Geotechnical stability analysis is traditionally performed by a variety of approximate methods that are based on the theory of limit equilibrium. Although they are simple and appeal to engineering intuition, these techniques suffer from a number of serious disadvantages, not the least of which is the need to presuppose an appropriate failure mechanism in advance. This feature can lead to 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 analytical or numerical methods, practical application of the limit theorems has been restricted by the need to develop specific solution strategies for each problem. Over the last decade, the Newcastle Geotechnical Research Group has developed powerful new methods for performing stability analysis that combine the limit theorems with finite elements and optimisation. These methods are very 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 loading, excavations, tunnels, mine workings and sinkholes.
This paper gives an outline of the new techniques and considers a number of practical applications. Future research developments will also be highlighted.