SEASONAL VARIATION IN THE SHEAR STRENGTH BEHAVIORS OF LANDSLIDE SOIL FROM THE SIWALIK HILLS OF NEPAL

The slope instability is widespread in the Himalayas during the Monsoon season. The slope's soil has various Geotechnical features that cause a landslide to occur. In this study, the shear strength characteristics of slope soil from 52 landslides in Nepal's Siwalik Hills were assessed. Soil samples were taken during two seasons: the wet season (December-February) and the dry season (March-May) (January- March). The shear strength was obtained through the direct shear test under ASTM standard. The stress/ strain curve was obtained from each sample and the shear strength parameters were also obtained.

In the rainy season, shear strength values range from 16 to 27 kPa, while in the dry season, it ranges from 22 to 38 kPa. Between the two seasons, there is a large variation in shear strength. In the rainy season, the angle of internal friction of the soils ranges from 7^o to 14^o, whereas in the dry season, it ranges from 17^o to 33^o. The strain/stress curves of soils show considerable peaks in the dry season, while no peaks are visible in the wet season. The result demonstrates that during the wet season, the shear strength and shear strength parameters of slope soil decreased and became minimum. The decline in shear strength behaviors eventually results the landslide in the Siwalik Hills of Nepal.

Key words: seasonal variation, shear strength, Siwalik, Nepal