|Environmental Handbook Volume I: Introduction, Cross-sectoral Planning, Infrastructure (GTZ/BMZ, 1995, 592 pages)|
|16. Road building and maintenance, building of rural roads|
The term "road building" covers public roads and footways of all kinds inside and outside built-up areas that are used for the movement of persons and goods.
The roads involved will generally be two-lane rural roads or single or two-lane rural footways that are intended to establish a traffic link between two specified points or to open up a region in a practical way by providing a new or expanded transport network. For this reason we will not be considering here the special problems of urban roads or developments of the motorway type (such as the stress on the population in centres of population).
As dictated by purpose, volume of traffic, and the given natural morphological conditions in the landscapes to be crossed, the design standards (right-of-way width, vertical and horizontal alignment, structures, and carriageway pavements) that the planners elect to adopt in the light of vehicle characteristics will be more or less generous.
Despite the wide variations in topography, climate, hydrography, soil types and vegetation that exist, there are relatively standard rules for planning and construction that govern the three main stages of building a road:
- deposit a fill of the minimum height required using suitable soil that if possible is available from nearby sources,
- protect this fill with a system of passages through it and ditches to drain away rainwater and any constantly occurring surface water,
- pave the carriageway with bound (with cement, bituminous material, etc.) or unbound mineral layers in a manner suited to the wheel loads anticipated and their frequency of passage.
All roads require constant maintenance, though the amount will vary with heaviness of use (crown to be kept free of vegetation, drainage system to be kept fully operational, erosive damage to be made good where cut and fill methods were employed, carriageway to be repaired).
The criteria that should be applied are those developed over the last few years for environmental protection in the road building field, as modified to suit different environmental conditions and the different weights given to the factors evaluated.
The present environmental brief will deal not with the effects stemming directly from the movement of traffic on roads and from the operation of vehicles (see the environmental brief Road Traffic for these) but simply with the impacts in both a wider and a narrower sense that, due to the interference there has to be with the natural environment, arise from the reasons for building the roads and from the subsequent maintenance these roads will need.
These impacts can be divided into
- widespread direct impacts
- linear indirect effects
- spot effects
- consequential impacts.
2.1 Widespread direct impacts
(a) Regional adverse developmental effects
The opening up by a new road of a region previously inaccessible to motor traffic, or simply the upgrading of an existing artery, causes changes to living conditions in the areas affected, these changes being of greater or lesser severity in line with the smaller or larger potential that there is for development. The range of more intensive or new human activities that may arise as a result is vast, and it is therefore quite impossible for their effects (migratory movements, uncontrolled growth of settlements, changes in land use, etc.) to be looked at in detail here. As well as this there will also be changes in socio-cultural and socio-economic conditions, and these too, in their capacity as possible secondary impacts, will require careful consideration.
Thought will also need to be given to the ways in which adverse developmental effects can be avoided or mitigated by collateral measures (land use plans, migration regulations, and monitoring of compliance with these). See also the environmental brief Transport and Traffic Planning in this connection.
(b) Adverse effects on areas deserving of protection
The technical and economic aim will be to produce the shortest possible link between two points but the full achievement of this aim will be frustrated firstly by the need to allow for items whose position cannot be altered (for natural reasons or as part of the transport system) and secondly by the need to divert round areas deserving of protection. Such areas include ones containing vegetation worth preserving, ones subject or due to be made subject to landscape protection or nature preservation orders, ones of high agricultural value, and ones containing high-density or historically important buildings, and also, in particular cases, specific, small biotopes, game reserves and fauna stocks.
2.2 Indirect linear impacts and spot impacts
(a) Effects on the appearance of the landscape
Applying the traditional rules of road building may help to moderate the look of the artificial changes made to nature. Where these rules are not followed as closely as they should be, such things as deep cuts and over-high fills will break up the natural lines and disrupt the appearance of the landscape.
(b) Fragmenting impacts of roads cutting across existing ecosystems, and changes to the microclimate
A road cuts the land it crosses into two, and it does so the more positively the greater is the difference in elevation between the carriageway and the natural terrain. It creates a barrier to humans and animals and stops geographical interchanges (e.g. migratory movements). By obstructing the movement of water and air, by causing a shadowing action, by preventing heat and cold from dissipating, by withdrawing or building up moisture, and by producing drifts of wind-blown material, it causes changes in the microclimate that may damage or destroy the existing systems.
