SHEAR BEHAVIOR AND DEFORMATION MECHANISMS OF REACTIVATED ANCIENT LANDSLIDES IN THE NEPAL HIMALAYA

The reactivation of old landslides has emerged as a major problem in the Nepal Himalaya in recent years. A significant number of individuals have resided atop the historical accumulation of landslide debris for a long time and are currently confronted with the threat of reactivation. This study examines the shear behavior and deformation mechanism of fourteen reactivated landslides in the Sindhupalchwok District of Nepal. Between 2019 and 2022, fourteen distinct ancient landslide deposits were reactivated in the study region during the monsoon period. The remolded soil samples from all fourteen reactivated landslides were obtained from the field. The multistage direct shear test of unsaturated soil was conducted by enabling separate regulation of metric suction. The curve depicting the relationship between shear stress and horizontal displacement was derived for each soil sample. The shear strength parameters of all soil samples were evaluated.

The soil samples exhibit cohesion value that ranges from 5 to 10, whereas the angle of internal friction varies between 21 and 27. The soil exhibited a plasticity index, ranging from 3 to 7. Out of all the samples, eleven are loose and non-compact, while three samples are in a dense state. The findings of the gradation study indicate that the soils are characterized as clayey sands (SC) with a relatively low plasticity index and a low cohesion. The average rainfall amount in the monsoon period from 2015 to 2023 ranged from 2300 mm to 2800 mm. The rainfall amount in the early monsoon period increased dramatically from 2019. The unconsolidated soil with low cohesion and low plasticity index deformed due to the 7.8 magnitude Gorkha earthquake and subsequent aftershocks. Furthermore, catastrophic rainfall during the pre-monsoon long-term dry season, causing the surface deformation in the study area.

Key words: Landslide reactivation, cohesion, soil deformation, Nepal Himalaya