A new concept of CSS surface for describing the thermo-mechanical volume change of unsaturated bentonite

Investigation on the thermo-mechanical volume change behavior of unsaturated bentonite is of great significance for successful design and construction of geological repositories. In this work, two types of thermo-mechanical volume change tests were carried out on unsaturated GMZ bentonite specimens. Results of temperature-suction controlled compression tests indicated that the virgin compression lines of unsaturated bentonite eventually converged toward that of saturated bentonite. With the concept of critical saturated state (CSS) curve, bilinear normal consolidation lines with consideration of temperature were proposed. Results of thermal loading tests indicated that the effects of over-consolidation ratio (OCR) on the thermal volume change behavior of unsaturated and saturated bentonite were similar, while, the suction effect on the thermal volume change behavior of unsaturated bentonite was different in unsaturated and critical saturated state. Based on the test results, a new concept of critical saturated state (CSS) surface was proposed in the stress space (s-p-T space) for compacted bentonite. In the new framework, the thermo-mechanical volume change behavior of unsaturated bentonite could be well described. The results obtained in this work could provide a new conception for developing the thermo-mechanical constitutive model for unsaturated bentonite.