Evaluating failure regime of an active landslide using instability and rockfall simulation, NW Himalaya

The NW Himalaya has been subjected to frequent disastrous landslides of different types owing to frequent extreme rainfall events and rock mass shearing caused by structural and/or lithological contrast. Though majority of the landslides in the NW Himalaya are of complex type comprising debris and loose rock mass that may result into debris flow and/or rockfall, their potential behavior is rarely explored. The present study aims to evaluate the recurrence of one such complex landslide (0.23 Mm2) of Yamuna Valley, NW Himalaya that is subjected to rock mass shearing and the region accommodating this landslide receives frequent extreme rainfall events. A huge slope failure in this landslide occurred on 12 September, 2017 damaging a 400 m stretches of the National Highway (NH) road. The landslide location has strategic significance, since up to 0.3–0.4 million pilgrims travel annually on the road passing through the landslide slope. To evaluate the potential behavior of landslide and to understand the factors causing this landslide (pre-failure analysis), slope stability analysis and rockfall simulation were performed. Pre-failure analyses indicated that the maximum shear strain of 0.14–0.18 and total displacement of 2–8 m likely developed parallel to the slope. The possibility of rainfall triggering is explored in view of increasing rainfall, soil moisture, and surface runoff conditions. Tectonic influences are also evaluated using joints and fracture patterns in rock mass. Post-failure analysis showed that though the maximum shear strain and the total displacement had reduced to 0.07–0.15 and 2–5 m, respectively after the failure, the slope is still unstable. Rockfall simulation revealed the potential for rockfalls having energy and velocities in the range of 900–4000 kJ and 18–75 m/s, respectively.