Mechanical and Acoustic Emission (AE) Characteristics of Rocks under Biaxial Confinements

The surrounding rocks of underground engineering are generally subjected to a biaxial compressive stress condition. The failure properties of rocks under such a stress condition are worthy of being studied to ensure the stability of surrounding rock. This study aims to investigate the mechanical characteristics and acoustic emission (AE) properties of granite, marble, and sandstone in biaxial compression tests. Under biaxial confinements, it is evident that the elastic moduli of the three types of rocks decrease, and the plasticity increases monotonously with the increase of the intermediate principal stress σ2. As σ2 increases, the biaxial compressive strength σbcs of rock increases initially and subsequently decreases. The lateral strain ε2 of rock under biaxial confinement is controlled by both σ1 and σ2, and the restrain degree in the development of microcracks and the constrain extent in the expansion along the direction of σ2 are both enhanced gradually with increase in σ2. The sharp increase points of AE hit and AE count indicate that the failure will occur soon. The AF-RA distribution of AE signals shows that the increase of σ2 causes more tensile cracks in rock. According to the dip failure angle of macro-cracks in rock under biaxial confinement, the failure modes of granite and marble are slabbing, while failure mode of sandstone is shear. In addition, the σ2 has a positive effect on the mass ratio of large size fragments after rock failure. An exponent relationship between the σbcs and σ2 was found, and the inner apices–inscribed Drucker–Prager criterion can be used to predict the σbcs of rock.