Experiment on deformation and failure characteristics of sandstone at different unloading rates

The unloading effect is an important factor for the failure of surrounding rock in deep underground engineering projects, especially under high-stress conditions. To investigate the deformation and failure characteristics of sandstone at different unloading rates, the evolution law of the surrounding rock stress caused by excavation was clarified. Then, the single-side unloading test of surrounding rock at different unloading rates was conducted using a true triaxial rock mechanics test system. During the test, the acoustic emission (AE) signals were monitored using an AE monitoring system. The test results show that excavation disturbance leads to complete single-side unloading at the boundary of the surrounding rock, and partial single-side unloading occurs as the depth of the surrounding rock increases. The ultimate strength of the sandstone specimen decreases as a power function with the increasing unloading rate. The unloading rock mass is mainly subject to shear failure. However, the increasing unloading rate raises the proportion of tension cracks. The sudden high strain rate on the unloading side can be used as precursor information of rock fracture, which can effectively prevent accidents caused by loss of rock bearing capacity. The AE signal is active and releases less energy in the unloading stage. At this time, the internal fractures are in the development stage, and the energy is still mainly accumulated. The unloading and failure are not synchronized but suffer from the hysteresis effect. Additionally, the AE and the strain rate change on the unloading side are consistent. Due to the hysteresis effect of damage on unloading, anchor bolts (cables) should be installed to support the surrounding rock immediately to compensate for the stress loss caused by excavation in engineering practice.