Compound slope failure mechanisms in layered undrained clays

This note examines compound failure mechanisms in three- and four-layer undrained slopes using the finite-element method. While two-layered slopes have been widely studied, conditions leading to failure in slopes with additional layering have received limited attention. The results focus on the variation of the critical failure surface based on undrained strength ratios of successive layers, identifying the constraints required for compound failure to occur. The influence of slope angle on these relationships is examined, and a generalised procedure is proposed to define the necessary undrained shear strength ratios to characterise all permissible failure mechanisms. The prevalence of each mechanism as a function of slope angle is further discussed. In the case of a four-layer slope, it is demonstrated that no combination of undrained shear strengths can produce certain failure modes, revealing inherent constraints based on the combination of geometry and mechanical properties. The findings provide new insight into the governing behaviour of stratified undrained slopes and establishes a foundation for interpreting the stability of increasingly complex systems.