Role of adsorbed water in the thermo-hydro-mechanical modeling of unsaturated expansive soils

Precise prediction of the thermo-hydro-mechanical (THM) behavior of clay-rich soils with a significant amount of adsorbed water requires explicit consideration of its distinct properties compared to capillary or free water. This research develops a fully coupled THM model that accounts for adsorbed water with higher density than free water, as well as its conversion into capillary water at elevated temperatures. After validating the model against experimental results from the literature, the model is employed to simulate a non-isothermal infiltration test on expansive soil. Parametric studies are conducted to analyze the influence of adsorbed water density and its conversion into capillary water on the THM response of unsaturated expansive soils. Results show that neglecting the higher density of adsorbed water and its conversion into capillary water leads to considerable differences in predicted relative humidity and significantly influences predictions of water content distribution, pore pressure, phase change rate, water intake, and swelling pressure. Water intake differs by up to 33% when adsorbed water density is neglected, while swelling pressure increases by as much as 50% with changes in conversion coefficient. These findings highlight the critical importance of accurately considering the distinct properties of adsorbed water in THM models to improve the prediction of unsaturated soil behavior under non-isothermal conditions.