Low-cost road construction and rehabilitation in unstable mountain areas

Abstract

Rationale 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 pavement provides a useful overview of stability, and the reaction of slopes and drainage channels to road works in general. These benefits obviously do not accrue in the case of new road construction. The design case studies for a proposed new road in Nepal and the rehabilitation of an existing road in the Philippines are described below to illustrate the value of land surface evaluation techniques in each case.