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Surface Options

Until recently, the standard response to identified problems of rural access in developing countries was to provide gravel roads. The attractions looked convincing; low initial road cost, all-weather passage and technology so simple that even communities could be organised to build the roads themselves. Gravel became the universal solution. So what went wrong?

There are still many situations where gravel is still an appropriate and affordable rural road surface option. However, in recent years research in Africa and South East Asia has identified some limits to the sensible use of gravel and also highlighted a range of alternatives that can provide the desired improved access, but in a more sustainable way and probably cheaper in whole-life-cost terms; each option's suitability depending on the local circumstances and environment.

The old 'rule of thumb' used to be "Up to 50 (motor) vehicles per day (vpd) then use earth surface, 50 - 200 vehicles per day use gravel, and above that seal.". Unfortunately such a simplistic approach can be very misleading and even downright wrong.

At the very basic level, some soils are just not capable of bearing any sort of traffic, especially when wet. Furthermore as natural materials can be found that graduate from very weak clays and silts right through to solid rock, their corresponding strength and performance characteristics mean that some natural soils are very easily able to carry quite substantial volumes of traffic in their natural state if adequately shaped and drained. So that demolishes the 50 vpd benchmark!

Research in the SADC region has also shown that sealing can be justified at motor vehicle flows of as low as 70vpd or less (see below). Furthermore, the range of surface options at the disposal of the engineer or community makes bitumen 'sealing' only one of the techniques to be considered.

Anticipated traffic generated benefits are also not the only possible justification for improving access to poor rural communities.

The first weapon in the engineer's armoury that is often neglected is the 'Engineered Natural Surface (ENS)' or earth road. This uses the in-situ natural material of the road, shaped up to form a camber and drainage provided to ensure that rainwater flows off and away from the road. In the early 1990s a pilot project in Kenya - Roads 2000 - showed that many sections of rural road could provide satisfactory access if the earth was shaped up, side drainage, and cross drains were provided (at the rate of about 1 culvert every km) and difficult sections such as poor soils or steep gradients were provided with an improved surface. Roads 2000 has now developed into a national programme of network improvement and maintenance.

The recent document Behaviour of Engineered Natural Surfaced Roads discusses the considerable potential for the use of this widely under-rated technique. We hope to return to this subject soon.

Concerns about the performance of gravel roads led to the preliminary work on surface options under a DFID-funded Knowledge and Research project and the publication of the Low Cost Surfacing Working Paper No 1. This set out the rationale for restricting the application of gravel as a surfacing material and investigation and promotion of surface options.

A paper by Johnston and Salter in 2001 highlighted the sustainability problem regarding continued investment in gravel road networks without the necessary maintenance capability being in place to preserve the considerable investment.

In Southern Africa the SADC Low-Volume Sealed Roads Guideline  has made a very strong case for the sealing of roads at quite low traffic volumes. The paper by Hongve and Paige-Green describes the experience with Labour Orientated use of the guideline and also provides indications of the expected lives of various surface options.

SEACAP 4 in Vietnam studied over 700 road sites where gravel surfacing had been used in current and previous rural road projects throughout the country. The study led to recommendations on the restriction of the use of gravel, particularly for locations with adverse factors of material haul distance, traffic, rainfall or flooding, gradient, material quality and maintenance capability.

SEACAP 1 has been a major complementary programme of research and trials into alternative rural road surfaces in Vietnam. It drew on local experiences and earlier surfacing trials in Cambodia to develop recommendations on a range of surfacing and paving techniques, many of which are suitable for construction by local contractors and communities themselves, thus creating new employment opportunities as well as the economic benefits of improved access.

A brief overview of the SEACAP 1 and 4Vietnam work is available.

Rural road surfacing trials programmes have now extended to Lao PDR (SEACAP 17), Afghanistan, Sri Lanka and other countries.

Other important initiatives or documentation on the rural road surfacing topic include:-

We are compiling information on relevant projects under our website gallery of Project Profiles.

Research, knowledge compilation and dissemination efforts continue and we hope to be able to identify and add to this listing of useful documents in the coming months with your assistance.

Contributions

If you would like to make a contribution to this topic or help to 'sign-post' any key documents on the topic for gTKP partners and users, please contact Rob Petts: rob.petts@gtkp.com

Updated March 2010