Mechanism of activation of the Lanta Khola landslide in Sikkim Himalayas

The Lanta Khola is a major landslide on the North Sikkim Highway in the Indian state of Sikkim. The abnormally low width-to-length ratio and slope instability in spite of the gentle surface slope (24°) make this slide unique. Geological, geophysical, and geotechnical studies reveal that a major Himalayan discontinuity daylights within the slide. At the contact, the schist is weathered to fine sand and silt with lower shear strength and permeability. The overlying gneiss is less weathered and exposed at the contact. Surface runoff enters the contact zone through crevices in the overlying gneiss, and debris material is extruded laterally from within this zone rendering instability, with blocks collapsing and eventually rolling down the slope after cloud bursts. Numerical modeling of the slide confirms this mechanism of instability. Diversion of runoff, plugging of crevices, and construction of pipe piles and horizontal drains are suggested as remedial measures.