GEOPHYSICAL CHARACTERIZATION OF SOME ACTIVE LANDSLIDES OF GARHWAL HIMALAYA, INDIA

Abstract

Traditional methods of landslide hazard analysis in Garhwal Himalayan region are based on geomorphic observations and sparse subsurface data obtained from boreholes or excavations. They are neither informative in understanding the causes of landslides nor useful for undertaking suitable remedial measures. Further, these methods can not cope up with active land slide challenges. Here, an integrated geophysical methodology involving ERT, IPI and gravity is proposed for debris type active landslide characterization in terms of causes, mechanism and dynamics. The use of gravity along with ERT and IPI is attempted in a novel way for characterizing three active landslide sites of Garhwal Himalaya. They are respectively, Naitwar Bazar landslide, Salna village sinking zone and Narayan Bagar landslide. Electrical resistivity tomography and induced polarization imaging methods were helpful in ascertaining the depth of slip surface of Naitwar Bazar landslide. They could locate saturated sandy zones or locked ground water pockets within the sliding mass and predict slopes of future failure. The utilization of gravity method has inferred the depth of hard rock surface and facilitated a better understanding of the possible causes of mass movement. The deployment of gravity method has inferred the presence of several neotectonically activated faults in Salna village sinking zone site, which are found to be parallel to the major lineaments of the region. The crisscross network of these inferred faults has divided the entire region into several blocks. Application of electrical resistivity tomography supported by gravity method has revealed these inferred faults to be vertical to sub-vertical in nature. Integrated approach has clearly explained the reason for sinking of the site, which is currently taking place along these vertical to sub-vertical faults. in For the landslide at Narayan Bagar, gravity study has revealed the presence of faults in the subsurface, which may not be tectonically active now. However, these faults are found to be parallel to thrusts close to the landslide site. These faults seem to have severely crushed the subsurface rock formations. These crushed zones in turn could have acted as conduit for draining water from the upper reaches of the slope. The electrical resistivity tomography supported by induced polarization imaging reveals the presence of low resistive clayey and sandy clay horizons at a greater depth within the fault zone. An attempt also made to evaluate the stability of slopes through geotechnical methods for all the three sites as per norms. The results of geotechnical investigations corroborate well with geophysical findings. Three models of failure mechanism are inferred from the geophysical investigations. The first model is based on a simple sliding mechanism, where incompetent layer slides over competent formation due to gravity. The second model that is based on a fault mechanism explains land subsidence due to neo-tectonically activated faults. The third model which is also based on fault mechanism, explains the role of subsurface drainage in the mass wasting processes of Garhwal Himalaya. The present study reveals the importance of integrated geophysical approaches for active landslide characterization. The strength of chosen combination of electrical resistivity tomography, induced polarization imaging and gravity method illustrated in characterizing three active landslides of Garhwal Himalaya. The same may be deployed for landslide characterization of debris type slides in other similar mountainous regions of the world.