Characteristic Features of Deep-Seated Landslides in Mid-Nepal Himalayas: Spatial Distribution and Mineralogical Evaluation

Landslides, primarily occurring in deep-seated landslide areas, frequently damage important roads in Nepal and contribute to massive economic losses and public suffering every year. Efforts to understand these landslides, which are often only regarded as geological phenomena, through material shear behaviour however remain insignificantly reported. In order to narrow this research gap, this paper first addresses spatial distribution of these landslides in about 150-km arterial road corridors in central Nepal, and then discusses their occurrence mechanism in terms of a relation between material shear strength and mineralogical composition. The deep-seated landslide inventory prepared out of the aerial photos and topo-sheets revealed that the landslide distribution is dominant in phyllitic and slate zones of geology while the laboratory tests on clay materials, sampled from 15 locations in the study area, in ring shear machine and x-ray diffractometer exhibited that the landslide soils, especially from the phyllitic area, possess comparatively high angle of shear resistance in a range of 20–30 degrees with remarkable influence of mica and chlorite composition, whose increased relative amount was found to result in notable decrease of the angle of shear resistance.