Effect of Water Content and Soil Texture on Consolidation in Unsaturated Soils

Soil consolidation, involving time-dependent coupling between deformation of a porous medium and interstitial fluid flows within it, is of relevance to many subsurface engineering problems. In the current study, we apply the consolidation theory of poroelasticity developed by Lo et al.(2014) to investigate the effect of soil texture and initial water saturation on one-dimensional consolidation in unsaturated soils. Our model constitutes two coupled diffusion equations, where coupling occurs only in the time-derivative terms with symmetric coefficients. As simplifying to water-saturated soils, these equations exactly reduce to those in the classic Biot (1941) model of consolidation. Closed-form analytical solutions describing the excess pore air and water pressures along with the total settlement in response to external loading are obtained for both permeable and semi-permeable boundary drainage conditions by the application of the Laplace transform.

Numerical calculations are then implemented for unsaturated sand, loam, silt, and clay with respect to three initial water saturations (0.7, 0.8, and 0.9) as representative examples. Our results reveal that the excess pore water pressure and total settlement are significantly sensitive to both soil texture and initial water saturation. For a given initial water saturation, the rate of dissipation of excess pore water pressure is smallest in clay, with the highest total settlement being induced, followed by silt, loam, and sand. In the early stage of consolidation, unsaturated soils bear smaller excess pore water pressure, but its dissipation is completed faster in saturated soils. Two important parameters for characterizing consolidation processes are quantitatively examined. The coefficient of consolidation for water is shown to increase with an increase in initial water saturation, taking a value in saturated soils approximately four to five orders of magnitude greater than that in unsaturated ones. The loading efficiency for water exhibits a concave upward relationship with initial water saturation in clay, whereas a positively-correlated relationship between the efficiency and initial water saturation is observed in silt, loam, and sand.