Engineering Geology and Stability of the Laprak Landslide, Gorkha District, Western Nepal

Abstract

The Laprak landslide is a complex consisting of translational and rotational landslides, debris flows, and rockslides triggered by exceptionally heavy rainfall in July 1999, with continuing movement during subsequent monsoonal wet seasons. The active portion of the complex is a structurally constrained wedge of colluvium with a triangular or trapezoidal cross section, the geometry of which is controlled by two or three sets of foliation and joint planes, derived from phyllitic and quartzose metamorphic bedrock of the High Himalayan Crystalline Sequence. Stability analyses, constrained by field observations, limited geotechnical testing, and finite element simulation of steady-state seepage, show that the landslide most likely moves as three sections, with the middle section being more stable than the upper and lower sections. Non-slope-parallel seepage simulated by the finite element model appears to locally decrease instability by allowing drainage into the discontinuous bedrock. The stability analyses also suggest that the landslide will continue to move during the wet season when pore-water pressures are moderately high (although complete saturation of the slope is not required). Moderate earthquakes would exacerbate wet season instability but do not appear sufficient to trigger movement during the dry season, when pore-water pressures are low. The remote location of Laprak, which is a two- to three-day walk from the nearest road-head, and lack of money limit remedial options to those that can be undertaken using manual labor and native materials, for example, replacement of dry stone masonry buildings with lightweight wooden structures and surface drainage improvement.