Geotechnical Stability Analysis: New Methods for an Old Problem

Prof. Scott W Sloan, Geotechnical Research Group, University of Newcastle

In practice, geotechnical stability analysis is usually performed by a variety of approximate methods that are based on the notion of limit equilibrium. Although they appeal to engineering intuition, these techniques have a number of major disadvantages, not the least of which is the need to presuppose an appropriate failure mechanism in advance. This feature can lead to grossly inaccurate predictions of the true failure load, especially for realistic problems involving layered materials, complex loading, or three-dimensional deformation.

A much more rigorous method for assessing the stability of geostructures became available with the advent of the limit (or bound) theorems of classical plasticity in the 1950s. These theorems can be used to give upper and lower bounds on the predicted collapse load (a most valuable property in practice), do not require assumptions to be made about the mode of failure, and use only simple strength parameters that are familiar to geotechnical engineers. Although many ingenious bound results have been derived using both analytical and numerical methods, the true power of the limit theorems has been greatly restricted by the need to develop specific solution strategies for each practical problem. Over the last decade, the speaker and his group at the University of Newcastle have developed powerful new methods for performing stability analysis that combine the limit theorems with finite elements and optimisation. These methods are extremely general and can deal with layered soil profiles, anisotropic strength characteristics, complicated boundary conditions, and complex loading in both two and three dimensions. Indeed, they have already been used to obtain new stability solutions for a wide range of practical problems including soil anchors, slopes, foundations under combined loads, excavations, tunnels, mine workings, and sinkholes. The Lecture gave a brief outline of the new techniques and consider a number of practical applications. Future research developments were also be highlighted.

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