STABILITY ASSESSMENT OF KOTROPI LANDSLIDE USING NUMERICAL MODELING AND ESTABLISHMENT OF REGIONAL RAINFALL THRESHOLD, HIMACHAL PRADESH, INDIA

Abstract

Landslides have affected at least 15% of the country's land area, which is more than 0.49 million km2. Various types of landslides occur in both geodynamically active and stable parts of the Indian subcontinent, such as the Indian Himalayas (North-Eastern and North-Western) and the Western Ghats, Nilgiri Hills. From various research works carried out by many researchers, it is evident that Himalayan landslides put up the biggest threat and loss to the Indian government, especially the landslides in North-Western region of Himalayas. So, the landslides occurring in the North-Western Himalayan states, especially in Himachal Pradesh and Uttarakhand, need to be well researched. However, landslides in Uttarakhand have been studied more often than the ones in Himachal Pradesh. Hence, it’s time that researchers should focus on detailed study of landslides in Himachal Pradesh as the state carries so much importance in the field of tourism than that of any other Indian states and such frequent sliding activities hamper the development work in the region. Therefore, in-depth study of the landslides in this state is of paramount importance and hence should be taken forward by the researchers. There are enough instances of some major landslide occurrences within the state and Kotropi being the most recent one. Kotropi landslide is a complex deep-seated debris slide having considerably large dimension and took more than 46 human lives, which is a testament of the gravity of this particular incident. Thus, this particular slide acts as a suitable test site to study the stability and mechanism of such debris slides in mountainous regions like Himachal Pradesh. The current work describes the recent devastating landslide of Kotropi, which had claimed many human lives and properties along its course on 13th August 2017. This event was the third reactivation of the slide as per data available. The study for this landslide involves a preliminary investigation of the site and comparison of pre- and post-event slope stability analysis. Other than rainfall, the combined effect of differential weathering rate of the debris and soil layers, low material strength, and the presence of a fault zone was found to be the primary cause of instability. The analysis was carried out with the geotechnical investigation and numerical modeling with the help of finite element model-based Phase II simulator. The significant findings of this paper include numerical simulation of pre- and post-event slide as stated under unsaturated and saturated condition. The study finds a low factor of safety of the present slope. The unstable condition of the slope may trigger another event of the same intensity shortly. 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 field visit in April 2022. This study provides useful insights for examining the failure mechanism of deep-seated landslides in the Himalayan terrain.