Study on Transient Unloading Response Characteristics of Brittle Coal Samples Under Different Stress Paths

During the unloading process of engineering excavation, the high energy storage in brittle coal is induced to release, which leads to the failure of surrounding rock. The response characteristics of brittle coal samples under instantaneous unloading are analyzed by numerical simulation of different unloading stress paths in this paper. The results show that under the condition of limited lateral displacement, there is a certain linear relationship between the lateral stress and the vertical stress in a certain range before unloading. With the increase of the level of vertical stress loading, the maximum displacement increases obviously after the instant unloading and the failure is more serious. The critical impact stress value of the brittle coal sample is 17 MPa. Under the condition of equal lateral pressure on both sides, the maximum displacement and the vertical stress increase in the form of quadratic polynomial after instant unloading. The relationship between vertical stress and lateral stress on the critical failure envelope is a fourth-order quartic polynomial. With the increase of lateral stress, the impact liability of brittle coal samples is changed from being controlled by vertical stress to being controlled by lateral stress. Under the condition of unequal lateral pressure on both sides, the burst liability of brittle coal samples after instantaneous unloading is most affected by the vertical stress, followed by the minimum stress, and least affected by the intermediate stress.