RISK EVALUATION-A CASE STUDY, CHALNAKHEL LANDSLIDE, KATHMANDU VALLEY, CENTRAL NEPAL

The Chalnakhel landslide is located 9.5 km southwest of Kathmandu valley, central Nepal. This landslide poses a serious collapse threat to 0.07 km² of agricultural land and a road section that connects Pharping to Kathmandu. The main objectives of the study are to 1) assess the stability of the slope, 2) define possible hazards if the whole landmass collapsed, 3) recommend the appropriate preventive measures, and 4) evaluate future risk in the area. The present study outlines an overview of the geotechnical and geophysical methods that were used in the investigation of the slope failure. The integrated approach applied in this investigation was designed to evaluate the present situation of the landslide and to determine the nature and geotechnical parameters of the sediment at the site. In particular, the nature of subsurface conditions and field methods appropriate for preventive and mitigative methods were investigated. The slope instability (rotational soil creep) at this site is a combined effect resulting from unconsolidated residual-colluvial quaternary deposits that overlie more impermeable low grade metamorphic (calcareous) rocks, occurrence of sporadic subsurface silty-clayey soil, and presence of smectite minerals at or near the slip surface. Perceptible movement occurs typically after intense rainfall. A factor of safety (FOS) was calculated for three profiles within the landslide based on geotechnical parameters. FOS calculation for natural moisture content during dry season predicted stable slope in all profiles. In contrast, when the slope is fully saturated following extended periods of rainfall FOS in all profiles predicted unstable slope. To date, the landslide has destroyed one house; an additional three houses and the road were partially damaged. The ongoing landslide stabilization and mitigation activity includes construction of a shallow drainage system. Based on the results of this study deep drainage wells and lime-sand piles would further stabilize the landslide.