Deep geological sequestration is widely recognized as a reliable method for nuclear waste management, with expanded applications in thermal energy storage and adiabatic compressed air energy storage systems. This study evaluated the suitability of granite, basalt, and marble as reservoir rocks capable of withstanding extreme high-temperature and high-pressure conditions. Using a custom-designed triaxial testing apparatus for thermal-hydro-mechanical (THM) coupling, we subjected rock samples to temperatures ranging from 20 degrees C to 800 degrees C, triaxial stresses up to 25 MPa, and seepage pressures of 0.6 MPa. After THM treatment, the specimens were analyzed using a Real-Time Load-Synchronized Micro-Computed Tomography (MCT) Scanner under a triaxial stress of 25 MPa, allowing for high-resolution insights into pore and fissure responses. Our findings revealed distinct thermal stability profiles and microscopic parameter changes across three phases-slow growth, slow decline, and rapid growth-with critical temperature thresholds observed at 500 degrees C for granite, 600 degrees C for basalt, and 300 degrees C for marble. Basalt showed minimal porosity changes, increasing gradually from 3.83% at 20 degrees C to 12.45% at 800 degrees C, indicating high structural integrity and resilience under extreme THM conditions. Granite shows significant increases in porosity due to thermally induced microcracking, while marble rapidly deteriorated beyond 300 degrees C due to carbonate decomposition. Consequently, basalt, with its minimal porosity variability, high thermal stability, and robust mechanical properties, emerges as an optimal candidate for nuclear waste repositories and other high-temperature geological engineering applications, offering enhanced reliability, structural stability, and long-term safety in such settings. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

In the high-level radioactive waste (HLW) deep geological repository, bentonite is compacted uniaxially, and then arranged vertically in engineered barriers. The assembly scheme induces the initial anisotropy, and with hydration, it develops more evidently under chemical conditions. To investigate the anisotropic swelling of compacted Gaomiaozi (GMZ) bentonite and the further response to saline effects, a series of constant-volume swelling pressure tests were performed. Results showed that dry density enhanced the bentonite swelling and raised the final anisotropy, whereas saline inhibited the bentonite swelling but still promoted the final anisotropy. The final anisotropy coefficient (ratio of radial to axial pressure) obeyed the Boltzmann sigmoid attenuation function, decreasing with concentration and dry density, converging to a minimum value of 0.76. The staged evolution of anisotropy coefficient was discovered, that saline inhibited the rise of the anisotropy coefficient (Delta delta) in the isotropic process greater than the valley (delta(1)) in the anisotropic process, leading to the final anisotropy increasing. The isotropic stage amplified the impact of soil structure rearrangement on the macro-swelling pressure values. Thus, a new method for predicting swelling pressures of compacted bentonite was proposed, by expanding the equations of Gouy-Chapman theory with a dissipative wedge term. An evolutionary function was constructed, revealing the correlation between the occurrence time and the pressure value due to the structure rearrangement and the former crystalline swelling. Accordingly, a design reference for dry density was given, based on the chemical conditions around the pre-site in Beishan, China. The anisotropy promoted by saline would cause a greater drop of radial pressure, making the previous threshold on axial swelling fail. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).