A New Capillary Stress Model for a Proper Effective Stress in Unsaturated Soils

Abstract

This paper presents a new framework to reproduce the behaviour of unsaturated soils based on the effective stress concept in the original sense of Terzaghi. The solid skeleton constitutive models are only directly dependent on the effective stress tensor and not the suction or the degree of saturation. The proposed framework incorporates a solid skeleton constitutive model, a soil-water retention model and a new non-monotonic capillary stress model, supported by experimental evidence, showing the viability of using a proper effective stress framework to reproduce unsaturated soil response. An isotropic capillary stress model (CAPSM) is proposed that includes the main characteristics of the capillary stress in regular sphere arrangements. It is a function of the degree of saturation, incorporating hysteretic behaviour between drying and wetting paths and dependence on specific volume. Using the original effective stress concept, together with the proposed capillary stress model and advanced solid skeleton constitutive models, several complex unsaturated laboratory tests are simulated, accurately reproducing diverse relevant aspects of the unsaturated soil's behaviour. The main conclusions are that an effective stress in the original sense can be defined for unsaturated soils and, with an adequate effective stress model, there is no need to modify the solid skeleton constitutive models (which can be formulated, updated and calibrated independently for saturated conditions) by introducing a direct dependence of model parameters on suction or degree of saturation.