A Back-Analysis Method for Shear Strength Parameters of Double-Layered Slopes

As a supplement to probabilistic back-analysis methods, traditional deterministic methods are limited by the homogeneous slope assumption and nonunique inversion results. Constrained by the actual slip surface and critical height equation, this paper establishes a back-analysis method for shear strength parameters of double-layered slopes using the upper-bound method. Case studies show that, compared with traditional homogeneous methods, the proposed method better reflects the layered distribution of strength parameters. By virtue of its rigorous theoretical foundation and kinematic characteristics, this method yields more reasonable and uniquely determined results. This paper reveals a special influence mechanism of unit weight: the unit weights of upper and lower layers have completely monotonic but opposite effects on back-calculated parameters. Specifically, if an increase in upper-layer unit weight causes a strength parameter to increase, an increase in lower-layer unit weight necessarily causes the same parameter to decrease, and vice versa. This mechanism determines two limit states of unit weight, transforming its inherent dispersion into controllable dispersion of inversion results, providing a reasonable parameter range for engineering applications. Based on slip surface continuity, a new error analysis principle is proposed: internal friction angles of different layers vary synchronously, while cohesion exhibits opposite trends. These findings provide a theoretical basis for rationality assessment and error control in slope back-analysis and serve as a reference for parameter selection in landslide remediation design.