Influence of Axial Stress on Borehole Rockburst: Insights from True-Triaxial Tests using Granite Specimens

In this study, structural model tests using cubic granite specimens of 100 mm × 100 mm × 100 mm with a centered horizontal circular hole of 50 mm diameter were used to investigate the influence of axial stress (5–160 MPa) on borehole strainburst under true-triaxial stress conditions. A camera and an acoustic emission (AE) monitoring system were used to capture the failure process. The experimental results indicate that the micro- and macroscopic failures are significantly influenced by the axial stress. As the axial stress increases, the vertical stress increment and the duration of particle ejection (from particle ejection to spalling) and spalling failure (from spalling to rockburst), as well as particle ejection to the occurrence of rockburst tend to decrease first and then increase. However, the cracking mechanisms around the V-shaped failure zone and the features of the trans-granular and the extensile micro-cracking in the cross-section perpendicular to the borehole axis are unaffected by the axial stress. For the specimens tested, the rock failure stresses corresponding to crack initiation, particle ejection, crack damage, spalling and rockburst, as well as the cumulative AE absolute energy during the entire test increase as the axial stress increases from 5 to 100 MPa and then they all decrease as the axial stress continues to increase from 100 to 160 MPa. It was also observed in our tests that an increase in the axial stress suppressed the propagation of radial cracks but promoted the development of axial extensile cracks. As a result, rock fractures were developed more in the axial direction in a smoother and more planar fashion and rock fragments with elongated dimension along the borehole axis were produced in the rockburst process. These results suggest that a higher axial stress enhances axial fracturing or spalling during strainburst.