GEOTECHNICAL MODELLING OF SURBEE LANDSLIDE

Abstract

"Surbee Landslide" is located just below the Surbee Resort on Mussoorie-Kempty Fall road. The initial failure of this slide had occurred during the Uttarkashi Earthquake in 1991. It had again occurred effectively during the heavy rains during July, 2003. This slide is one of the biggest slides in the Mussoorie hills. It 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 the stability analysis of the landslide has been carried out using probabilistic method. Deterministic analysis is usually done to get the value of the factor of safety mathematically, wherein single fixed values (typically, mean values) of representative orientation and strength parameters are used in the analysis. The stability analysis is normally carried out with one set of geotechnical parameters. Factor of safety, based on limit equilibrium analysis, is widely used to evaluate slope stability because of its simple calculation and results. However, most input values measured in the field or obtained by laboratory tests which are used subsequently to calculate a safety factor show a wide scatter across a significant range rather than being a fixed single value. Therefore, each parameter should be considered as a random variable and the analysis involving different values for each parameter will result in different factors of safety. Therefore, the factor of safety itself is a random variable, depending on many input values. The deterministic analysis is therefore unable to account for variation in rock mass properties and conditions. Hence, the probabilistic method is used to take account of the variation in rock properties and conditions. The variation in the attitude of the three joints present in the site and their shear strength parameters are analyzed by probabilistic method in this study. Further, the wedges formed by the combinations of these three joints have been analyzed for their factor of safety by Monte Carlo simulation using computer program. Monte Carlo simulation is also applied to analyze the stability of debris, present at the site, by considering shear strength parameters as variable. Graphs have been developed to find the probability of factor of safety.