Effective Stress Evaluation of the Thermal Volume Change of Swelling Soils

A high-temperature (up to 200°C) and high-pressure (up to 20 MPa) thermal triaxial testing device was used to investigate the volume change behavior of compacted granular MX80 bentonite in constant water content conditions under staged temperature increases from 25 to 150°C after isotropic compression to 2 MPa. Pore air pressure was monitored and changes in degree of saturation and suction were estimated to permit evaluation of the results in terms of effective stress. After an initial thermal expansion during heating, contraction was observed at a transition temperature of approximately 88°C, followed by relatively linear contraction during cooling, with the specimen accumulating shrinkage strain after the full heating–cooling cycle. An effective stress-based interpretation of the results using pore air pressure and relative humidity highlights transition points in the volume change response that can be linked to thermo-elasto-plastic behavior. The recognition of a distinct transition temperature under extreme thermal and mechanical conditions provides new insights into the coupled thermal-hydro-mechanical processes in bentonite. These findings are significant because engineered bentonite barriers in nuclear waste repositories are expected to experience elevated temperatures for extended periods, where volumetric stability directly affects sealing capacity and hydraulic performance. The experimental evidence and effective stress framework developed here contribute unique data and interpretation tools that support constitutive model improvement and strengthen confidence in long-term performance assessment of geological disposal systems.