(c) Landscape uptake and demand for land
A road takes up a strip of land of greater or lesser width which is demarcated physically by the road fill or cut including the ditches and other ancillary works and which is legally defined by the ground acquired (the right of way). It is also standard practice in many countries for the soil needed for the fill to be dug by an excavator or bulldozer from shallow trenches immediately adjacent to the road in a process known as "side borrowing". This method saves on transport but it also creates a demand for fairly large amounts of additional land and leaves behind it, unless levelling up is explicitly demanded, troughs following the line of the road that are at risk from erosion and that may also harbour pathogens if for example they turn into mud holes. Even when soil or rock borrowing is confined to pits or quarries, these should be left in as near natural a state as possible after work at the site has been completed.
Similarly, site waste, residual quantities of unused soil and building materials and so on should disposed of with due care for the environment.
After a new road has been completed, animal-drawn farm carts and herds of livestock are sometimes relegated from the carriageway to an unpaved strip by the side of the road. Depending on the type of soil and the amount of use they receive, such cattle tracks may be further deepened by wind and water erosion or may become impassable at certain times of the year. Also, constant use will make them wider and wider. To prevent this happening, they should receive the appropriate maintenance in the same way as the road.
Newly built roads should be positioned an adequate distance away from the traditional tracks used for livestock herds.
(d) Removal of vegetation and top-soil
In some cases, the corridor cleared for the building of the road will be made sufficiently wide at the outset to stop trees and bushes from growing back to the edges of the road too quickly. It also needs to be ensured that, when the margins of forests or woodland are cut back, trees brought down by the wind cannot fall onto the road. Ground is occasionally cleared by burning off and this, combined with the physical stress on the sensitive, thin layer of humus, can give rise to soil erosion during the building period.
Proper treatment of vegetation (and particularly forest and woodland), preservation of existing forest margins or isolated trees, re-afforestation, laying of humus cover, and planting of newly created areas of exposed soil together with reasonable long-term care for such areas must have compatible technical details specified for them in the plans and must be monitored by the authorities supervising the building, as also must the setting up of storage for the stripped top soil, the upkeep of the latter and the subsequent relaying of the soil.
(e) Production of soil erosion, and changes to soil structure
Improper land clearance, failure to observe the soil mechanics characteristics of the soils present, mis-assessment of the stability of cut or deposited slopes, or total neglect of this basic question, may lead to progressive damage to the road pavement itself and to its immediate or more distant surroundings. In the worst case, and particularly where the road is exposed to adverse meteorological conditions, such damage may be so severe that the road is totally destroyed and rendered impassable.
As well as this, there is also the possibility that earth may slip, settle, or be washed away over large areas, and this can produce extensive erosion which, particularly in hilly or mountainous country, may extend well beyond the area of the road proper and take in neighbouring slopes or valley floors. Even rock slopes that appear solid can become unstable if roads are cut in them without paying proper attention to the geological structure or by excessively heavy blasting.
Although soil erosion is usually caused by the action of water as a transporting medium, in arid and semi-arid zones it is necessary to pay heed to wind erosion, which is triggered or at least encouraged by removing even very sparse vegetation or by placing roads at an aerodynamically unfavourable elevation (dunes may be created!). Where use is made of fine-grained types of soil to form unbound base courses, simply the movement of traffic may enable erosion by wind (that creates dust) or water (that washes soil away) to destroy the carriageway.
As well as expert soil surveys, proper planning, and official supervision of the earthworks and rockworks to reduce vulnerability to damage to a minimum, the road and its surroundings must receive constant maintenance to remedy minor damage in its initial stages and prevent it from spreading.
It is particularly important for areas of exposed soil to be planted as quickly and as permanently as possible with native plants (grasses, ground cover plants) in order to obtain biotechnical stabilisation of the soil by deep-rooted plants, hedges, wattle fences, etc. A further important contribution will be made by the careful, non-destructive drainage of surface and infiltration water.
Particular care should be taken to stop erosion when building roads of tertiary status such as rural roads, feeder roads and developer roads. In this case road-building comes down essentially to questions of hydraulic engineering and earthworks. The road alignment should be along the crest of elevations and it should largely follow the terrain. Where watercourses have to be crossed, this should preferably be done by means of wholly or partly flooded fords which in no way obstruct water runoff and thus help to prevent erosion.
(f) Effects on the water balance
It is inevitable that there will be interference, some of it considerable, with the existing water balance in the course of road-building operations. If watercourses have to be diverted to prevent them crossing the line of a road, then if the laws of hydrology are not observed this may result in adverse changes to the discharge regime of the watercourses (caused by changes in the roughness, gradient or cross-section of the bed). There are many different ways in which the water balance may be affected and among them are deliberate or accidental impoundment by road fill, draining of wetlands and swamps, lowering of the groundwater level, deepening of watercourses by the extraction of excessive amounts of material from their beds, bridgeworks built in watercourses with the risk they create of blockage by flotsam, and the creation of reservoirs for livestock watering by exploiting the effect of the road fill, with its attendant problems of infection and pest breeding.
