Fracture and evolution characteristics of specimens containing double holes

Crack initiation, propagation and coalescence, induced by mining stress disturbance are the direct causes of engineering geological hazards which posed a great threat to the safety of coal mining and the stability of engineering rock mass. With the aid of Mining-Induced Stress Test System and Acoustic Emission(AE) monitoring system, the uniaxial compression test of prefabricated large-size double-hole specimens were constructed. The field mining height, the borehole diameter and borehole spacing were reduced in equal proportion. The strength, AE characteristics and fracture evolution process of specimens under different conditions were systematically studied and the dimensionless damage risk coefficient was constructed based on the current research. The formula for the theoretical calculation of reasonable borehole spacing for pressure relief is further revised. Results show that: compared with the complete specimen, the peak strength, elastic modulus and crack initiation stress of the double-hole specimens are significantly decreased. Basically the increase of the hole spacing shows an increasing trend first and then it is decreased. The first tensile crack in the specimen with holes, is initiated close to the hole and it is propagated along the direction of the maximum principal stress. The secondary tensile crack and shear crack are produced with the increase of load and finally the tensile failure mode is formed. An obvious corresponding relationship is shown by the stress curve and AE energy curve. The shear crack always accompanied with a large energy emission rate. The specimens show obvious brittle failure, and internal cracks of the specimens create sudden changes in the rate of AE energy curve and energy accumulation curve. The dimensionless damage risk coefficient can quantitatively characterize the stability of surrounding rock, the revised calculation formula accomplished the field requirements. The research results strengthen the understanding of the mechanism of crack initiation and provide theoretical support for improving the reliability of engineering disaster prediction.