The relation between the soil-water characteristic curve (SWCC) and the soil freezing characteristic curve (SFCC) is investigated based on the similarity between the freezing/thawing and drying/wetting behaviors of soils. The SWCCs of clay and silt are obtained by the pressure plate extractor and vapor pressure method, while the SFCCs are determined from the unfrozen water content measurement using the nuclear magnetic resonance (NMR) and temperature measurement. The pore water potential of the frozen soil is derived from a generalized Clapeyron equation, which addresses the effects of capillarity, sorption, and osmosis, and compared to that of unsaturated soils. Our experimental results show that at low water content the matric potential in the soil saturated with pore water and pore ice is generally different from that in the soil saturated with pore water and pore gas. A series of experiments on the soil samples saturated by NaCl solutions of different concentrations were performed to determine the osmotic potential and the water retention characteristics. Based on the principles of surface chemistry, a relationship between the adsorptive forces and the water film thickness is developed, which is capable of describing the soil water characteristic when the sorption effect is significant. Our results indicate that the freezing-point depression method should be applied with caution in determining the soil water retention characteristics of unsaturated soils at lower water content.

Hydraulic hysteresis, including capillary soil water retention (SWR), air entrapment SWR, and hydraulic conductivity, is a common phenomenon in unsaturated soils. However, the influence of hydraulic hysteresis on suction stress, and subsequently slope stability, is generally ignored. This paper examines the influence of each of these three types of hysteresis on slope stability using an infinite slope stability analysis under steady infiltration conditions. First, hypothetical slopes for representative silty and sandy soils are examined. Then a monitored hillslope in the San Francisco Bay Area, California is assessed, using observed rainfall conditions and measured hydraulic and geotechnical properties of the colluvial soil. Results show that profiles of suction stress and the corresponding factor of safety are generally strongly affected by hydraulic hysteresis. Results suggest that each of the three types of hydraulic hysteresis may play a major role in the occurrence of slope failure, indicating that ignoring hydraulic hysteresis will likely lead to underestimates of failure potential and hence to inaccurate slope stability analysis.

The soil shrinkage curve (SSC) defines a constitutive relationship between soil volume and water content under drying conditions. Nearly all work to date considers a zero volume shrinkage for water content below a certain value such as shrinkage limit, which contradicts some recent experimental evidence of pervasive nonzero shrinkage for clayey soils. This paper uses a soil-water retention (SWR)-based conception to theorize such nonzero shrinkage behavior by considering the mechanical effect of capillary and adsorptive SWR mechanisms. A mathematical equation for SSC in the full water content range is developed, which conceptualizes a linear relation for adsorption shrinkage and a sigmoid relation for capillary shrinkage. The equation is validated with test data for a variety of soils. For given soils, the maximum adsorption water contents inferred independently from the SWR and SSC data are highly correlated, with a factor of 2.47. The analysis shows that the adsorption shrinkage rate is strongly correlated with the SWR characteristics of specific surface area (SSA) and cation exchange capacity (CEC). A universal exponential form between the adsorption shrinkage rate and SSA and between the adsorption shrinkage rate and CEC is discovered for silty and clayey soils.

The pore-water status, including adsorptive and capillary water, in expansive clay with different dry densities was investigated under the effect of temperature and saline solution via the nuclear magnetic resonance technique. Combined with a method of distinguishing the adsorptive and capillary water, the variations in the adsorptive water content with the thermochemical effect were obtained quantitatively. The experimental results show that the adsorptive water content decreases as the temperature increases, consistent with the theoretical conclusion, which is considered to be attributed to the increase of the chemical potential and the activity of the pore water. On the other hand, the volume of micropores decreases while the volume of macropores increases with the increasing temperature or concentration, resulting from the flocculation and agglomeration of soil particles. Meanwhile, the increase of salt concentration can suppress the electric double layer and decrease the micropore volume, resulting in a decrease in the adsorptive water content. In addition, due to ion hydration, the relaxation of water molecules accelerates, shifting the water distribution curves of soil leftward as concentration increases. This effect overshadows the suppression of electric double layers and the decrease of the micropores, rendering higher adsorptive water content. The microstructure and the pore size distribution, obtained by the scanning electron microscope and mercury intrusion porosimetry experiments, were used to validate the variation of the adsorptive and capillary water in the pores.