Limitations in the Back-Analysis of Strength from Failures

Stability failures are often "back-analyzed" in an attempt to estimate the operative shear strength. In fact, back-analysis is commonly believed to be one of the most reliable ways to estimate soil and/or rock strength. However, this paper provides examples to illustrate specific situations in which back-analysis of failures can lead to misinterpretation of strength. Examples from earth and concrete gravity dams are used, and consideration is given to both two- and three-dimensional idealizations. The cases demonstrate that interpreted strength can be in significant error, and in practically all cases the errors are unconservative. Finally, this paper illustrates that back-analysis is reliable only when the model and all assumptions are reasonable and accurate representations of the real system.