TRL – Overseas Road Note 31 : A guide fo the structural design of bitumen-surfaced roads in tropical and sub-tropical countries

Road Note 31 was first published in 1962 and revised in 1966 and 1977 to take account of advances in our understanding of the behaviour of road-budding materials and their interaction in composite pavements. Many of these advances have been made by engineers and scientists working in temperate climates but a considerable amount of both fundamental and applied research has been necessary to adapt and develop the knowledge so that it can be used with confidence in tropical and subtropical regions where conditions are often very different. In addition to differences associated with climate and types of materials, problems also arise in some countries from uncontrolled vehicle loading and unreliable road maintenance. At the same time, the level of technology available for construction and maintenance can be relatively low. All this has presented a unique challenge to the highway engineer. This edition of the Road Note has drawn on the experience of TRL and collaborating organisations in over 30 tropical and sub-tropical countries. Most of this experience has been gained in carrying out a research and development programme on behalf of the Overseas Development Administration, with additional projects for other aid agencies. The new edition extends the designs of previous editions to cater for traffic up to 30 million equivalent standard axles and takes account of the variability in material properties and construction control, the uncertainty in traffic forecasts, the effects of climate and high axle loads and the overall statistical variability in road performance. The range of structures has been expanded and the chapters on the different types of materials have been enlarged to provide more detailed advice on specifications and techniques. Nevertheless, there will be situations and conditions which are not covered in the Note and there will be many examples where local knowledge can be used to refine and improve the recommendations. Indeed, the role of local engineering knowledge and the judgement of experienced engineers should never be underestimated and should always form an important part of the design process. The Note has been produced in response to a continuous demand from engineers worldwide and I am confident that the new edition will prove to be as popular with practitioners as its predecessors