Investigation on fracture mechanism of anchored rock with anchor defect and existing joint under uniaxial compression based on acoustic emission

The anchor defects weaken the anchoring effect on jointed rock mass. In order to investigate the influence of anchor defects on the anchoring effect of joint specimens, uniaxial compression tests were conducted on rock specimens with different anchor defects. Combining with acoustic emission, distributed fiber optic, and Digital Image Correlation (DIC) monitoring technologies, the acoustic emission information of the bolted rock specimens at different stages of fracture evolution was analyzed to explore their precursor characteristics. The results indicate that the anchor defects reduce the uniaxial compressive strength, peak strain, and acoustic emission energy release of the bolted rock joint specimens. In the later stages of loading, both high and low-frequency signals of main frequencies of acoustic emission increase in fully anchored rock joint specimens. In contrast, main frequencies of acoustic emission of rock joint specimens with anchor defects primarily increase in high-frequency signals. Moreover, rock specimens with intermediate defects show higher frequency bands in main frequencies of acoustic emission when compared to rock specimens with defects at the ends. The fully anchored rock joint specimens experience macroscopic shear failure along the joint,while the bolt on the rock specimens with intermediate anchor defects result in a delayed crack control effect,leading to macroscopic tensile failure along the joint. When the anchor defect is on one side of rock specimen, the deformation of the bolt on that side is greater than that of the symmetric side. The antiwing crack at the joint tip deviates towards the side with the anchor defects and penetrate with the anchor defects,eventually leading to specimen instability and failure. It can be served as the precursor information for the unstable fracturing of rock specimens with anchor defects through continuous high-energy acoustic emission, an increase in the main frequency range of acoustic emission peak distribution with a decreasing trend, and a continuous and fluctuant decrease in the b-value of acoustic emission. The research findings can provide constructive reference for grouting reinforcement of bolts in jointed rock masses.