Site-specific geological and geotechnical studies on the Lanta Khola landslide, North Sikkim Highway, India

Abstract

In the Indian state of Sikkim, the North Sikkim Highway (NSH) has a sector that is exceptionally susceptible to landslides. The road transects a major Himalayan discontinuity called the Main Central Thrust, which is a tectonic contact between mica schists and quartzofeldspathic gneisses. Site-specific studies on the debris slide at Lanta Khola, located on the contact zone, show that weathering of the contact zone produces debris with a higher silt fraction than in the rest of the slide. This zone (called the soggy zone) is water saturated, but is mineralogically identical to the debris in other parts of the slide. Direct shear tests show that the soggy zone debris has lower shear strength; additionally, the increased silt fraction reduces the permeability and is responsible for its higher water content. Increase in pore pressure in this zone following periods of persistent rainfall makes it increasingly susceptible to failure. This zone is parallel to the geological foliation and daylights' along suitably oriented slopes at Lanta Khola. Material is extruded to the surface from within this zone, and subsequently moves downslope. Mass removal is responsible for widespread sinking of the NSH road stretch along which the gneiss-schist contact is exposed.