Stability assessment of a recurring rockslide on Dehradun – Mussoorie highway in the Uttarakhand state of India

The Himalayan geology is complex and fragile in nature. Many landslides are witnessed every year in the region and the occurrence of landslides drastically increases during monsoon. An alarming increase in landslide occurrences has been noticed, which sometimes are attributed to anthropogenic activities. In the present article, stability assessment of a road-cut rock slope site has been discussed. The most recent landslide at the slope site occurred on July 9^th, 2022. The rock slope site is situated on Dehradun – Mussoorie highway, and is 29 km away from Mussoorie and 22 km from Dehradun in Uttarakhand state of India. The location is strategically important and poses risk of landslide hazards that can cause loss of human lives and resources. After the landslide occurred, the scarp of the rock slide was of the height of nearly 80 m above road level (1370 m above MSL) with slope height more than 150 m. The entrainment of the slided debris mass was of the extent of 70 m below road level. The preliminary data after the rock-slide at the slope site were recorded and collected such as slope images, slope attributes, joint attributes, joint roughness, and rock chunks. Google earth imageries of past years were observed for the slope site and 1^st instance of instability was recorded in 2017, followed by another slide in 2019. Lastly, a major slide occurred in 2022. Point-load strength index (PLSI) tests on irregular rock chunks were performed as per IS – 8764 (1998) and UCS of the intact rock was derived using empirical correlation. Normal distribution was applied on the PLSI results to better assess the UCS of the intact rock. The probabilistic mean of the UCS was 24.21 MPa. Rock mass at the slope site was classified using Rock Mass Rating (RMR) and Q-slope classification system. Evaluated RMR and Q values were 37 and 0.073 respectively. A general impression from the derived values was that the rock mass was of poor quality with calculated safe cut slope angle of 42.3°. In the present case, the slope inclination lies between 60° - 70° on an average and can be termed under the category of unstable slopes. Further, Slope Mass Rating (SMR) was implemented on the slope site. The site was categorized in class IV and termed as unstable with probability of failure of 0.6 and type of failure as planar or big wedges. To be more certain and specific about the type of failure, kinematic analysis was performed in DIPS. It was determined that the rock slope has the probability of direct toppling by 0.33 and wedge sliding by 0.33. Hence, proper mitigation and stabilisation measures should be adopted to avoid any potential hazard.