A multi-scale multi-physics model of soil drying

Drying shrinkage is modeled at a microscale as a two-stage process of deformation and evacuation of a two-tube vessel system, based on porosimetry evolution. In the first stage water flow is driven by an external evaporation flux. Flow is associated with a negative pressure (suction) gradient along the vessel. The amount of water evaporated in this stage equals to the vessel volume reduction through the deformation of its walls, as long as the suction constricting the vessel can be supported by water. At some point suction required to further deform the vessel reaches at the vessel exit the water tensile strength,. As a result liquid/gas interface penetrates the vessel via undistiguishable mechanisms of meniscus plunging or near-surface cavitation (isothermal evaporation). The subsequent process is modeled as a moving drying front, without much of deformation of the vessels. The resulting macroscopic effective stress is generally compressive due to large suction. However, locally around defects a relatively small tensile total stress is greatly amplified, and hence despite the large suction, the effective stress is tensile and may easily reach tensile strength. Thus, the condition for tensile failure involves microscopic characteristic size of the soil pore system, and its defects.