Use of finite element methods in geotechical ultimate limit state design

The finite element method (FEM) continues to gain importance as a tool for geotechnical ultimate limit state design (ULS). Suitable procedures have been proposed, especially within material factoring approaches: the complete stress and loading history is simulated using unfactored values of the material parameters and at each stage where the ULS requirements have to be checked, the shear strength parameters are reduced until a state of failure is reached (ϕ'-c' reduction from characteristic stress field). It is shown that such procedures are fully compatible with the Design approaches 1 and 3 of Eurocode 7. The failure load obtained with FEM is briefly compared with the upper and lower bound values obtained from classical plasticity. The effect of dilatancy is highlighted. Some requirements on numerical and modelling aspects are given to obtain numerically reliable and clearly defined "failure situations". The ϕ'-c' reduction is performed using a simple Mohr-Coulomb model even when the stress history is simulated using an advanced soil model. The advantages and limitations involved by the use of a simple model for the ϕ'-c' reduction are discussed. The value of the dilatancy angle during the ϕ'-c' reduction is discussed, as it may influence in some situations the value of the failure load, or the design values of forces in structural elements.