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Key engineering properties of unsaturated soils such as volume change and shear strength can be defined using the effective stress principle. Several problems like prolonged drought, high-level radioactive waste, buried high voltage cables can subject surface and near-surface unsaturated soils to elevated temperatures. Such elevated temperatures can affect the hydraulic and mechanical behavior of unsaturated soils. It is very important to develop a closed-form model that can reasonably estimate the effective stresses under different elevated temperatures. For this purpose, the current study incorporates the temperature effect into a suction stress-based representation of Bishop’s effective stress. The proposed model accounts for the effect of temperature on matric suction and degree of saturation. A temperature-dependent soil water retention curve is used to account for thermal effects on surface tension, contact angle, and enthalpy of immersion per unit area. The proposed effective stress model is then used to calculate the effective stress for two soils, Pachapa loam, and Seochang sandy clay, at various temperatures ranging from 25°C to 100°C. The validity of the model is examined by comparing the predicted effective degree of saturation and suction stress values against the experimental data reported in the literature for GMZ01 bentonite. At a constant net normal stress, the results for both soils show that the impact of temperature on effective stress can be significant. The proposed model can be used for studying geotechnical and geoenvironmental engineering applications that involve elevated temperatures. 

Summary This study presents a thermo‐hydro‐mechanical (THM) model of unsaturated soils using isogeometric analysis (IGA). The framework employs Bézier extraction to connect IGA to the conventional finite element analysis (FEA), featuring the current study as one of the first attempts to develop an IGA‐FEA framework for solving THM problems in unsaturated soils. IGA offers higher levels of interelement continuity making it an attractive method for solving highly nonlinear problems. The governing equations of linear momentum, mass, and energy balance are coupled based on the averaging procedure within the hybrid mixture theory. The Drucker‐Prager yield surface is used to limit the modified effective stress where the model follows small strain, quasi‐static loading conditions. Temperature dependency of the surface tension is implemented in the soil‐water retention curve. Nonuniform rational B‐splines (NURBS) basis functions are used in the standard Galerkin method and weak formulations of the balance equations. Displacement, capillary pressure, gas pressure, and temperature are four independent quantities that are approximated by NURBS in spatial discretization. The framework is used to simulate strain localization in an undrained dense sand subjected to plane strain biaxial compression under different temperatures and displacement velocities. Results show that an increase in the displacement rate leads to reduction in the equivalent plastic strain while an increase in the temperature leads to an increase in the equivalent plastic strain. The findings suggest that the proposed IGA‐based framework offers a viable alternative for solving THM problems in unsaturated soils.