One disadvantage that will become apparent is that reliable data on the volume, frequency and duration of precipitation is seldom available and hence reliance has to be placed on empirical data combined with observations of nature and experience gained in comparable situations. Here too, great importance attaches to constant maintenance being put in hand at an early stage.
The risk of surface water and groundwater being polluted is particularly serious in countries where it is difficult for drinking water to be checked due to the special nature of the supply (e.g. decentralised withdrawal). In countries of this kind where the winters are cold, pollution from road salting should be prevented by selecting non-polluting materials for salting use.
(g) Reduced safety due to the risk of accidents
More serious and more frequent accidents may be caused by the higher speeds that become possible after road development, by inexperienced drivers, by pedestrians and animals crossing roads, and by the absence of a parallel route for slow-moving traffic (e.g. pedestrians, non-motorised transport, driven cattle).
Road building should therefore always be accompanied by or prepared for by instruction in road safety and monitoring and inspection of vehicles for safety.
(h) Effects on settled populations in towns and villages
Existing towns and villages are normally too densely built-up to allow the building of a direct through-road. If such a road is built nevertheless, the detrimental effects it will have are noise, vehicle exhaust emissions and a greater risk of accidents (see the environmental brief Road Traffic).
However, even with effects of this kind caused by more intensive land use, the density of building often rises in such areas, with attendant environmental impacts in the fields of, amongst others, hydrology and local climate.
If an attempt is made to stop such effects by building a by-pass, mature existing social structures are often damaged or destroyed and new buildings soon spring up along the by-pass, though these can be prevented from having any detrimental impacts by proper environmental planning (see the environmental brief Provision of Housing).
(i) Detrimental impact of lack of maintenance
A road that is not maintained will not last. A suitable technical design for the road and good-quality building work will serve to keep down the amount of maintenance needed, at least in the first few years. However, at that stage maintenance work will certainly need to be done, and if it is not the damage and adverse environmental impacts described above will inevitably ensue.
2.3 Consequential impacts
(a) Building noise
Where the volume of the works involved is small and the techniques employed are simple, building noise is only a minor consideration in road building.
(b) Re-use of bitumen
When the time comes to replace bituminous carriageway pavements, the binder should be recovered from the old pavement courses. Where the pavement is relatively thin (bituminous treatment mainly of surface layers), the old pavement will normally be needed to improve load-bearing capacity and it will be impossible for it to be re-used.
(c) Seismic damage
In the event of earthquakes, roads having only a few, simple structures (bridges and tunnels) along their routes will be far less susceptible to damage than ones with more complicated, highly engineered structures. Fissures and slips in the earthworks will be limited too due to the smaller dimensions.
(d) Beneficial impacts
Some additional measures with beneficial environmental impacts which can be taken in the course of road-building are as follows:
- Retention basins for the population/drinking troughs for livestock can be created by raising the vertical alignment of the road at watercourses.
- Borrow sites can be developed into biotopes/amenity ponds.
- Where roads passing through villages are paved and if there is an unpaved market place the road paving can be extended to include it.
- In the course of the building work, preparations can be made at the points where material is borrowed to ensure that sufficient material will be available for maintaining the road.
- The road can be used as a fire-break to stop large fires.
- Erosion control can be improved (especially in the course of maintenance).
Binding, universal guidelines for any complex analysis of the many and diverse aspects of the possible environmental impacts generated by the building of roads do not exist and given the many different sides there are to the problems it would be virtually impossible for them to be satisfactorily drafted.
Even the evaluation criteria developed for German conditions have so far proved impossible to define in quantifiable terms, except that is for the criteria for noise emission and the emission of harmful air pollutants (see the environmental brief Road Traffic). In view of the number of variables that there are, any ranking assigned to possible alternatives will continue to be largely subjective. The only answer to this problem is more thorough planning which brings out particular critical issues with greater clarity.
Preliminary regional planning and a detailed feasibility study including a comparative evaluation of all the adverse and beneficial impacts may therefore be a useful way of obtaining the information needed for assessing the widespread effects of a road-building programme.
An advance study of this kind will be made at an early stage and it is at this stage that a check should always be made to see whether the alignment corridors
- are suitable
- are suitable with the proviso that corrective measures still to be defined are carried out, or
- are not suitable.
For this purpose it will be necessary to draw up appropriate maps of protected areas, plans of the natural landscape, geological surveys, registers of land used for agriculture and forestry and maps of forests and woodlands, surveys of hydrological resources, and master building plans, to define the possible alignment corridors, and to evaluate alternative possibilities by a process of comparison.
