Combined Gravity and Electrical Imaging in Landslide Investigations at Narayan Bagar, Garhwal, Himalaya, India

In N-W Himalaya, landslides lead to frequent disruption in communication network, which requires a lot of resources for the maintenance and up keep. One of such important transportation networks is the motor road between Karnaprayag and Gwaldam of Chamoli district in the central part of the state connecting Garhwal and Kumaun physiographic division of Uttarakhand Himalaya, India. Among many, the landslide affecting this road is located at Narayan Bagar (30º08΄27˝ N & 79º22΄27˝ E) on the left bank of Pinder river. The upper reaches of the slope is covered with cultivated land of terrace farming. Here, periodically during every rainy season, the soil mass moves down leading to repeated failure of controlling measures. This study area forms a part of Garhwal lesser Himalaya comprised of medium to low grade metamorphic rocks, which occur as Nappe / Klippen over metamorphosed to un-metamorphosed sedimentary sequences. The geophysical investigations (three Electrical Resistivity Tomography (ERT), Induced Polarization Imaging (IPI) and gravity profiles) at the landslide site were carried out during July, 2007.The three ERT and IPI profiles, overlain on the map of gravity inferred faults parallel to the existing regional thrusts in the vicinity of the study region, and their lateral extensions helped us to infer the sources of failure mechanism. The crushed zone in the subsurface, as a consequence of a fault zone, serves as a conduit for water to flow from the upper reaches of the slope, leading to periodic instability during the rainy season.