The length of pre-existing fissure effects on the dilatancy behavior, acoustic emission, and strength characteristics of cracked sandstone under different confining pressures

It is significant to study the dilatancy behavior and strength characteristic of cracked rock to ensure the stability and safety of engineering rock mass. Consequently, the effects of the length of pre-existing fissures and confining pressure on the strength characteristic of sandstone specimens were analyzed, and the dilatancy and acoustic emission (AE) behaviors of sandstone specimens with different lengths of pre-existing fissures under different confining pressures were discussed. The rationality of the stress of dilatancy onset can be as the strength parameter of rock is explained from the cohesion and internal friction characteristics of rock material and its dilatancy behavior. The results show that the strength parameters of sandstone specimens are negative related to the length of pre-existing fissures, but positive related to the confining pressure. The difference between the stress of dilatancy onset and peak strength decreases with the increase of length of pre-existing fissures, but increases with confining pressure, which is related to the internal friction characteristic of rock material. Under the condition of uniaxial compression or low confining pressure, the obvious stress drop easily occurs after the dilatancy onset, and the frequency of this phenomenon is positive related to the length of pre-existing fissures. However, this phenomenon gradually decreases with the increase of confining pressure, it correspondingly transforms to the emergence of yield platform. No matter what occurrence of stress drop or yield platform, all the cracked rock specimens show the special characteristics of drastic increase of volume dilatancy, active AE signals, and full evolution of cracks. After the dilatancy onset, it seems to be more insecure from the way to obtain the higher strength parameters by the frequent occurrence of cohesive particles fracture in rock, and the rock material can still rely on crack friction to load. In addition, from the dilatancy behavior of rock material, it seems that the large deformations always appear after the dilatancy onset, which is more likely to cause the potential engineering danger.