Multiphase soil mechanics for landscape protection: from testing to modelling

The stability of geotechnical structures is often driven by the superficial layer of soils subject to cycles of drying, wetting, and temperature variation. This shallow layer, interacting with the atmosphere, is often in unsaturated conditions due to the depth of the water table and the change in water content with climate loading. The unsaturated soils comprise at least three phases: the soil particles, the wetting phase, and the non-wetting phase. To tackle this multiphase system, the classical soil mechanics must be generalised, accounting for different phases and the interaction between them. In this study, a review of the principal experimental evidence on unsaturated soils is first presented, followed by a definition of the stress variables used to describe the observed behaviour. Later, a summary of the different classes of constitutive models is depicted. The role of the water retention curve (WRC), linking the amount of water to the suction, is discussed with an emphasis on its dependence on void ratio and microstructure. Further, an extension of the unsaturated soil mechanics to describe the behaviour of frozen soils is treated. Finally, using the framework of unsaturated soil mechanics, a back analysis of different engineering applications spanning from shallow landslides induced by rainfall to frost heave is performed.