Abstract When designing the construction or rehabilitation of roads in unstable mountain regions subjected to floods, landslides, erosion and earthworks failures, it has become usual practice to employ a selection of the techniques described in Section 2. The need for adequate land surface evaluation in this context is obvious: (i) the information so produced is essential to many if not most design processes; and (ii) proper evaluation of topography, materials and geohazard is central to the performance of the road construction, and its maintenance in the longer term. These concepts are embodied in Fookes et al . (1985) and in Overseas Road Note 16 ( Transport Research Laboratory 1997 ) which deals specifically with geohazards and road design in unstable mountain areas. The selection of the techniques, and the manner in which they are applied, will depend on project area conditions, the availability of the necessary data or documents, and the nature of the engineering scheme being proposed. The techniques are usually applied in a progressive manner, with the need to refine and detail ground conditions more closely as the site selection and design procedure takes place. The conclusion to this process is the usual inevitability that final design will not be established until ground conditions are fully defined during construction. In the case of road rehabilitation projects, existing cut slope exposures provide a more or less continuous record of soil and rock conditions above the road, while the stability of slopes and the observed distress to the existing road

Abstract Soils in hilly or mountainous areas are normally divided into two types: in situ weathered soils and transported soils. Table A3.1 provides a simplified classification and description of the common soils encountered in the humid tropics and subtropics and their engineering behaviour, based mainly on Fookes (1997) .

Abstract Retaining structures are a common feature of road construction in hilly and mountainous areas and can account for up to 20% of the total construction cost. Retaining structures comprise: gravity walls, where the weight of the wall and its backfill provide most of the stabilizing force (masonry, gabion and reinforced concrete cantilever walls are typical examples); embedded walls, where the soil in front of and behind the structure and anchors (if any) provide the stabilizing force (sheet pile or bored pile walls are typical examples); reinforced soil, where the in situ soil mass is reinforced with nails or dowels (usually behind a protective face); reinforced fill, where steel or geosynthetic geogrids or straps are embedded into the fill during its emplacement. Due to cost considerations on most low-cost roads, retaining walls are usually designed as gravity structures. Consequently, this chapter focuses on gravity walls constructed from masonry, gabion, mass concrete and reinforced concrete. However, consideration is given to the use of soil nails to strengthen cut slopes (Section C5.2.5) and to the use of reinforced fill structures (Section C5.2.6), as these can provide useful alternatives under certain circumstances. Walls are constructed in above-road and below-road locations; see Figure C5.1 for illustrations of these terms.