Desiccation shrinkage of non-clayey soils: multi-physics mechanisms and a microstructural model
Analysis of macroscopic desiccation shrinkage experiments indicates that most, but not all of the shrinkage during drying occurs while soil is still saturated. Shrinkage practically ceases and air starts to penetrate the soil, when the water content is still quite high, e.g. above 20% for the tested soils. The remaining, unsaturated drying process occurs with a much-reduced shrinkage rate. In this context we examine data of the pore system evolution as represented by the mercury porosimetry experimental results. The process is then modeled as a two-stage process of deformation and evacuation of a two-tube vessel system driven by the external evaporation flux. In the first stage, Poiseuille flow occurs through the vessels. The amount of water evaporated in this stage equals to the reduction of volume of the vessel through the deformation of its walls. This stage ends when a negative water pressure (suction) required to further deform the vessel reaches a critical value at which air enters the pore space. Two physical interpretation of such threshold are discussed. In the subsequent stage, evaporation proceeds with a receding liquid/vapor interface starting from the open end, incrementally emptying the vessel but with a marginal water flow and vessel deformation. The leading variables of the process are identified and a quantifiable multi-physics meso-scale scenario of models is established.