Experiments and numerical simulations of rockburst prevention based on the slotting method

Rockburst prevention is an urgent task in deep-rock engineering, as support systems alone are often insufficient for resisting high geostresses. Cutting slots in surrounding rock can directly reduce the circumferential stress of a tunnel, but it also destroys rock integrity and reduces rock mass strength, which leads to a contra-indication for using this method to prevent rockbursts. How to balance these two aspects and truly reduce the risk of rockburst is the topic of this study. Rockburst prevention experiments were implemented on both intact and slotted granite rock samples using the “slotting before loading” mode. Considerations included the loading force and deformation by the testing machine, sample surface displacement fields, acoustic emission signals, and fragment ejection energy. These were recorded during experiments and the influencing factors and precursory characteristics of a rockburst analyzed, with the stress-release effects of different slotting schemes also evaluated. The experimental results demonstrated that cutting slots on granite samples under eccentric loading conditions leads to direct damage, regardless of the depth, width, and number of slots implemented. And then, numerical simulation of rockburst experiments were performed based on an improved particle-flow program (PFC2D), in which a weakening mechanism was introduced, resulting in the macro compression-tension ratio (up to 25.8/1) approaching real granite. On this basis, numerical slotting tests were conducted and it revealed that cutting slots could lead to sudden tensile failure or fragment ejection under high eccentric stress. These findings were further validated through a physical experiment. Cutting slots in a tunnel model indicated that the energy released by two slots was 37.7% higher than one slot and triple slots 114.4% higher than one slot under a uniform geostress field of 25 MPa, revealing that the energy released by slotting did not increase linearly with slot number. However, the released energy almost increased linearly with increased slotting depth within a certain range and the lateral stress coefficient had an important influence on energy release. Both physical and numerical tests showed that the cutting-slot method could release strain energy and reduce stress, but appropriate support schemes must be matched with existing conditions when using this method to prevent rockbursts or the process might trigger early rockbursts and run counter to the goal of preventing rockbursts.