| Summary: | 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 <i>σ</i><sub>2</sub>. As <i>σ</i><sub>2</sub> increases, the biaxial compressive strength <i>σ</i><sub>bcs</sub> of rock increases initially and subsequently decreases. The lateral strain ε<sub>2</sub> of rock under biaxial confinement is controlled by both <i>σ</i><sub>1</sub> and <i>σ</i><sub>2</sub>, and the restrain degree in the development of microcracks and the constrain extent in the expansion along the direction of σ<sub>2</sub> are both enhanced gradually with increase in <i>σ</i><sub>2</sub>. 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 <i>σ</i><sub>2</sub> 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 <i>σ</i><sub>2</sub> has a positive effect on the mass ratio of large size fragments after rock failure. An exponent relationship between the <i>σ</i><sub>bcs</sub> and <i>σ</i><sub>2</sub> was found, and the inner apices–inscribed Drucker–Prager criterion can be used to predict the <i>σ</i><sub>bcs</sub> of rock.
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