EFFECTS OF GRADATION AND INDEX PROPERTIES ON THE SHEAR STRENGTH BEHAVIORS OF LANDSLIDE SOILS FROM THE SIWALIK HILLS OF NEPAL

This paper presents the relationship between gradation, geological units and shear strength behaviors of landslide soils from the Siwalik Hills of Nepal. Polygon based landslide inventory map and geological map of the study are were prepared. After layering the landslide inventory map on the geological map in the GIS layer, the landslide sampling locations were chosen. Thirty-four landslides with an area greater than 10,000 m² were chosen from each geological units in such a way that the distribution of sampling points covers the whole research region to compare the test findings. Altogether 124 representative landslides from the entire study area were selected to collect the soil samples for laboratory analysis. The box shear test was conducted under the normal stresses of 20, 40 and 60 kPa to explore the soil shear strength characteristics and associated parameters. The sieve size ranges from 12.5 mm to 0.075 mm was taken for sieve analysis. The uniformity coefficient of each soil was obtained.

The study reveals that the landslide soils of Middle Siwalik are substantially compacted, allowing for the determination of residual shear strength and well graded soils are dominant. Similarly, the shear strength values in the geological units differ greatly. The mean shear strength value in Middle Siwalik is significantly greater compared to Lower and Upper Siwalik. The soils of Lower and Upper Siwalik are loose and poorly graded and mean shear strength value is less. Shear strength rises as cohesion and internal friction rise, but falls when the void ratio and plasticity index rise. There is significant positive correlation between uniformity coefficient and shear strength value. The result concluded that the geology, soil gradation and shear strength are positively associated to each other.

Keywords: Landslides, soil gradation, shear strength, Siwalik, index properties