Mechanism of Structural–Slip Rockbursts in Civil Tunnels: An Experimental Investigation

Structural–slip rockbursts are frequently faced in civil tunnels, which pose huge threats to the safety of personnel and equipment in tunnels. In this study, the core features of structural–slip rockbursts were summarized qualitatively based on the many structural–slip rockburst cases encountered in civil tunnels. Direct shear tests on splitting granite joints were performed to reproduce structural–slip rockbursts in the laboratory, and stress–deformation relationship, sound waves, acoustic emissions (AEs), ejected fragments and failure characteristics of the structural planes were investigated to comprehensively make clear the mechanisms of structural–slip rockbursts. Two generation modes of structural–slip rockbursts were revealed in the experiments. One occurs in the early stage of structural plane slip and shows stress drops resulted from the brittle rupture of asperities on structural surfaces, and the other is that a part of ruptured asperities are gradually pulverized into powder shapes and the powdery debris impels the movement mode of structural planes suddenly from sliding friction to rolling friction, which lead to sudden stress drops and cause the rockbursts. Further, the normal stress effect on the rockburst intensity was discussed. It was found that the larger the normal stress, the greater the intensity of rockbursts. Moreover, the characteristics of rockburst intensities in the tests were analyzed by means of the sound waveforms, and it was induced that the rockburst intensity presents alternating circulation laws. The findings from this study may be valuable for understanding the structural–slip mechanism in civil tunnels.