Crack coalescence and brittle failure characteristics of open rock bridges

The coalescence of rock bridge of fractured rock mass with end cracking is the main cause of the sudden damage of the rock mass, and the brittle failure is obvious. The uniaxial compression tests are conducted on the open rock bridges with different lengths by using the acoustic emission tester and the rock mechanics rigidity tester. By use of the information recorded by high-speed camera, the crack coalescence laws and acoustic emission characteristics of open rock bridges are analyzed. The mechanism of crack initiation of open rock bridges is revealed by the theory of three-dimensional fracture mechanics. The results show that compared with the closed crack, the main crack of an open rock bridge starts from the tip of the crack at the lower part, and the main crack turns to the upper part expanding through the rock bridge and the top end. During the coalescence of the main crack, the rock bridge presents successive multi-stage failure. The secondary crack propagates along the inflection of the main crack and extends through the rock bridge and the lower end. The stress curve shows the "fluctuation rise" before the peak and "stagnation-sudden" after the peak, and the "multi-peak" phenomenon of the acoustic emission is obvious, with obvious stage and sudden features. Simultaneously, the three-dimensional theoretical analysis shows that the crack initiation direction is independent of the length of the rock bridge and extends along the direction of the maximum compressive stress. The obtained crack propagation characteristics of the open rock bridge may provide a theoretical basis for the sudden failure of the fractured rock mass.