Experimental study of factors affecting fault slip rockbursts in deeply buried hard rock tunnels

As civil tunnelling and mining have progressed to ever greater depths, the magnitudes of the stresses resulting from these activities have also risen significantly, leading to increasingly frequent excavation-induced seismicity and rockbursts that pose a great threat to workers and equipment on site. Although considerable research effort has been devoted to understanding the factors that influence strain bursts, few studies have addressed the factors affecting fault slip rockbursts triggered by slip on discontinuities such as structural planes. Thus, in the present work, shear tests were performed under constant normal load (CNL) conditions on joints with rough surfaces and interlocked asperities to study their shear behaviour and acoustic emission characteristics. The effects of rock type, normal stress, surface morphology, infilling, and shear history on slip rockbursts were investigated. The test results indicated that slip bursts occur more easily in granite joints because of either sudden and violent post-peak stress drops or stress drops during stick slip. Static shear failure dominated in marble and cement mortar joints, except when the joint surface was extremely irregular, and rockbursts took place when asperities were sheared off or when tensile ruptures occurred on the joint. The value of the stress drop immediately after peak stress and the value of the average stress drop during stick slip of granite joints both increased with normal stress; thus, the probability and intensity of rockbursts rose with normal stress. The failure modes of the joints were strongly influenced by the normal stress level; fillings and previous shearings (i.e. a shear history) reduce the risk of rockbursts because they reduce the amount energy released.