Slope Mass Assessment of Road Cut Rock Slopes Along Karnprayag to Narainbagarh Highway in Garhwal Himalayas, India

Abstract

Slope instability is a major problem in hilly terrains. Stability assessment of road cut rock slopes is of paramount importance for planning and construction of infrastructures in hilly terrain. Slope Mass Rating (SMR) technique developed by Romana (1985) is a geomechanical method to assess the stability of rock slopes, that in principle uses basic Rock Mass Rating (Bieniawski 1979, 1989) and geometrical relationship between slope and rock discontinuities. In the present study, 39 rock slopes along Gopeswar-Almora road from Karnprayag to Narainbagarh in Chamoli district of Garhwal Himalayas have been studied in detail for their slope mass assessment using the mentioned technique. Based on the field observations, quartzite is the dominant lithotype with maximum of three sets of discontinuities in the study area. Meta Volcanic and Meta Sedimentary rocks are also present, but within narrow patches along the road, with phyllitic and schistose rocks. The parameters pertaining to Rock Mass Rating (RMR) and Slope Mass Rating (SMR) techniques were collected from field and laboratory studies for all the rock slopes. The basic RMR values of quarzitic rocks range between 50 and 88 whereas the SMR values vary from 07 to 84 depending upon the geometrical relationship between orientation of Slope face and discontinuities. For most of the schistose and phyllitic rocks basic RMR values are observed to be below 50 and the SMR values lie between 0 and 38. Based on the SMR values, these cut slopes were categorized into five different failure potential classes (Romana 1985). From the results, it is inferred that out of total 39 rock slopes, six slopes are completely unstable, 17 slopes are unstable, another six slopes are partially stable, nine slopes are stable, and only one slope is completely stable. From the kinematic analysis of slope face in relation to discontinuities, it was found that planar mode of failure is the most predominant type followed by wedge mode of failure in these rock slopes. Out of total slopes, 19 slopes are prone to planar failure, 8 slopes are prone to wedge failure, 3 slopes are more likely to fail by planar mode but has significant wedge failure component, 1 slope is prone to topple failure and 8 stable slopes.