Seepage characteristics in unsaturated dispersive soil considering soil salinity and density impacts: Experimental and numerical combined study

Abstract

Seepage in dispersive soils has contributed to inducing severe damage to hydraulic structures worldwide, whereas its targeted quantitative and refined characterizations have long been overlooked. This study jointly adopted the laboratory constant-head seepage test and Hydrus-1D simulation to explore the dynamic seepage behavior in compacted unsaturated salt-affected dispersive soil columns (lean clay with sand). The impacts of two crucial factors including initial soil soluble salt content (η0, 0.3%, 1.0%, 2,0%) and dry density (ρd, 1.44, 1.53, 1.62 g/cm3) were considered. The matric suction of each experimental case was measured to characterize initial unsaturation, and the post-seepage particle and pore distributions of a representative case were measured to clarify the seepage consequence. Furthermore, the spatiotemporal evolutions of water, soluble salt, and hydraulic conductivity during seepage were captured. Laboratory results suggested that the advance of the wetting front conformed to a power function. Increasing η0 and ρd reduced and prolonged the total test duration by 19.2%-38.2% and 4.4%-38.6%, respectively. Since dispersive soil features a water-sensitive skeleton, fine particles could be dragged downward by water flow, thus simplifying the pore microstructure near the soil column top. Under such a low head of 2 cm here, this finding implied that the threshold hydraulic gradient corresponding to the formation of internal erosion in dispersive soil can be quite low. Based on simulation results, rapid salt accumulation formed where the wetting front arrived in contrast to the following slower leaching process. Higher η0 contributed to stronger permeability. The magnitude order of hydraulic conductivity ranged between 10−16 and 10−7 m/s from initially unsaturated state to saturated state, and that of saturated soil was mainly 10−8 m/s, reflecting the poor permeability of dispersive soil. These novel findings are expected to deepen the soil–water interaction understanding and further guide the rational treatment and application of this problematic special soil in engineering practice.