GEOTECHNICAL CHARACTERISATION OF A LANDSLIDE NEAR MUSSOORIE

Abstract

The landslide known as "Surbee Landslide", which is located just below the Surbee Resort on Mussoorie-Kempty Fall road had occurred during the heavy rains during July, 2003. The initial failure of this slide had occurred during the Uttarkashi Earthquake in 1991 which was observed to be a small scar. This slide which is now one of the biggest slides in the Mussoorie hills has caused huge losses to human livings, agricultural fields and the cattle sheds of villages located immediately down slope of this hill due the debris and big rock boulders that were carried away along the slope during failure. In the present study various field and laboratory studies have been carried out for knowing the ground characteristics, geotechnical characterisation of the slide. The strength assessment of the rock mass has been worked out using different concepts and data obtained from the field studies and the laboratory experimental studies. The shear strength parameters of the overburden soil mass have been assessed from the laboratory tests. The stability of the existing slope has been carried out using kinematic approach through stereo net plots and also by using the Slope Mass Rating system approach. It is the analysis indicate slope to be stable except the formation of a wedge at a location which is nearly 40-45m below the top. The overburden soil stability has been carried out for varying depth of water table and effect of water table on the stability has been assessed. As it was observed that a rock block due to formation of wedge may be carried out during the monsoon periods, a trial has been made for modelling the rock fall subjected to the gravity in UDEC. For this, trials have been made on the data of block size observed in the field at different locations for obtaining the coefficient of rebound for the existing ground that has to be considered for the rock fall. Using this coefficient of rebound value a programme was developed for the block falling down the slope.