Numerical Analysis of the Effects of Cracks on the Moisture-Diffusion Coefficient of Unsaturated Soils

In an unsaturated soil with desiccation cracks, the overall moisture-diffusion coefficient can be considered as consisting of two components, the diffusion coefficient of the intact (uncracked) soil matrix and the diffusion coefficient of the cracks developed during the desiccation process. Several models are available for obtaining the diffusivity of the intact soils, whereas the models considering the effects of cracks on the diffusion coefficient are either complex or insufficient. In this paper, a numerical model is presented to investigate the influence of the desiccation cracks on the moisture-diffusion coefficient of unsaturated soils. The analyses of the results indicate that the diffusion coefficient component, due to the desiccation cracks, dominates the overall diffusivity of the soil matrix, whereas the diffusion coefficient of the intact (uncracked) soil can be ignored for all practical purposes when the cracks occur. The main factor controlling the diffusion coefficient due to the cracks is the geometrical dimension of the cracks. Thus, the variation of the diffusion coefficient with the crack width and the crack depth is also evaluated. Unlike the diffusivity of an intact (uncracked) soil, which is nearly constant for a specific soil over a small suction change, the diffusivity of a cracked soil varies with depth due to the changes in the crack dimensions. Also, the effect of the cracks on the diffusion coefficient at shallow depths over a deep soil layer is limited and can be ignored when the depth reaches a specific value. Practical guidelines are provided to estimate the field diffusion coefficient when the crack width and depth are given.