Mechanical properties and failure mode of sandstone specimen with a prefabricated borehole under true triaxial stress condition

With the continuous demand for mineral resources, the construction design and stability of underground rock engineering have been a frontier research topic worldwide. Stability analysis of underground caverns, tunnels, boreholes, and cavities is of critical significance for underground engineering. To investigate the mechanical properties and failure mode of sandstone specimens with prefabricated cavities, experiments were conducted on sandstone samples with the borehole axis parallel to different principal stresses and for varying borehole depths and diameters using a novel true triaxial geophysical apparatus. Computed tomography (CT) was used to analyze the crack development around the borehole wall. The experimental results indicated that the direction of the borehole and the principal stress axis, as well as the borehole depth and size, significantly affected the strength and damage mode of the rock samples. For a given σ3, with increasing principal stress coefficient β, the strength of the samples with the borehole axis parallel to the different principal stresses first increased and then decreased, and the maximum strength appeared at β=0.5. The highest stability of the rock samples appeared when the borehole was parallel to σ1, and the lowest stability occurred when the borehole was parallel to σ2 or σ3. The strength of the sandstone sample decreased as the borehole diameter increased. Four types of typical damaged modes were observed in the CT scan results. Moreover, the intermediate principal stress had a significant effect on the damage-zone distribution around the borehole. This work can help to understand the damage area and failure characteristics of the borehole surrounding rock, also make a significant contribution to support the design of deep rock engineering.