Abstract: The determination of shear-strength parameters in geotechnical materials is crucial for analyzing slope stability and designing landslide reinforcement. The back-analysis method effectively compensates for the limitations of laboratory and in-situ testing and is therefore widely used due to its practicality and reliability. For gravelly soil slopes, stability and reinforcement analysis must focus on two key conditions: (1) the reduction in shear strength under water-saturated conditions, and (2) the need to adopt residual (post-peak) strength after sliding occurs. The variation of strength parameters under these conditions depends on factors such as gravel content and soil density, and no unified theoretical method currently exists. Thus, back-analysis can be used to determine these parameters. Based on an engineering case study, back-analysis was applied to quantify the reductions in cohesion c and friction angle φ after saturation, as well as the reduction from peak strength to residual strength. Results show that the shear strength of gravelly soil decreases significantly upon saturation, with cohesion exhibiting the largest reduction. After sliding deformation, the residual strength is approximately 75% of the peak strength. These findings provide useful guidance for the design and reinforcement of similar slopes.

Key words: Shear Strength, Inversion Analysis Method, Debris Slope, Landslides, Yield Strength