Evaluating stability of perilous slide zones occurring in Lesser Himalayan schuppen belt between Tiuni and Taluka route, Uttarakhand, India

Abstract

The Lesser Himalayan belt, being the most complex litho-tectonic units of Himalaya, is in frequent danger of slope destabilisation. The present study intends to assess geotechnical characteristics of identified perilous slide zones for their detailed slope stability analysis in one of the highly complex structural entities of Lesser Himalaya, named as Purola schuppen zone. It involves imbricate thrusting and embraces a number of slide zones in weathered quartzite, phyllite, schist and shales. On the basis of discontinuity characteristics, viz. their orientations and interrelationship with slope, structural attributes and debris materials, slide zones are showing planar, wedge or circular failure patterns. Kinematic analysis is attempted to identify the possible mode of structurally controlled failure in rock mass. As per their failure types, these slide zones are analysed for stability using geotechnical parameters evaluated in lab. For circular failure zones, angle of internal friction and cohesion, are attained quantitatively by direct shear test and varying from 30.8° to 36.8° and from 0.04 to 0.41 kg/cm2, respectively. In case of planar and wedge failures, modified slope mass rating technique is used that assigns class V status for Kothgaon slide zone indicating very bad rock mass and unstable to completely unstable slope conditions with probability of failure as high as 0.9 while Taluka slide is partially stable with normal rock mass falling in class IV. Factor of safety, estimated by circular failure charts using slope, soil and strength characteristics, is varying from 0.75 to 1.50 in dry conditions, signifying critical stable state while it reduces less than unity in wet conditions as saturation upsurges due to nearby percolation of rainwater through discontinuities and cracks. Imbricate thrust zone, substantial weathering and jointing in rock masses, steep slopes and nearby vicinity to stream channels are major contributory reasons of instability in the area, while road widening and rainfall are acting as catalysts prompting the failure. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.