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This study investigated the macroscopic physical and mechanical properties of Guiyang red clay during surcharge loading, lateral excavation and lateral unloading with axial loading, and clarified the failure mechanism of microstructure before and after shear under different stress paths of CTC, RTC and TC. Consolidated undrained triaxial shear permeability, SEM scanning, XRF fluorescence spectrum analysis and XRD diffraction tests were conducted to simulate the actual engineering conditions. The stress–strain curve, shear strength, pore water pressure variation rule and macroscopic failure mode of soil samples under different stress paths were analysed. In addition, Image Pro Plus 6.0 and PCAS were used to study the relationship between the macro mechanical properties and micro microstructure failure under different stress paths. The stress–strain curves from CTC, RTC and TC in CU tests were different, with the peak values of shear stress under the three stress paths being P-increasing, equal P-path and P-decreasing path. Moreover, the internal friction angle and cohesion of the increasing P path were higher than those of equal P path and decreasing P path, hence, the influence of stress paths on the cohesion is greater than that of internal friction angle. The pore water pressure is strongly dependent on the stress path, and the variation characteristics of pore water pressure are consistent with the change in the law of the stress–strain curve. Under the same confining pressure in the P-increasing path, the shear failure zone runs through the whole soil sample, and the shear failure zone is significant, whereas under the condition of the P-reducing path, the shear failure angle of soil sample is about 65°, 55° and 45°, and in the equal P path, the soil sample is dominated by the confining pressure, with no obvious microcrack on the surface of the soil sample. The difference is that the distribution of pores in the path of increasing P and equal P is directional, and the anisotropy rate is small, while the distribution of pores in soil samples with shear failure and before shear is random and the anisotropy rate is high.

A good understanding of thermal effect on physical properties (especially the permeability) of host rock is essential for a high-level radioactive waste (HLW) deep geological repository. In this study, we carried out comprehensive tests (permeability measurement, p-wave velocity measurement, mercury intrusion test, and nuclear magnetic resonance [NMR]) to study the macroproperties and microstructures of Beishan granite after different temperature treatments. We found the permeability decreased after treatment at 300°C then increased sharply with increasing temperature as temperature exceeded 400°C. Results from the mercury intrusion test and NMR measurement showed that the pore size and pore volume both decreased for granite after being treated at 300°C, which indicated that pores closured around 300°C for Beishan granite. From 400°C upward, the pore size and volume gradually increased with temperature. Based on the relationship of measured T2 data from NMR with Pc curves from mercury intrusion measurement, the synthesized Pc curves was constructed using NMR-measured data of thermally treated Beishan granite. The fractal dimension of pore structure was studied based on pore size distribution from NMR data. Consistent with the evolution of permeability, the fractal dimension decreased for granite after treatment at 300°C, then increased with temperature from 300°C to 700°C. Because the fractal dimension of thermally treated granite is effective in characterizing the variation of pore structures, a semiempirical formula is proposed to predict the permeability using the fractal dimension and porosity.