Experimental Study on the Free-Face True-Triaxial Rockburst Test Considering the Influence of Joints.

To investigate the influence of joints on the rockburst process in the surrounding rock of deep tunnels, a free-face true-triaxial test was conducted on the rockburst of the jointed surrounding rock of a tunnel. The rockburst ejection failure process (i.e., failure mode, rock strength, and deformation characteristics), rockburst debris characteristics, and acoustic emission evolution characteristics of red sandstone specimens under different joint conditions were investigated. The experimental results indicate that the failure modes of red sandstone specimens with a joint condition perpendicular to the tunnel direction generally exhibit 'internal shear and external splitting', which can suppress the occurrence of rockbursts, whereas the failure mode of red sandstone specimens with a joint condition parallel to the tunnel direction is mainly shear fracture, which can also promote the occurrence of rockbursts. The variation trend of the peak strength and the joint angle of the specimen exhibits a 'U' type shape, and the debris pertaining to the joint specimen's rockburst exhibits fractal characteristics. With the increase of the joint inclination angle, the fragment's ejection kinetic energy exhibits a trend of decreasing first and subsequently increasing. There are significant differences in the total number pertaining to impacts of acoustic emissions during rockburst under different joint conditions. The distribution range pertaining to the dominant frequency signal of the acoustic emission signal at the time of rockburst is mainly a low-frequency signal, and the intermediate-frequency, high-frequency, and ultra-high-frequency signals also exist, thus indicating that the rockburst damage exhibits strong complexity. Finally, the strain rockburst mechanism of jointed rock is significantly different from that of jointless intact rock. The rockburst mechanism is mainly divided into two typologies: shear fracture and slab buckling-shear fracture. The research results contribute to a deeper understanding of the underlying mechanisms of rockburst disasters in jointed rock formations. Highlights: Successfully performed a free-face true-triaxial rockburst test in the jointed surrounding rock of a tunnel. The presence of different joints under true-triaxial conditions significantly alters the mechanism of strain-induced rockburst. Joints perpendicular to the tunnel direction reduce the occurrence of rockbursts, whereas joints parallel to the tunnel direction promote the occurrence of rockbursts. With the increase of the joint inclination angle, the fragments' ejection kinetic energy exhibits a trend of decreasing first and subsequently increasing, where the joint inclination angle of 30° is the transition's inflection point. [ABSTRACT FROM AUTHOR]