Effects of interfacial properties on soil water infiltration

Abstract

Hydrophobic soils have a low affinity for water and inhibit water infiltration. Soil hydrophobicity is related to undesirable consequences such as severe soil erosion and increased risk of debris flows. However, the water repellent properties of hydrophobized soils deliver favorable applications such as shallow covers or barriers. The application of hydrophobized soils in geotechnical engineering is constrained by the water entry pressure and long-term durability and stability. This thesis aims to pave the way to the effective application of hydrophobized soils by providing an insight into the effect of soil interfacial properties on water infiltration. The specific objectives are to (1) determine the water entry pressure of hydrophobized soils and the influence of hydrophobizing agents and soil characteristics; (2) assess the change of saturated permeability in hydrophobized soils; (3) investigate the wetting behaviour of aqueous surfactant solutions on hydrophobized soils; (4) trigger water infiltration through electrowetting. The water ponding method was used to measure the water entry pressure of hydrophobized granular materials. Standard quartz sand and manufactured glass materials were hydrophobized with dimethyldichlorosilane to address the first objective. By altering the material physical properties while maintaining the same degree of intrinsic hydrophobicity, the water entry pressure of hydrophobized materials was measured to quantify the impact of particle size distribution, particle size, particle shape and dry density and to establish its inter-dependencies. A modified empirical equation was proposed based on the experimental data. To address the second objective, potential factors affecting the saturated permeability of hydrophobized soils were identified in a literature review. Hydrophobized sands were synthesized by dimethyldichlorosilane, Tung oil, and wax. Tests on saturated permeability of the hydrophobized sands with known physicochemical properties were conducted and supported by the use of optical techniques (e.g., interferometry) to assess the particle surface characteristics. Results highlighted the important role of surface roughness of hydrophobized sand at a near-saturated state. Methods to trigger water infiltration in hydrophobized soils by manipulating soil-water interfacial properties were investigated. For the third objective, a nonionic aqueous solution (Triton X-100) was applied to wet the hydrophobized quartz sand (treated with dimethyldichlorosilane and Tung oil). Water entry pressure remains constant before reaching a critical concentration due to the increased contact areas and limited adsorption of surfactants at both the sand-solution and solution-air interfaces. However, above this critical concentration, the water entry pressure decreases rapidly. The last objective was met by a preliminary study of electrowetting on hydrophobized soils. A classical electrowetting on dielectric setup was adopted. The feasibility of applying electrowetting to trigger infiltration in hydrophobized soils (sand, silt, clay) has been validated. For all materials, water infiltrated into soils under an external electric field. Contact angles were altered. The droplet volume kept decreasing during the electrowetting process. This thesis provides various insights into the effect of interfacial properties on soil water infiltration. The results provide a fundamental basis to model and design hydrophobized soils for ground applications. Uniquely, it also demonstrates the conditions under which water infiltration can be controlled in soils, by testing methods that promote or hinder water infiltration in hydrophobized soils.