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

Abstract

The negative pore-water pressure in unsaturated soils increases the inter-particle force and small-strain stiŠness, 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 runoŠ 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 stiŠness and corresponding degree of saturation are continuously monitored during evaporation for both specimens. The peak value of maximum shear stiŠness is captured at a degree of saturation S¿5.5z for hydrophilic specimen, while the hydrophobic specimen shows a quasiconstant small-strain stiŠness 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 identiˆed depending on the contact angle and volume of liquid bridge. The measurement of small-strain stiŠness combined with the numerical simulation of both hydrophilic and hydrophobic specimens clariˆes the governing factors to determine capillarity in the granular materials and provides insight into the phenomenological observation of capillary pressure for unsaturated soils.

Water‐Entry Pressure and Friction Angle in an Artificially Synthesized Water‐Repellent Silty Soil

“…Afterward, the treated sample was washed with deionized water to remove any remaining reactive compound. This process imparts water repellency and has been successfully used in other studies that have characterized the mechanical, thermal, and electrical properties of water‐repellent soils (Kim et al, 2011, 2013; Truong et al, 2011).…”

Experimental Investigation of Evaporation and Drainage in Wettable and Water-Repellent Sands

“…The scale of the specimen may correspond to that of stage 1 processes, in which the evaporation rate is constant. Therefore, the effective and macroscopic behavior is governed by wettability-dependent fluid configuration at the microscopic pore scale which is relevant to the physico-mechanical properties of the soil [26,27], despite the fact that the level of saturation observed at bulk scales is similar in different soils. Also, the unique evaporation is pronounced even at low levels of water repellency (e.g., contact angles of close to 90°).…”

“…In sands, the lowering of friction coefficients on the surfaces of hydrophobic particles results in low strength and stiffness of these sands [26]. In hydrophilic sands, capillarity among inter-particle contacts exerts maximum shear stiffness at 5%-10% saturation, whereas in hydrophobic sands, no such unique peak values of shear stiffness are observed [27]. With consideration of the distinct hydrologic and mechanical properties of water-repellent sands, we aim to experimentally explore detailed configuration of liquid phase and the drying rate resulted from evaporation as well as water retention during cyclic imbibition depending on the initial wetting condition in order to obtain a comprehensive understanding of surface wettability effects.…”

Hydromechanical behaviour of hydrophobised soils of varying degrees of saturation: a comprehensive review

“…Effects of S on G max of untreated and treated glass beads were also studied by [15]. A clearer trend can be identified (not shown in the present paper).…”