Investigation carried out on twenty-four landslide areas of Guwahati, a premier city of Northeast India, has revealed that natural slopes comprising soil mass with rock mass overburden generally fail causing devastation. Failure of such slope may be triggered either by the failure of top rock mass, or by the failure of soil portion bellow, or both soil and rock mass may fail simultaneously. The existing methods or stability analysis are not applicable in such complex failure mechanism. A new method of stability analysis has therefore been developed for analysis of such slope and for deciding on effective location of preventive measures. This method involves determination of factors of safety for various possible failure surfaces following different sequences of progressive failure. The values of factors of safety thus computed indicate the actual failure mechanism. A computer program has also been developed to expedite solution of the problem.
A number of approaches have been suggested for the stability analysis of soil slope by Fellinius (1939), Taylor (1948), Bishop (1955), Janbu (1954), Greenwood (1983), Mongenstern and Price (1965), Spencer (1967) etc. Similarly, the stability of a rock slope can be investigated by a number of methods such as plane failure analysis. Wedge Method of analysis etc. Thus when the slope consists either of soil or of a rock mass, those method can be readily applied. However, in the case of natural slope, one hardly finds a slope consisting of a single material. Such slopes are usually made of soil and rock masses together and analysis of stability of such slopes calls for a special approach. So far very little work has been done in this direction. Authors have suggested a method of analysis applicable to such natural slopes consisting of soil with an overburden of rock mass. A computer program has also been developed for quicker analysis and solution of the problem. The formulation of the approach was necessitated by a study of failure of an actual hill slope in Guwahati, a city which has witnessed an unprecedented pace of urbanization of-late with indiscriminate use of hill slope for building activities. This has resulted in many devastating landslides with repeated loss of life and properties. It is felt that whenever conditions are similar with increasing pressure on land necessitating use of hill slopes, such approach would have special practical relevance and urgency. Investigation carried out on twenty four landslide areas of the city has revealed that landslide become specially devastating when failure occurred in slope of soil with overburden of rock masses. The multiple landslides that occurred at Kacharibasti of Guwahati constitute a typical example of this kind. A sketch of pre-failure stage of the slope is given in the fig-I. In the situation represented by Fig-1, a question of interest is whether the failure of the composite slope is triggered by the rotational failure of the top rock mass caused possibly by rain softened weak underlying soil or is triggered by failure of soil portion bellow.
