Slope stability analysis is a critical component of geotechnical engineering applied to heap leaching. For decades, analytical techniques and software based on the limit equilibrium method (LEM) have been the basis for slope stability analyses. It is well understood that these techniques do not account for the deformational behavior of the slope materials and many complex failure mechanisms cannot be modeled by the LEM. However, the calculated factors of safety (FOS) are generally conservative. This positive experience has given the LEM somewhat of a model stature despite its limitations.

A more rigorous method using finite element or finite difference techniques and a process labeled as shear stress reduction (SSR) is gaining some ground. In this method, shear strength is incrementally reduced to induce failure and the ratio of the actual shear strength to reduced strength at failure is taken as the FOS. For slopes with uniform material properties, the SSR and LEM have produced similar FOSs. However, in cases of slopes with complex profiles including multiple materials, geosynthetic or other types of liners, speed bumps or anchor trenches, etc., both techniques can produce anomalous results and raise some questions about the reliability of the calculated safety factors.

For example, in a complicated slope profile, LEM could result in forces implying movements contrary to intuition and reality. On the other hand, in the SSR process, selective or indiscriminate application of strength reduction to some or all materials could significantly impact the calculated FOS. There is some indication in the literature that the strength reduction should be applied indiscriminately to all materials. However, there is no theoretical basis for this. On the contrary, in case of a slope with a liner, often observed or proclaimed reduction in strength at the liner interface relative to the rest of the profile, favors selective application. This paper discusses some of these implications.