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.

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.