Capillary Induced Small-Strain Stiffness for Hydrophilic and Hydrophobic Granular Materials: Experimental and Numerical Studies

Abstract

ABSTRACT The negative pore-water pressure in unsaturated soils increases the inter-particle force and small-strain stiffness, though this concept is only valid in wettable soils. The non-wetting nature of soils originating from the organic contamination of geoenvironments and natural hazards causes unexpected geo-events such as impermeation of water and hillslope runoff due to the changes in soil wettability. This study presents an experimental and numerical investigation to understand the evolution of capillary force and pressure for unsaturated soils whose surface wettability is wettable (hydrophilic) and water-repellent (hydrophobic). Hydrophobic granular materials are synthesized by the silanization technique with 0.5 mm diameter glass beads. The small-strain shear stiffness and corresponding degree of saturation are continuously monitored during evaporation for both specimens. The peak value of maximum shear stiffness is captured at a degree of saturation S~5.5% for hydrophilic specimen, while the hydrophobic specimen shows a quasi-constant small-strain stiffness during evaporation. The minimization of free energy for the liquid bridge between the two-particle system allows the attractive and repulsive capillary force and pressure produced between particles to be numerically estimated. The regime of zero-capillary pressure is identified depending on the contact angle and volume of liquid bridge. The measurement of small-strain stiffness combined with the numerical simulation of both hydrophilic and hydrophobic specimens clarifies the governing factors to determine capillarity in the granular materials and provides insight into the phenomenological observation of capillary pressure for unsaturated soils.