Effect of water content and soil texture on consolidation in unsaturated soils

• Exact solutions are derived for two semi-permeable boundary drainage conditions.
• Consolidation is dominated by the coefficient of consolidation and loading efficiency.
• The coefficient of consolidation for water increases with increasing water content.
• The loading efficiency for water depends strongly on water content and soil texture.
• The time-dependent total settlement is controlled by soil bulk modulus.

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. A comprehensive model of poroelasticity for consolidation in unsaturated soils has been recently developed by Lo et al. (2014), but it still remains elusive how variations in soil texture and water content affect consolidation behavior, and the underlying parameters deriving this behavior.In the current study, a boundary-value problem is first setup corresponding to two symmetric semi-permeable drainage conditions, and then solved analytically for describing the excess pore air and water pressures along with the total settlement in response to time-invariant external loading using the Laplace transform. These solutions are numerically calculated for unsaturated soils with eleven texture classes as a function of three initial water saturations as representative examples. Our results reveal that the excess pore water pressure and time-dependent total settlement are indeed significantly sensitive to both soil texture and initial water saturation. We demonstrate that the coefficient of consolidation for water and its loading efficiency are two important physical parameters controlling consolidation behavior. With respect to the same soil texture, the coefficient of consolidation for water increases with an increase in initial water saturation, taking a value approximately four to five orders of magnitude greater in saturated soils than that in unsaturated ones. For a given initial water saturation, the rate of dissipation of excess pore water pressure is smallest in clay, followed by silty clay, silty clay loam, sandy clay, clay loam, silt loam, loam, sandy clay loam, sandy loam, loamy sand, and sand. A comparative study shows that in the early stage of consolidation, unsaturated soils bear smaller excess pore water pressure, but its dissipation is completed faster in saturated soils. The loading efficiency for water exhibits a concave upward relationship with initial water saturation in silty clay and clay, whereas a positively-correlated relationship between the efficiency and initial water saturation is observed in other soil textures. Unlike saturated soils, a considerable amount of total settlement is shown to occur immediately after application of external loading.