The measures that will have to be taken to blend the road into nature and the landscape should be realistically shown and described in words in a special drawing (e.g. to a scale of 1 : 5000). After a comparative appraisal of the possible alternatives, all the details of these measures should be worked out and incorporated in the design drawings (e.g. on a scale from 1 : 2000 to 1: 500 with appropriate detail drawings).
Where it proves virtually impossible in the different planning phases for harmful factors to be quantified (and their impact to be rated or even costed) in relation to evaluation criteria and standards arrived at by analysis or ones for which standard procedures are defined, then qualitative approaches to evaluation may be successfully adopted.
There are a number of references in the above text to the direct interactions that exist with the road traffic sector.
There are also close relationships with projects in the field of rural, village and urban development, particularly with regard to the developmental effects that follow on from road building.
The benefit to road users conflicts with the interests of those using the land occupied by the road and its close and more distant environs.
There may in particular be conflicts for agricultural or forestry reasons.
Questions to be considered in this case are not simply the actual land occupied but also the functional disruption that road-building causes to existing, and in some cases sensitive, systems (e.g. the disruption caused by the road crossing irrigated rice-growing land, the heavier felling of commercial timber now that transport is easier, the erosion caused by different farming methods adopted for new crops made more attractive by transport, and the greater mobility and production levels encouraged by easier access to markets).
Impacts encouraging the growth of trade and industry which would not have occurred without the new road are generated in the same way (better locational advantages give rise to extra production).
Finally, there are often reciprocal impacts which a road-building project generates with respect to other impending infrastructural projects (e.g. where the building of an earth dam in a remote region cannot go ahead until access facilities exist; the commercial activity triggered by the building of a road will create a need for the disposal of waste water and solid waste; the use of fertilizer as a result of a recently built road will create stress on the environment that did not exist previously).
Road-building projects result in the opening up of regions that were difficult or impossible of access previously, or they at least improve conditions of movement on existing routes. This being the case, it is inevitable that they will always interfere with the natural situation in a region. As well as this presence-related impact there is also an operating impact due to the traffic the roads carry. And over and above these there will be direct and indirect impetuses for changes to take place in the socio-cultural and socio-economic status quo.
The variety of different questions that arise from this complex pattern is enormous and they all need to be considered in the course of carrying out a road-building project. What this means is that each individual project must be treated as a special case, in that the weight attached to the different environmental impacts of greater or lesser seriousness may vary and will thus have to be re-assessed in each individual case.
The object in making a summary assessment of a road building project is
- to minimise the stresses on the environment in the primary areas of soil protection, noise control, and preservation of clean air and water, and
- to preserve or reshape the natural environment by means of the building work specific to the project or in parallel with it, with major importance being attached to an alignment which fits as neatly as possible into the natural environment.
Asian Development Bank: Environmental Guidelines for Selected Infrastructure Projects; Manila, 1988.
Burger, R., Heider, O., Kohler, V., Steinlin, H.: Leitfaden zur Beurteilung von Straßenbauvorhaben unter Gesichtspunkten des Natur- und Landschafts- schutzes, Schriftenreihe des Instituts für Landespflege, Universität Frei- burg, Heft 10, 1987.
Der Elsner: Handbuch für Straßen- und Verkehrswesen, Teil M Straßenbau in Entwicklungsländern, Teil E, 34 Umweltgerechte Straßenplanung, Darm-stadt, 1986 edition.
Forschungsgesellschaft für Straßen- und Verkehrswesen: No.352, Umweltgerechte Straßenplanung, Cologne, 1981.
Forschungsgesellschaft für Straßen- und Verkehrswesen: Merkblatt: Berücksich-tigung von Umweltkriterien bei der Straßennetzplanung (in preparation)
Krämer: Forschungsberichte des BMZ, Möglichkeiten der genauen Dimensionie-rung beim Straßenbau in Entwicklungsländern, Munich, 1982.
Ministère de la Coopération BCEOM: Manuel sur les routes dans les zones tropicales and désertiques, Paris, 1972/1981.
Sandleben: Forschungsgesellschaft für Straßen- und Verkehrswesen, Heft 398, Entwicklung eines Bewertungssystems für die Berücksichtigung von Umweltkriterien im Straßenbau, Cologne, 1983.
UN: Appropriate Technologies in Civil Engineering Works in Developing Countries, New York, 1976.
Unitar: Protecting the Human Environment, New York, 1976.
United States Agency for International Development: Environmental Design Considerations for Rural Development Projects; Washington, 1980.
United Nations Economic and Social Commission for Asia and the Pacific: Environmental Impact Assessment of Road Transport Development, Bangkok, 1986.