Liquid bridge contribution to shear behavior of unsaturated soil: modeling and application to a micromechanics model

Liquid bridges in unsaturated soils attach to grain contacts and contribute to strengthening microscopic bonding forces, which leads to macroscopic high strength and stiffness compared with dry or saturated soils. This study investigated microscopic liquid bridge behaviors in monodisperse granular media by combining X-ray micro-computed tomography and image analyses. Evaluating the anisotropy of grain contact orientation by fabric tensor revealed that fabric transitions under a triaxial process for both grain contacts and that with liquid bridges are equivalent, which indicates that the orientation of the liquid bridge contribution does not have unique anisotropy. The liquid bridge ratio (LBR) was defined as the ratio of the number of grain contacts with liquid bridges to the total number of grain contacts, representing the magnitude of the liquid bridge contribution. LBR distributions—the relationship of LBR to degree of saturation obtained by water retention tests for both dense and loose specimens—exhibited different peak positions depending on the void ratio and constant dispersion, which was then modeled using β distribution. A constitutive model on an experimental micromechanics basis for unsaturated soils was proposed by applying the liquid bridge contribution model as a combination of the LBR distribution model and the soil water characteristic curve model to a micromechanics model, in which spherical and monodisperse grains are assumed and the pendular saturation regime is focused. The proposed model qualitatively reproduced the macroscopic suction effect by describing the microscopic liquid bridge contribution.