Low-Cost Road Surfaces
Surfacing is not always necessary to provide an adequate or "good-enough" road. In some circumstances the in-situ soil along the road line may be sufficiently strong to support low levels of light traffic flow in areas with modest rainfall (up to about 1,000mm/year). The soil must be shaped into a camber to shed rainwater to each side and then consolidated, as a minimum, with hand rammers. The passage of vehicles will also help. Compaction is always necessary to ensure its durability, and it must be maintained thus with regular reshaping or grading. Such a road is referred to as an 'Engineered Earth Road' or 'Engineered Natural Surface'.Soils with California Bearing Ratio (CBR) strength of about 15 or more can usually be used in this way. Strength may be gauged by using simple low cost apparatus such as the Dynamic Cone Penetrometer (DCP).
However, for many circumstances it is necessary to stabilize or improve the in-situ soils mechanically, either with other selected soils/aggregates or with cement, bitumen or chemicals, or construct a stronger pavement on top to support heavier vehicles or higher traffic flows. This will help spread vehicle loads so that they can be carried on the alignment soils without causing deformation.
There is also a range of surfacing or paving options that can be used. Natural gravel surfacing is generally used as the principal low cost solution in many developing countries. This material provides an intermediate surface between basic engineered earth and higher cost, often bituminous, paving. Gravel can be appropriate where suitable material is available and laid to strict surfacing specifications and procedures, gravel haul distances are short (usually less than 10 km), road gradients are less than about 6%, rainfall is low or moderate (less than 2,000mm/year), traffic is relatively low (usually less than about 100 motor vehicles/day), finance and resources are available for periodic re-gravelling, and dry season dust generation is not severe.
Unfortunately, these requirements are not met in many locations. Naturally occurring lateritic and other suitable gravels tend to be rare with any good quality deposits remaining often located far from the roads. Transport is expensive. Gradients can be steep on low volume roads to minimise overall construction costs. When rainfall is intense, concentrated within relatively short periods of the year as is frequently the case, the gravel surface will be quickly washed away. Dry season dust loss leads to the surface disintegrating, and again washed away during the rainy season, particularly on steep sections. Maintenance is expensive, especially for periodic re-gravelling, which is typically required at three to five year intervals, Therefore gravel roads are rarely maintained systematically and many revert eventually anyway to earth standard roads. There are also environmental problems. Un-reinstated borrow pits, which fill with water, form loci for erosion and disease while the clouds of dust thrown up by motor vehicles during the dry season are a health and safety hazard, as well as affecting nearby crops and property. In many countries, gravel roads are meeting increasing resistance because of this.
Fortunately, there are alternative surfacing and paving options already tried in various countries that could provide appropriate, economical and sustainable alternatives. Suitability will depend on local circumstances. These alternatives, involving the appropriate use of available materials, may be cheaper in whole-life-cost terms (covering all construction and maintenance costs and vehicle operating cost savings). Many can be carried out by small local enterprises using labour-based methods and light equipment. They could have lower maintenance requirements than gravel, not only in terms of cost but also by reducing the need for heavy equipment to transport and compact, and the resulting damage to haul routes. The many considerations influencing the choice of surface are discussed in this page.
In summary, the "window of opportunity" for gravel in developing countries is being squeezed from below by earth roads, and from above by the increasing awareness of low-cost surfacing alternatives. The cost of the gravel surface has proved to be relatively high, taking account of its need for regular replenishment, low durability, especially in many less-developed countries, and the rapidly declining level of service it provides to users as it deteriorates. As countries, especially in Asia, get richer, dust becomes more unpopular, to the point where communities finance street paving on their own even if the roads leading to their town are unpaved. Alternative surfaces, many perfected years ago, can now be justified at lower traffic levels than hitherto thought. These include water-bound macadam, hand-laid stone, fired brick paving, concrete paving and low-cost seals. These surfaces are more durable and allow year-round mobility to all types of vehicle.
The Global Transport Knowledge Partnership is presently compiling information on these alternative surfaces. More documentation about this topic can be found by clicking on the logo below:
Contributed by Robert Petts, gTKP Rural Transport Theme Champion.