Mountainslope failures usually occur during heavy rainfalls. On granite slopes, these failures occasionally appeare in a thin layered form. In order to predict the most dangerous site of these failures, the failure potential layer had been defined by employing the results of field investigations, examinations and laboratory tests. These results have already been reported elsewhere (Okimura and Tanaka, 1980). Using a distribution of the said layer and the longitudinal cross section of this slope, the most dangerous site and the length of the failure was analyzed by a slope stability analysis method. This method is a multi-planar sliding surface method which is developed from a tri-planar sliding surface method (Chowdhury, 1978).
This method is applied to the data obtained from a granite test field in which slope failures had occurred in 1972. The analysis results showed that the assumed sliding block, which presented the minimum safety factor, appeared at the same place where failure had occurred. Failured slopes and non-failured slopes could be grouped by using the modified safety factors which were calculated by multiplying the shape factor of the catchment area by original safety factor. Therefore, if we obtain the distribution of the failure potential layer, we can point out the most dangerous site of failure on a slope by using the multi-planar sliding surface method.