Landslides are very common in high altitude regions of the Himalayas. The Chandmari landslide is a recurrent landslide located in the Eastern part of Gangtok, Sikkim—a Northeastern state in India. Intense rainfall in the monsoon is the main triggering factor for landslides in this region. This paper enumerates the contributing parameter for rainfall induced landslides, viz., bedrock hydrogeology and geotechnical properties of soil and studies their impact on the same. A geophysical resistivity survey was carried out to identify the subsurface structure and to get an insight of the water saturation profile below the landslide surface. The result of the resistivity study showed high to moderate conducting zone in the lower elevations of the landslide site. This was contrary to what we measured at the higher elevations, which showed higher resistive zones. This anomaly was profound and revealed the existence of water saturated zones below the surface of landslide at these lower elevations. In addition, resistivity results revealed that the weak water saturated debris layer was parallel to the geological contact that favored movement. Results of geotechnical investigation for soil showed that an average maximum dry density was 1.8 kg/m3 and average optimum moisture content was 31.84%. The results showed that the value of Coefficient of uniformity (Cu) was 6.4 and the value of Coefficient of curvature (Cc) was 0.894. The angle of internal friction and average cohesion values were calculated as 30.57° and 0.2 kPa respectively. A combined hydrological-slope study was performed using SLOPE/W software to understand the relationship of pore water in variations with the aforementioned soil parameters to determine the probability of a landslide occurrence. Finally, the identified conductive feature of the water saturated debris layer was correlated with the calculated slide mass using a slope stability model.
Dr. Thambidurai P. and Dr. Maneesha V. Ramesh, “Slope Stability Investigation of Chandmari in Sikkim, Northeastern India”, 4th World landslide Forum Ljubljana, Slovenia, May 29 – June 2, 2017, 2 vol. Springer International Publishing, pp. 363-369, 2017.