Effect of geological structures, rock weathering, and clay mineralogy in the formation of various landslides along Mugling-Narayanghat road section, Central Nepal Himalaya

The present study was conducted on the landslide prone area around Mugling-Narayanghat road section that consists of Lesser Himalayan and Siwaliks rocks. From more than 250 mapped landslides, ten were selected for detailed study that are supposed to the representative of the whole area. Detailed study showed that large and complex landslides are related to deep rock weathering followed by the intervention of geological structures as faults, joints, and fractures. Large landslides formed by gravitational deformation are related to the rock structures, while rock weathering plays a minor role. Rotational types of landslides are observed in weathered rocks, where the dip direction of the foliation plane plays a principle role. Shallow landslides are common in slopes covered by residual soil or colluviums. Some shallow landslides (rock topples) occur in less weathered rocks where the attitude of the foliation plays a major role, while others (rock plane failure) occur in cut slopes with less weathered rocks. Debris slides/flows occur in colluviums or residual soil covered slopes. In few instances, rock fall may occur on the upper slope, which then is mixed with the colluviums, residual soil, and other materials lying downhill and come down as debris flow. Rock falls are mainly related to the joint pattern and the slope angle and are found in less weathered rocks