Rock slope instability assessment using spatially distributed structural orientation data in Darjeeling Himalaya (India)

We discuss a geographic information system (GIS)-based methodology for rock slope instability assessment based on geometrical relationships between topographic slopes and structural discontinuities in rocks. The methodology involves (a) regionalization of point observations of orientations (azimuth and dip) of structural discontinuities in rocks in order to generate a digital structural model (DStM), (b) testing the kinematical possibility of specifi c modes of rock slope failures by integrating DStMs and digital elevation model (DEM)-derived slope and aspect data and (c) computation of stability scenarios with respect to identified rock slope failure modes. We tested the methodology in an area of 90 km² in Darjeeling Himalaya (India) and in a small portion (9 km²) within this area with higher density of field structural orientation data. The results of the study show better classification of rock slope instability in the smaller area with respect to known occurrences of deep-seated rockslides than with respect to shallow translational rockslides, implying that structural control is more important for deep-seated rockslides than for shallow translational rockslides. Results of scenario-based analysis show that, in rock slopes classified to be unstable, stress-induced rock slope instability tends to increase with increasing level of water saturation. The study demonstrates the usefulness of spatially distributed data of orientations of structural discontinuities in rocks for medium-to small-scale classification of rock slope instability in mountainous terrains.