface for later sinking into the bottom section of the well;
· should allow for water to enter the well, as discussed previously;
· should have the capacity to be firmly attached together;
· should have an outside diameter 10cm less than the inside diameter of the lined middle section of the well;
· can be very heavy and awkward to move and so may require a special lowering device;
· are most commonly made of reinforced concrete or mortar which should be cured at least 3 days after pouring before being lowered into the well (see Cement Appendix, p. 221.)
· are usually set on a cutting ring which can also be poured at the ground surface although it will require a different form.
NOTE: A cutting ring may not be a necessity in loose sandy aquifers but is useful. The cutting edge could also be cast on to the bottom lining ring. A flat edge will tend to be caught more easily, preventing further sinking~as opposed to a cutting edge which will tend to cut through loose material and funnel it toward the center for easier removal from the well. (See Fig. 9-4.)
FIG. 9-4. CUTTING RING EFFECT
b. Construction Procedure
1) The general construction procedure to be fallowed when all materials are assembled and ready for use is as follows.
· stack several (usually 2 to 4) lining rings on top of a cutting ring which has been centered and levelled in the bottom of the hole.
This initial column should be 2 to 3 meters high whether it is composed of 1/2 met:er high rings or 1 meter high rings. If holes have beer made in the rings to allow water entry. make sure that the holes slant up toward the middle of the hole. Firmly attach rings together by whatever means you have chosen.
Unstable ground can make the initial leveling/ plumbing of rings very difficult. Where you can anticipate this, place a layer of gravel on the bottom before lowering the cutting ring into the hole.
· after the lining rings have been stacked on the cutting ring in the hole, well diggers will fill in the space between the lining and the middle section lining with gravel. (See Fig. 9-5.) Instead, some have placed several boards vertically in the" space to act as a guide to help the column sink straight. (See Fig. 9-6.) Gravel is easier and better to use because it will tend to fill in around the rings as they are sunk, filling up any voids that might be created by over-pumping. Gravel is also a convenient filler that is normally used to fill that space once the bottom section is sunk.
FIG. 9-5. GRAVEL USED TO GUIDE SINKING RINGS
FIG. 9-6. BOARDS USED TO GUIDE SINKING RINGS
· Remove soil and water from inside the cutting ring thus causing the column to sink. Begin digging in the middle and gradually work out to the edges of the column being careful to remove soil evenly so as not to encourage the column to sink out of alignment with the rest of the hole. (See Fig. 9-7.)
· Add more rings as necessary as the column sinks. It is always necessary to have some overlap of the sinking column and the middle section, and helpful to have up to 2 meters of overlap to provide extra weight which will aid sinking and help to guide the column down.
· Plan to add more rings when it won't interrupt the sinking process. If it requires many people to lower the rings, lower as many as you need in one or two sessions to avoid the need to regather and reinstruct a large group of people.
· Continue digging and sinking until water can no longer be removed from the hole fast enough to allow further sinking. This will occur when well workers are digging in as much as 1 meter of water. Remember,the farther this column is sunk into the water bearing layer without going beyond it, the more water will be available from that well. In commonly-found loose sand aquifers, the column of rings is usually sunk between 2 and 4 meters beneath the bottom of the middle section lining. In more consolidated aquifers, where water generally enters the well more slowly, a greater depth can and should be reached.
· When sinking is stopped it is necessary to have some overlap between the respective linings of the middle and bottom sections. If the well has been completed in other than the end of the dry season, it may be useful to leave several extra ring sections stacked up into the middle section lining. If necessary, the well can then be easily deepened at the end of the dry season when the water table is at its lowest level. This will help to provide a permanent supply of water.
Digging steps while sinking lining
a. Dig a cone-shaped hole down to a convenient depth.
b. At four evenly spaced points in this cone-shaped hole, widen the hole so that it reaches the width of the cutting ring.
c. Widen the hole at four more places between the areas already widened.
d. Continue enlarging and widening the hole until the column of rings sinks.
It is likely that this method will only be considered when:
· The middle section of the well has been dug but not lined because there is little danger of soil collapse. (A temporary lining could be used to reinforce the section or help prevent caving.)
· The water beating soil is firm enough to permit digging and removing water without great danger of collapse.
Under these conditions an open hole can be dug down into the aquifer. Even so, digging beneath the water table must be done very carefully and can only reach a limited depth because of the increased danger of cave-ins and collapse as the hole reaches deeper and deeper into the aquifer. The hole should be dug as deep as is safely possible.
In the traditional well, the lining is begun by stacking rounded stones to a 'neighs of between .5 and 1m. (See Fig.98.) A rock or brick masonry wall is then built on top of the loose rock wall. Alternately, pre-cast rings can be lowered in place or the lining could possibly be poured in place. If, however, one has the equipment and supplies to cast concrete walls, it is usually possible to sink the lining into place. This is much safer and can reach a greater depth where there is any danger of cave-ins. Whatever lining method is used is then continued up to the ground surface and the top section of the well constructed.
