Massive Varunavat Hill Landslide Overlooking Uttarkashi Town in Uttarakhand Himalaya: Its Treatment Vis-à-Vis Stability Analysis

Abstract

Uttarkashi, a town in Uttarakhand Himalaya, India, experienced massive flow of rock mass debris due to landslide of Varunavat Parvat on 24th September, 2003. The landslide was caused by two major earthquakes in the area and triggered by incessant rains during the period. Post-landslide remedial measures included several geological and geotechnical aspects like surveying, geological and structural mapping, and demarcation of lineaments/faults by visual interpretation of IRS LISS II, followed by geo-mechanical testing of overburden mass, slope stability analysis, monitoring of slope movement by Automatic Target Recognition. The slope stabilization measures included re-profiling by benching, drainage of seepage water through weep holes and interconnected drains, placing of retaining walls, barricading and application of cable anchors, rock bolts, shortcrete wiremesh, geotextile, geogrids and bio-restoration. Stability analysis using numerical modeling was conducted to evaluate the performance of supports provided in the slope. Model without supports revealed that the maximum displacement of 0.255 m occurred in re-profiled slope soil mass below EL 1,630 m. The factor of safety (FOS) was found to be 0.98–4.5 in this region, indicating presence of weak zone at 20–30 m inside the soil mass. Application of support system reduced displacements by an average of 53 % throughout the slope profile. The entire slope face was found to have FOS value above five indicating no failure along the surface. The studies established that application of support system along with re-profiling of failed slopes and other water drainage works have resulted in Varunavat Parvat being safe with regards to long term stability.