Fracture mechanism of naturally cracked rock around an inverted U-shaped opening in a biaxial compression test

To determine the fracture mechanism of the hard and brittle surrounding rock containing natural cracks, a biaxial compression test was performed on a rhyolite, which was extracted from the deep buried Niba Mountain of the expressway in Sichuan Province, China. An inverted U-shaped opening was machined in the specimen to investigate the initiation, propagation and coalescence of cracks around the opening. The fracture modes, tangential strain and principal strain around the opening were monitored and discussed. According to the measured strains in different locations of surrounding rock, the floor was firstly tensile fractured and the roof followed. The "dumbbell-like" compressive strain concentration was found in the sidewall. However, the intense influence region of stress was limited in the shallow rock which was about 0.5r in this experiment, since the deep surrounding rock was in the state of slight compression and tension. Considering the fracture process, "V-shaped" shear wedge was formed to lead to shear slipping under the "dumbbell-like" compressive strain concentration in the sidewall. It indicated that the pre-existing natural cracks in the hard and brittle rock played an important role in consuming strain energy by initiating and propagating during biaxial compression. Slight fracture-induced damage, shear slipping in the sidewall, was occurred instead of common splitting and rockburst. On the basis of the experimental achievements, it was important to strengthen the shallow surrounding rock and prevent the hysteretic failure in the fractured sidewall of the inverted U-shaped opening.