Publication

Abstract : In the Indian state of Sikkim, the North Sikkim Highway (NSH) has a sector that is prone to landslides. The road transects the geological contact between mica schists of the Lesser Himalayan Sequence (LHS) and quartzofeldspathic gneisses of the Higher Himalayan Crystalline Sequence (HHCS), which is a major Himalayan discontinuity called the Main Central Thrust. The MCT zone is a ductile shear zone characterized by a consistently oriented schistosity and grain-size reduction of the sheared rocks. Site-specific studies at the Lanta Khola slide, located on the contact zone, show that weathering of the contact zone leads to debris production with an increased silt fraction. 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, but the increased silt fraction reduces the permeability and is responsible for its higher water content. The resulting increase in pore pressure in this zone makes it increasingly susceptible to failure. Since the zone nucleates at the base of the gneisses of the HHCS, the zone parallels the geological foliation and 'daylights' along suitably oriented slopes. Material is extruded laterally from within this zone and subsequently moves downslope. Material removal is responsible for widespread sinking of the NSH road stretch along which the gneiss-schist contact (the MCT zone) is exposed.