Limit Equilibrium Method based Slope Stability analysis in the vicinity of Kotrupi area, Mandi

Landslides are prevalent in the Himalayan region that consists of a complex geological setting and presence of many thrust zones, lineaments, and faults. The areas in the vicinity of thrusts experiences many landslides and their slopes are very unstable. One such area where sliding is dominant is the Mandi-Joginder Nagar area in Himachal Pradesh which lies on the contact zone between Main Boundary Thrust (MBT) and Main Central Thrust (MCT). Though the geological formation remains the same, the topography varies rapidly, and hence the instability of slopes increases. However, slope stability analysis of different profiles in this section is not studied in detail. Hence, the present study uses Limit Equilibrium Method (LEM) to estimate the Factor of Safety (FoS) along different profiles in the Kotrupi area where a massive landslide happened on 13th August 2017 killing 46 persons.

In this work, 8 different profiles (Figure 1) are drawn and their elevation was extracted from TanDEM-X Digital Elevation Model (12 m resolution) showing convex slopes. Based on the earlier work, the geotechnical parameters were given to the slope geometry and the FoS was calculated using LEM methods like Bishop, Janbu, Spencer, and GLE (General Limit Equilibrium). The results show that the different LEM methods give almost identical FoS for circular slip surfaces. It is seen that the value determined by the Janbu method is the lowest among all 4 methods (Figure 1). It is observed that the steeper slope geometry has an FoS value lower than 1, as seen for profiles 1, 2, 4, 5, and 6 indicating a certain portion of the profile is unstable. Among these, profile 5 shows the lowest FoS, which is just next to the Kotrupi landslide area. The main reason for this instability is attributed to the weaker lithology and close vicinity to MBT and road widening. We observed that a certain portion of the slopes has very critical FoS values which need due attention. Similar work may be carried out in other areas to identify the zones where slopes are unstable. These areas should be analyzed for applying proper stability measures to reduce the risk of slope failure.