The unified effective stress equation based on suction stress, a widely accepted method for calculating effective stress in unsaturated soils, provides a closed-form solution that enables the characterization of soils in both saturated and unsaturated states. The effect of desaturation on the water content of natural and treated soils was studied with respect to unconfined compressive strength (UCS) and indirect tensile strength (ITS). The soil's moisture-dependent behavior was characterized by the van Genuchten (Soil Sci Soc Am J 44:892-898, 1980. https://doi.org/10.2136/sssaj1980.03615995004400050002x) and Lu et al. (Water Resour Res, 2010. https://doi.org/10.1029/2009wr008646) models and implemented using the equation. Suction tests were conducted using the dew point and filter paper methods, alongside UCS and ITS tests, on silty clay soil and microsilica-treated soil with microsilica contents of 5%, 10%, and 15%. The equation was validated by comparing mean total stress (p) and mean effective stress (p ') to deviatoric stress (q) and analyzing the friction angle at different suction levels. It proved applicable to both natural and treated soils, with valid moisture content ranges of 4-17.5% and 6-20%, respectively. This study experimentally confirms the equation's effectiveness in characterizing the hydro-mechanical behavior of soils under varying moisture conditions.

An understanding of the mechanical properties and macroscopic behavior of unsaturated soil can be improved through an in-depth microscopic insight of the variables controlling the soil-water characteristic curve (SWCC). In this study, the effects of the initial conditions on the pore structure and SWCC of silty soil was examined. Their relationships to the soil behavior during water loss was addressed from both macroscopic and microscopic perspectives. In this study, patterns different from those of previous studies were revealed; this especially pertained to the effect of the initial water content on the SWCC. The SWCC was obtained using repeated centrifugation and filter paper tests. Mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) were performed to collect the microstructure information. The results showed that soils compacted using the optimal dry side conditions had double S-shaped SWCC for their bimodal pore size distributions, and these pores were classified as intra-aggregate and inter-aggregate pores. Thus, these soils underwent two distinct stages of water loss during drying, and water loss occurred more easily in the first stage because of the presence of many large macropores or inter-aggregate pores. However, soils compacted at the optimum water content produced a single S-shaped SWCC for the multimodal pore structure. Water drained from these soils at a relatively constant rate from a more homogeneous and uniform pore system. This study has provided a comprehensive set of macroscopic and microscopic experimental data and well-established relationships among the PSD, SWCC and initial state of the silty soil.

Featured Application The findings of this study establish the behavior of sanitary landfill cover materials, such as compacted clay and compacted polyurethane-clay, in unsaturated conditions under several wet-dry cycles, which would aid in predicting the performance of the material under varying environmental conditions. By predicting the unsaturated hydraulic conductivity and understanding the effects of environmental stresses, the findings can aid in the design and implementation of more durable and efficient landfill liners and covers.Abstract Sanitary landfill covers are exposed to varying environmental conditions; hence, the state of the clay layer also changes from saturated to unsaturated. The study aimed to predict the unsaturated hydraulic conductivity of the locally available compacted clay and clay with polyurethane to determine their behavior as they change from wet to dry using matric suction and empirical models proposed through other studies. The specimens underwent three wet-dry cycles wherein the matric suction was determined for several moisture content levels as the specimen dried using the filter paper method or ASTM D5298. The results showed that the factors affecting the soil structure, such as grain size difference between clay and polyurethane-clay, varying initial void ratios, and degradation of the soil structure due to the wet-dry cycles, did not affect the matric suction at the higher suction range; however, these factors had an effect at the lower suction range. The matric suction obtained was then used to establish the best fit water retention curve (WRC) or the relationship between the matric suction and moisture content. The WRC was used to predict the unsaturated hydraulic conductivity and observe the soil-water interaction. The study also observed that the predicted unsaturated hydraulic conductivity decreases as the compacted specimen moves to a drier state.

Soil-fluid retention (SFR) is considered a fundamental unsaturated property with a wide application from transient flow analysis, and strength prediction to constitutive modeling. Previous research has led to a consensus that soil microstructure can significantly affect retention properties. Furthermore, the soil microstructure is subject to changes due to variations in the sampling method and pore fluid chemistry. Therefore, this study aims to explore the influence of multiple factors, including sample preparation technique, dissolved salt concentration and cation type, on soil-fluid retention and volumetric behavior in a systematic manner. The results of tests conducted by employing the axis translation and filter paper methods are interpreted in detail based on some complementary scanning electron microscopy and micro-CT scan experiments. Furthermore, the repeatability of the test results is validated through some control tests. The results reveal a higher retention capacity for reconstituted samples compared with the compacted ones independent of the solute concentration. In contrast, both the SFR and shrinkage are suppressed by the addition of sodium chloride to the pore fluid independent of the sample preparation method. It is also found that the SFR capability of potassium cation is less than that of sodium cation at the same concentration. The SFRs are quantified and compared in terms of air entry/air expulsion suction, hydraulic hysteresis and desorption/adsorption rates. The interpretation of the results in light of the diffuse double-layer theory is supported by microstructural observation and provides insights into the SFR of lean clays exposed to saline environments with different types of salt species.