Influence of tunnel axis stress on mechanical response and fracture characteristics of rock surrounding tunnel subjected to gradient stress

The excavation of deep underground engineering leads to a gradient stress within surrounding rocks in the radial direction of the tunnel. In this study, we investigated the mechanical response and fracture characteristics of rocks subjected to gradient stress under different axis stresses. During the experimental process, an acoustic emission (AE) system was used to capture the AE signals. The obtained results showed that the influence of axis stress on mechanical response of rock subjected to gradient stress is similar to the test results obtained by classical true triaxial compression tests. When the axis stress is below the critical strength, as the axis stress increases, the characteristic stress increases, resulting in an increase in rock strength. However, once the axis stress exceeds the critical strength, localized failure occurs during the application of the axis stress, leading to a decrease in rock strength. In this case, only part of the cracks coalesces to form oblique macro cracks that do not penetrate the entire rock specimen. The AE signals indicate that the proportion of shear cracks decreases as the axis stress increases. The axis stress significantly suppresses the formation of macro shear cracks resulting from the coalescence of micro tensile cracks.