A soil-shrinkage curve (SSC) can be defined as the characteristic volume–water content relation under free external stress conditions. A SSC is traditionally divided into three characteristic states: normal shrinkage when soil is saturated, residual shrinkage when soil is unsaturated and subjected to volume reduction, and zero shrinkage when soil approaches the dry state. Using the measured SSC, soil water-retention (SWR) curve, and suction stress characteristic curve of various silty and clayey soils, the traditional states are found to be not well defined and incorrect, particularly when soil approaches the dry state. A conceptual model for SSC is proposed in which SSC can be divided into four states: capillary, pendular, adsorbed, and tightly adsorbed; each is governed by one or/and two SWR mechanisms: capillary and adsorption. The shrinkage rate, defined here as the change in void ratio due to the change in moisture ratio, is found to be not universally zero in adsorptive states. It is shown that the shrinkage rate in the adsorption SWR regime is highly correlated to SWR characteristics—namely, the specific surface area—or to cation-exchange capacity, or that total amount of adsorption water follows the same exponential form with the shrinkage rate.

Although siting of thermal energy storage systems in the vadose zone may be beneficial due to the low thermal conductivity of unsaturated soils, water phase change and vapor diffusion in soils surrounding geothermal heat exchangers may play important roles in both the heat injection and retention processes that are not considered in established design models for these systems. To better understand these roles, this study incorporates recently-developed coupled thermohydraulic constitutive relationships for unsaturated soils into a coupled heat transfer and water flow model that considers time-dependent, nonequilibrium water phase change and enhanced vapor diffusion. After calibration of key parameters using a tank-scale heating test on compacted silt, the subsurface response during 90 days of heat injection from a geothermal heat exchanger followed by 90 days of ambient cooling was investigated. Significant decreases in degree of saturation and thermal conductivity of the ground surrounding the heat exchanger were observed during the heat injection period that were not recovered during the cooling period. This effect can lead to a greater amount of heat retained in the ground beyond that estimated in conduction-based design models.

The stress-dependent curve of shear modulus degradation with increasing shear strain amplitude is a fundamental mechanical property of soils. Although it is well known that the degree of saturation has an important impact on the small strain shear modulus of unsaturated soils, its role on the shear modulus evolution with strain has not been thoroughly investigated. A testing program has revealed strong correlations between two key parameters of the shear modulus degradation curve, the reference strain and the coefficient of curvature, and parameters of the soil water retention curve (SWRC). An SWRC model capable of distinguishing between soil water in the capillary and adsorption regimes was used to correlate the reference strain to the maximum adsorption water content and pore size distribution of a soil, and to correlate the curvature coefficient to the maximum adsorption water content. A hyperbolic equation for the shear modulus reduction curve using these correlations shows good performance in predicting the shear modulus under unsaturated small or finite strain conditions. The new model was validated using the shear modulus reduction curve of independent data sets measured at different shear strains.

The lowest matric potential is an important soil property characterizing the strength of retaining water molecules and a key parameter in defining a complete soil water retention curve. However, the exact value of the lowest matric potential is still unclear and cannot be measured due to the limitation of current experimental technology. In this study, a general theoretical framework based on metadynamics was proposed to determine the lowest matric potential in quartz minerals. The matric potential was derived from partial volume free energy and can be further calculated by the difference between the adsorption free energy and self‐hydration free energy. Metadynamics was employed to enhance molecular dynamics for determination of the adsorption free energy. In addition to the water‐mineral interaction, the adsorptive water layer structure was identified as an important mechanism that may lower the free energy of water molecules. The lowest matric potential for quartz mineral was found as low as −2.00 GPa.