International Research Journal of Earth Sciences______________________________________ ISSN 2321–2527 Vol. 3(8), 1-7, August (2015) Int. Res. J. Earth Sci. International Science Congress Association 1 Assessing Cohesion, Friction Angle and Slope Instability in the Shivkhola Watershed of Darjiling Himalaya Mandal Sujit Department of Geography, University of Gour Banga, Malda-732103, West Bengal, INDIA Available online at: www.isca.in, www.isca.me Received 14 th June 2015, revised 21 st July 2015, accepted 6 th August 2015 Abstract The study of soil strength properties is of very much important to assess the slope instability in the mountain area. Himalayan mountain range characterized by most fragile lithological composition. This fragile lithology is the outcome of heavy compressive forces resulting from the convergence of two solid slabs i.e. Indian Plate and Eurasian Plate. Hence, the upper part of the slope materials up to the weathered limit or weathered front is of low shear strength and very much prone to failure. Cohesion (c) and friction angle (φ) are the two significant parameters of soil and on the basis of these properties the stability and instability of the slope segment can be assessed. The present study area of the Shivkhola watershed of Darjiling Himalaya is dominated by slope instability. To indentify the potential landslide locations, a landslide inventory map was prepared in consultation with Topo-sheet, Google Earth Image, Satellite image (LISS III 2010) and intensive field investigation with GPS. The soil samples were collected from 50 locations considering 0.25 sq. km surface and tested in the laboratory to estimate cohesion and stress parameters. Based on cohesion and major principal stress and minor principal stress a Mohr Stress Circle was developed to determine friction angle. Finally the spatial distribution of cohesion and friction angle and their integration with landslide distribution was accomplished on GIS platform incorporating pixels affected by landslide and pixels not-affected by landslide in each class of the prepared data layers. Keywords: Cohesion, friction angle, landslide potentiality, shivkhola watershed. Introduction Each and every spatial segment of the earth surface possesses some physiographic aspects and the analysis of all the aspects enables us to predict an interrelationship between physical and cultural phenomena and as a whole. The study area, Shivkhola Watershed comprises a number of diversified physical aspects and there is a great diversity of forms and the complexity of interrelationships. The practical relevance of landslide can be recognized only by the systematic and thorough study of geomorphic attributes such as relief, geology, and soil. A detailed and integrated investigation of the geological structure of the area, the petrographical and physical properties of the rocks and the local hydro-geological conditions with changing slope of the Shivkhola watershed will help to prepare the corrective and preventive measures in a reasonable scheme. Barton and Choubey 1 studied the shear strength of rock joints and its impact on slope stability. The existence of finer to large size soil-rock composition has aggravated the problem of soil erosion and soil slip in the Shivkhola Watershed. Besides the size of the soil particles, the mineralogical composition of the soil changes all the physical and chemical properties within the soil. Keen and Raczkowski 2 propounded the relation between the clay content and certain physical properties of a soil. The amount of sand, silt and clay; porosity, water holding capacity and bulk density; cohesion; and saturated depth of the soil are some of the significant properties which continuously changing the actual nature of the soil-rock properties of the hill slope causing slope failure. The study of various soil strength parameters such as particles size distribution, cohesion and friction angle plays a significant role in slope instability. Landslide potentiality was estimated incorporating landslide inventory map (2) for all the geomorphic attributes by determining class/ranges wise Landslide Potentiality Index Value (LPIV) of each factor by means of a ratio between the number of cells/pixels disturbed by landslides and the total number of cells/pixels for that specific class. More details of these procedures were obtained in other studies 3,4 . Topographic Index (TI) Value was calculated in consultation with slope and upslope contributing area. The effectiveness of all these parameters were being influenced by hydrologic conditions and other atmospheric processes. Anderson and Burt 5 presented the role of topography in controlling through flow generation and related landslips. GIS tools were applied for the identification of topographic settings conducive to landslide occurrences 6 . Various geomorphic and hydrologic models were being introduced for understanding slope instability 7,8,9,10 . LPIV = (F2 ÷ F1) × 100 (1) Where, F1 = number of pixels/cells or grid without landslide. F2 = number of pixels/cells or grid with landslide.