The final disposal of high-level nuclear waste in many countries is preferred to be in deep geological repositories. Compacted bentonites are proposed for use as the buffer surrounding the waste canisters which may be subjected to both thermal and hydraulic loadings. A significant increase in the temperature is anticipated within the buffer, particularly during the early phase of the repository lifetime. In this study, several non-isothermal and non-isothermal hydraulic tests were carried on compacted MX80 bentonite. Compacted bentonite specimens (water content = 15.2%, dry density = 1.65 Mg/m³) were subjected to a temperature of either 85 or 150 °C at one end, whereas the temperature at the opposite end was maintained at 25 °C. During the non-isothermal hydraulic tests, water was supplied from the opposite end of the heat source. The temperature and relative humidity were monitored along predetermined depths of the specimens. The profiles of water content, dry density, and degree of saturation were established after termination of the tests. The test results showed that thermal gradients caused redistribution of the water content, whereas thermo-hydraulic gradients caused both redistribution and an increase in the water content within compacted bentonites, both leading to development of axial stress of various magnitudes. The applied water injection pressures (5 and 600 kPa) and temperature gradients appeared to have very minimal impact on the magnitude of axial stress developed. The thickness of thermal insulation layer surrounding the testing devices was found to influence the temperature and relative humidity profiles thereby impacting the redistribution of water content within compacted bentonites. Under the influence of both the applied thermal and thermo-hydraulic gradients, the dry density of the bentonite specimens increased near the heat source, whereas it decreased at the opposite end. The test results emphasized the influence of elevated temperatures (up to 150 °C) on the thermo-hydro-mechanical response of compacted bentonites in the nuclear waste repository settings.

This paper presents a newly designed column-type experimental device for investigating the coupled thermo-hydro-mechanical behavior of expansive soils. Sensors and monitoring instruments facilitated continuous and nondestructive measurements of temperature, relative humidity, water content, and total stresses in both axial and lateral directions at various preselected locations of the device. The design features and calibration of specific accessories are presented. System implementation and validation tests were performed to examine the performance of the device at an elevated temperature. In the context of underground storage of nuclear waste, a test was performed with a compacted Calcigel bentonite-sand mixture (50:50) to investigate the thermo-hydro-mechanical behavior of the material under an applied temperature gradient involving temperatures of 80°C and 20°C at the opposite ends of the sample. The test results highlighted the key features of temperature-driven processes in unsaturated compacted porous media. Thermal loading caused changes in the water content and relative humidity within the sample. During the temperature equilibration period, the thermal dilation phenomenon in the regions closer to the heat source was found to be responsible for the development of axial and lateral stresses in the sample. A longer duration of thermal loading caused volumetric shrinkage of the material, which in turn affected the magnitudes of both axial and lateral stresses.

During the postconstruction phase, the stress equilibrium conditions within the proposed underground nuclear waste repositories may alter because of various factors, such as far-field mining activity, fluctuation of the groundwater table, and seepage erosion. A reduction in the transmitted stress from the host rock may destabilize the isochoric condition, leading to swelling displacement of backfilling and buffer materials. In the current study, the swelling behavior of bentonite-based backfilling materials was studied. Laboratory tests were carried out on compacted bentonite ($\mathrm{dry}\mathrm{density}=1.26\mathrm{Mg}/{\mathrm{m}}^3$ ) and bentonite-sand (70% bentonite, 30% sand, $\mathrm{dry}\mathrm{density}=1.50\mathrm{Mg}/{\mathrm{m}}^3$ ) specimens in a strain-controlled oedometer. First, compacted specimens were hydrated with distilled water under constant volume condition. Next, predetermined displacements were introduced in a step-wise manner that caused expansion of the specimens. The swelling pressures of the specimens were measured under a constant-volume condition at each induced strain. The test results showed that induced strains caused a decrease in the swelling pressures of the saturated backfilling materials. A decrease in the swelling pressure for a maximum induced strain of approximately 1.6% was found to be approximately 19% for the bentonite specimen and approximately 30% for the bentonite-sand specimen. It was shown how the swelling index of backfilling materials can be used to assess the recoverable swelling pressure following any induced strain.