Identifying the dominant mode of moisture transport during drying of unsaturated soils

Diffusion of capillary water and water vapor during moisture loss in an unsaturated soil is impeded by the chemical and geometrical interactions between water molecules/vapor and the soil structure. A reduction in moisture content contracts the diffuse and adsorbed water layers in the partly saturated soil and disturbs the connected capillary network for flow of liquid water. With further drying, the dry soil layer expands and moisture is predominantly lost as vapor through continuous air-flow channels. The water-filled capillary network and air-filled channels are moisture conduits during different stages of soil drying. It is important to identify zones of dominant moisture transport and to select appropriate tortuosity equations for correct prediction of moisture flux. Laboratory experiments were performed to determine moisture flux from compacted soil specimens at environmental relative humidity of 33, 76 and 97% respectively. Analysis of the resultant τ - θ (tortuosity - volumetric water content) relations, illustrated the existence of a critical water content (θ_cr), that delineates the dominant zones of capillary liquid flow and vapor diffusion. At critical water content, the pore-size occupied by the capillary water is governed by the generated soil suction. Generalized equations are proposed to predict tortuosity factor in zones of dominant capillary liquid flow and vapor transport over a wide range of relative humidity (33 to 97%).