Effect of groundwater table fluctuation on slope instability: a comprehensive 3D simulation approach for Kotropi landslide, India

In the hilly terrain of the Indian Himalayas, prolonged severe rainfall and subsequent rise of the water table during the Indian monsoon season are the most prevalent prerequisites for the development of deep-seated landslides. The Kotropi landslide in the mountainous region of Himachal Pradesh represents such a suitable site; its location in the North-West (NW) Himalayas and varying depth of groundwater table (GWT) throughout the year along with several other geological factors resulted in the third reactivation of the slide on August 13th of 2017. To properly quantify and demonstrate the effect of GWT fluctuation on slope instability, this research proposes a comprehensive approach. It integrates the 3D model–building process of the entire slide with the simulation process of that model using FLAC 3D software. To determine the geometry of the slide and the depth of the GWT, a total station tacheometric survey and an electrical resistivity tomography (ERT) study were conducted respectively. When the model was simulated at different GWT depths of 15 m, 10 m, 5 m, and surface, the factor of safety (FoS) dropped from 1.21 to 0.86, indicating slope instability as GWT rises. The findings highlight the importance of groundwater fluctuation modeling in slope instability studies of deep-seated landslides. The simulated models show impending failure in the right flank, which was validated during a recent field visit in April 2022. This study provides useful insights for examining the failure mechanism of deep-seated landslides in the Himalayan terrain.