Fracture and tomography of velocity structures of sandstone under uniaxial loads

The extent of fracture in the loaded rock increases with the increase of the loads, accompanied by changes in velocity structure. To monitor the fracture laws of sandstone under uniaxial loads, the acoustic emission (AE) is adopted. According to the characteristics of the AE signals, the loading process is divided into three stages, and the fracture laws of loaded sandstone at different stages are analyzed. Besides, based on the time-lapse double-difference tomography, the velocity structures between and within each stage are inverted. The results show that within the three stages of the loaded sandstone, the velocity structures of sandstone increase slightly at first, then decrease gradually, and in the end decrease rapidly. Comparing changes of the velocity structures between the segments at each stage, the region increases most at the first stage with the largest increase, while the change laws between the two segments at the third stage are the opposite. In addition, due to the heterogeneity of the rock, the rock blocks in some areas are separated from the loaded sandstone to form a "single body" under the influences of fractures. As the loads continue to increase, the velocity structures will still increase. Based on the time-lapse double-difference tomography, not only the evolution of the velocity structures of the sandstone during the loading process can be reproduced, but also the fracture degree and influence range of the loaded rock at any section can be characterized. The research results may provide some references for the stability assessment and danger warning of the loaded rock mass.