Geological and geotechnical engineering aspects of low-cost roads in mountainous terrain

Abstract

The paper is concerned with design and construction of mountain roads in areas of high relief and steep mountain slopes, where the principal soil types are transported rather than residual and the landscape is shaped mainly by active erosion processes. In order to simplify the wide range of ground conditions encountered, the geology and geomorphology of typical mountain terrain are described and related to a five zone model. Geotechnical features within each zone are then considered so that the application of various investigation and construction techniques can be appreciated. The paper is primarily concerned with low-cost but well-trafficked primary roads where closure time due to slope failures or lack of maintenance are intended to be minimal. Village feeder roads and steep mountain haul roads, for example those used in forestry, are not the prime concern of the paper. Three aspects which distinguish mountain roads from typical lowland roads are the difficult ground access with numerous slope instability problems, the greater use of very local construction materials and the greater need for off-site drainage and erosion protection measures. In remote rugged terrain access for wheeled traffic may not exist. This severely curtails the use of conventional site investigation techniques, such as drilling, and means that greater emphasis is placed on remote sensing and on the surface assessment of ground conditions. Several relatively new low-cost techniques applicable to the investigation of mountain regions are described. A rational approach to alignment selection is developed which stresses the importance of stable ascent corridors, and practical guidelines are offered on the selection of the road cross-section and on the variety of techniques available to stabilise the cross-section chosen. The use of local construction materials is dictated largely on economic grounds and may place heavy constraints on construction standards and structural design. The use of gabion structures is a major construction technique examined in the paper. Drainage and erosion protection methods are important in areas of high relief and erosion potential and a range of design approaches are described. The bulk of stabilisation and control works described can only be attempted by labour