Investigation of hill slope stability and mitigation measures in Sikkim Himalaya

Abstract

Survey of India toposheets, cloud free satellite data and field survey with the help of handheld GPS were carried out along major road of Sikkim for landslide investigation. The geological and geotechnical analysis  carried out for the major trouble shooting spots in the eastern Himalayas along National Highway (NH-31A), North Sikkim Highway (NSH) and Singtam-Dikchu road, Gangtok-Nathula Highway, and a perennial trouble spot at Bojecck  on Jorthang-Hilly state highway  undertaken to understand the factors affecting triggering of  landslides in the respective areas. The study identified lithology or rock type, structure of the bedrocks (i.e. orientation of the rock foliations and faults), excessive rainfall and human interference like diversion of streams as the main factors triggering the landslides in the region. The areas with weak rocks which are susceptible to weathering, i.e. schist, mudstone, and slate and the areas, where rock and and/or fault foliations are favorable with respect to the road cuts, are found to be vulnerable to landslide. During continuous heavy rainfall, the weathering process is accelerated reducing schists and mudstones into silts and fine sands, and subsequent washout fine materials triggers landslides. Rainfall threshold study carried out for Bakthang slide using rainfall data for the year 2010 and minimum threshold for landslide initiation was 35mm/day. In the Bojeck slide the suggested slope mitigation measures include diversion of run-offs, plugging of cracks and crevices, construction of filters and horizontal drains at different elevations along the slopes to prevent washout of fines and safe passage of the water, rock bolting, avoiding human interference and judicious alignment of the roadways.