SHEAR BEHAVIOR ANALYSIS OF THE LANDSLIDE SOILS IN THE SIWALIK HILLS OF NEPAL

Siwalik Hill of Nepal is composed of weak, fragile, and highly weathered sedimentary rocks like sandstone, mudstone, shale, and conglomerates. The two major thrusts: Main Boundary Thrust in the North and Main Frontal Thrust in the South is causing the Siwalik Hills highly susceptible to landslides in the Monsoon period. Nepal is facing challenges for mitigating landslides due to complex and heterogeneous geology. To find out the shear behavior of soil from the active thrust zone is of paramount importance for understanding and mitigating landslides. In this study, the shear behavior and associated parameters of the soil from the landslides are investigated and analyzed.

All together 102 soil samples were obtained from the field. The direct shear test and index properties of soil were conducted in the Geotechnical lab by adopting the ASTM standard. The obtained data are analyzed by using descriptive statistics, principal component analysis, and multiple regressing. The principal component has a higher loading on shear strength so that shear strength is taken as a major response variable. The average value of shear strength is 35.26 kPa in the sandstone-dominated area, whereas the value is 22.69 and 29.35 kPa in mudstone/shale and conglomerates-dominated area. The shear strength has the negative relation to plasticity index and void ratio, whereas significant positive relation to the cohesion and angle of internal friction.

The shear strength of soil is higher in the sandstone dominant area could be the compactness and less void ratio of the soil. The number of landslides is also less in the sandstone zone. The number of landslides is higher in the mudstone zone with less value of shear strength depicted that the shear strength and landslide occurrence event is highly significant. The variegated and concretion rich with a high void ratio along with the highly weathered and fragile mudstone could be the cause of frequent landslides in the Siwalik Hills.

The result shows that the shear strength and its associated parameters of soil are mainly controlled by geology in the Siwalik Hills. This study is very useful to understand the Geotechnical nature of landslides in the Siwalik Hills for landslide mitigation and planning.

Keywords: Landslides; laboratory study, shear strength; direct shear test; Siwalik