The Effect of Pre-heating Treatment and Water–Cement Ratio on the Shearing Behavior and Permeability of Granite–Cement Interface Samples

The shearing behavior and permeability of the interfaces between rock and cement are critical for the stability analysis of supporting structures in underground engineering projects, especially after exposure to thermal loads. In the present study, an advanced test method was proposed to preform shearing tests on granite–cement interface samples after pre-heating treatment. The samples consisted of two semi-cylindrical parts. The first part was granite, and the other was cement with two different water–cement ratios (for example, 0.3 and 0.5). The shear stress–strain curves, peak shear strength at shear failure, as well as the residual shear strength at the residual shear stage, cohesion, and internal shear angle, were analyzed. The results were correlated to the pre-heating temperatures and cement–water ratios, while the initial permeability before shear loading, along with the permeability evolution during the shearing process, was also analyzed. It was found that after higher pre-heating treatment the lower the peak shear strength, cohesion, and internal friction angle of the samples would be. Meanwhile, the initial permeability was higher. In addition, during the shearing process, the shear stress and permeability levels increased rapidly, reaching maximum values when shear failure occurred. Then, the shear stress and permeability levels decreased gradually to stable values at the residual shear stage. The degradation in the shear strength, cohesion, and internal friction angle caused by the pre-heating treatment may be attributed to the microcracks induced by the thermal expansion differences between the granite and cement and the evaporation of the chemically bound water in the cement. The shearing behavior and permeability evolution of the granite–cement interface samples were determined to be closely related to the water–cement ratios. For example, under the same normal stress and pre-heating temperature conditions, when the water–cement ratio was higher, the porosity of the cement was greater and the adhesion between the granite–cement interfaces was lower. Therefore, the peak shear strength at the shear failure point was also lower. In addition, the shear stress–strain curves showed stronger ductile behavior, and the cohesion and internal friction angle were lower. Meanwhile, the reductions in the shear stress and permeability levels at the residual shear stage became smaller.