Probably the most reliable method of determining the strength of a soil and rock mass is to back analyze a failed or failing slope. 1, 2 This method is used in geotechnics for estimating the geotechnical parameters of the rock and soil mass, 3, 4, 5 but it requires that the failure mode is well established and that there is complete and precise information available on the slide surface and the sliding mass. 6 Measurement of the failure surface geometry combined with knowledge of the slope profile before failure provide the basic information required for a back analysis. 2, 7 The post-slide topography is another key factor.

The back analysis process for determining the actual strength parameters of soil or decomposed rock masses after a landslide requires knowledge of the sliding mass geometry, both before and after the sliding event, as well as the geometry or the real profile of the sliding surface. Many studies have reported in the literature the application of back analysis for determining geotechnical and geomechanical parameters of rock and soil mass, 3, 4, 5, 8 but they generally present little information about the geometry of the slide surface or about the weight of the rock and soil mass that had slid, even though these factors are the most important parameters for the back analysis process. Liang and Xue-song 9 have determined the slip surface having the minimum factor of safety (called critical slip surface in deterministic analysis of slopes) and the slip surface with the minimum reliability index (called critical reliability slip surface), but they have not considered the actual profile and geometry of the slide surface. Rocscience 10 proposes the back analysis option in the slide software for slope stability studies, but it requires an imposed slide surface as input; alternatively, one can estimate by a finite element analysis the surface with the minimum factor of safety. However, this estimated surface is very likely different from the actual sliding surface, considering the uncertainties in input data of the finite element method.