FIG. 9-8. TRADITIONAL WELL BOTTOM
3. Dig-and-Line-Short Sections
This particular method of digging into the water table is a variation of the "dig-a-meter, pour-a-meter" method sometimes used to line the middle section. The bottom section is dug and lined, usually 1/2 meter at a time. As the aquifer is penetrated increasing amounts of water will enter the well as the depth increases. Eventually a depth will be reached where the water is coming into the well so fast that 1) it will wash away the mortar or concrete before it has a chance to set, or 2) it will wash the walls in before there is time to set a form, re-rod, and pour. From this point, use the following procedure to extend the well as far into the aquifer as possible.
· Make sure that the vertical re-rods extend beneath the bottom of the last pour so that the subsequent sections can be attached to the existing lining.
· Dig in the center of the hole to form an inverted cone which is about 40 cm deep and sloped out to the bottom of the previous plastered section. (See Fig. 9-9a.) This tapered hole keeps sand from washing out from behind the sections already lined.
· Quickly dig a short section on one side down to about 30 cm. (See Fig. 9-9b.)
· Roughly plumb and smooth the newly dug short section.
· Throw handfuls of dry quick-setting cement mix onto the moist slowly collapsing sand.
· Moisten small quantities of the mix and quickly plaster the short section. The mortar should set (in about 5 minutes) before the water flow causes it to collapse. (See Fig. 9-9c.)
· Continue digging and plastering short sections 30 cm deep around the hole until one complete 30 cm deep section is plastered. (See Fig. 9-9d.)
· Place re-rod; leave extensions below plaster limit for connection to next pour.
NOTE: A quick-setting mortar mix consists of 1 part quicksetting cement, 2 parts ordinary cement, and 1 part sand.
· Plaster over re-rod with same quick-setting mix.
· Continue digging and plastering short sections until 1) the water flowing into the well prevents further plastering, or 2) you have reached the desired depth into the water table. Do not leave any re-rod exposed to water. If necessary bend it up and plaster, or cut it off and plaster.
FIG. 9-9. DIG AND LINE BOTTOM SECTION
NOTE: Special bricks may be made for covering places where water is coming through the sand too quickly to permit plastering. These bricks are made outside the well by spreading a 1 1/2 cm. layer of mortar on a flat surface, and then cutting it into 15 cm.x 15 cm. squares with the edge of trowel before the mortar dries. Whenever a spot is found where the water flow is so rapid that it erodes even the rapid setting mix, one of these square mortar bricks can be used as a stopper. Throw dry cement mix freely on the place where the water is entering and slap a flat brick over it. Hold the brick in place until the cement sets. It can then be plastered over in the same manner as the rest of the short section being lined.
E. Bottom Plug or Filter
Most hand dug wells need a filter over the bottom of the well that will allow only water and no fine particles of soil to enter the well. It is especially critical where the aquifer is very fine sand. Only where wells are sunk into hard rock is this not a consideration.
There are two different materials that can be used separately or together: a gravel filter or a porous concrete plug.
1. Gravel Filter
A layer of gravel across the bottom of the well can serve as the filter.
A minimum depth of 20 cm is suggested. The filter can be made more effective by using 2 or more different sizes of gravel in separate layers with the smallest size gravel on the bottom and the largest size gravel on top.
2. Concrete Plug
a. This is a slab of porous concrete which fits closely to the inside diameter of the bottom lining. The slab can be cast in sections on the surface and later lowered and placed in the well. It is placed in the well on top of a shallow (10 - 15 cm) layer of gravel.
The slab should be made porous either by making holes in a regular concrete slab or using a concrete mixture with very little sand as described in the Cement Appendix. It can easily be cast on the surface digging a mold out of the ground.
b. Making a bottom plug with the ground as the form
· Choose a fairly flat section of ground that can be dug and will hold its shape.
· Draw a circle with a diameter a few centimeters less than the inside diameter of the bottom section.
· Dig out the circle to a depth of 6 cm and make the bottom surface fairly even and square with the sides. (See Fig. 9-10.)
· Put a divider across the center of the circle so that you will then pour two halves of the slab which can be more easily lowered and placed. The divider should be at least 6 cm high and be made from some suitable form material. (Alternately where a divider is not available or appropriate, two circles can be drawn but then only half of each dug out to form the two halves of the slab.) (See Fig. 9-10.)
· Place re-rod in each half of the form. Leave handles or lowering hooks. When using 10 mm re-rod it can be placed 20 cm apart or with 8 mm re-rod leave 15 cm between re-rods.
· Pour concrete - either standard or porous mix. If using non-porous concrete, make holes through the slab with a piece of re-rod when the concrete begins to set.
· Cure for about a week.
· Place in well on 10 to 15 cm of gravel.
FIG. 9-10. CASTING THE BOTTOM PLUG