Describing the influence of partial saturation on soil behaviour as realistically as possible is still a major challenge today. In particular, the question of how the effective stresses in partially saturated soils can be calculated has not yet been finally clarified. It is similarly challenging to capture the soil-structure interaction as accurately as possible in numerical calculations. The Coulomb friction model frequently used for this purpose can only approximate the contact behaviour under monotonic loading and can only approximate it poorly under cyclic loading; new, improved approaches are required.

By redefining the Bishop parameter, which determines the contribution of the suction stress to the effective stresses, known shortcomings are eliminated and the term ‘effective’ is given a clear meaning. Tests on K0-consolidated kaolin are used to show that the undrained strength can exhibit anisotropy at the contact. Approaches for consideration in simulations are presented.