Effect of thermal treatment and associated mineralogical transformation on the soil-water retention curve and unsaturated characteristics of expansive soil

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Expansive soils are spread across wide areas globally and require attention due to volume changes (swelling and shrinkage) induced by moisture fluctuations. Thermal treatment of these soils has been attempted to suppress their volume changes and render them safe for construction. Many geotechnical and geoenvironmental applications require a comprehensive understanding of the retention capacity and unsaturated characteristics of these materials to conduct advanced modeling and design based on unsaturated soil mechanics. In this study, the retention capacity and unsaturated functions of expansive soil treated at thermal gradients of 200, 400, and 600°C were examined and compared to natural specimens. Soil-water retention curves (SWRC) were measured in the laboratory using the axis-translation technique (ATT) and the vapor equilibrium technique (VET) across the entire suction range. Tests were conducted on specimens subjected to these temperatures for durations of 15, 30, and 120 min. An appropriate model was applied to fit the experimental data, allowing the retention capacity and unsaturated characteristics to be established and evaluated. The results indicate a significant reduction in retention capacity as thermal levels increase up to 600°C; the shape of the SWRC also shows thermal dependency in the low-to-moderate suction ranges. Finally, a new 3D response surface model is proposed to accurately predict SWRCs across a wide range of temperatures. The output of this research significantly benefits practitioners designing safe, advanced geotechnical and geoenvironmental systems. It also predicts soil behavior when exposed to extreme thermal sources like fires, heavy industry, and nuclear waste repositories.