Revisiting stability charts for homogeneous undrained slopes by finite element methods

Abstract

Slope stability charts are essential engineering tools that enable rapid calculation of the factor of safety; hence, they are frequently used during the early design phase of a project to compare design options. This thesis revisits various chart solutions for undrained slope stability by numerical analyses using state-of-the-art programs based on (i) elastic-plastic finite element analysis (EPFE) and (ii) rigid-plastic finite element limit analysis (FELA). In this study, parametric studies were performed using a wide range of soil and geometrical parameters, and the results were compared with chart solutions based on thousands of different cases.

Finite element methods do not require prior assumptions about the shape or location of the critical failure mechanism which simply follows the path of least resistance through the slope. The charts considered in this thesis for comparison, however, use limit equilibrium methods based on circular and composite circular assumptions. The finite element results observed several points of disagreement with the charts because of the availability of more realistic failure surfaces. Based primarily on the FELA analyses, stability charts in terms of dimensionless stability numbers were proposed and thoroughly validated using EPFE analyses. Then, they were extensively compared to the charts in the literature. It was found that the proposed charts, derived from finite element methods, improved the estimations of: (i) the factor of safety particularly for slopes having a large foundation layer or relatively flat gradients; (ii) two previously accepted parameters, namely the minimum stability number that can be obtained through a deep mechanism and the slope angle where the slope stability becomes independent of the depth of the foundation layer; and (iii) the factor of safety for slopes having a small foundation layer in the presence of reservoir water.