Study on anchoring performance and failure mechanism of rock bolts under different stiffness conditions of surrounding rock

This study investigates the anchoring performance and failure mechanisms of rock bolts under varying surrounding rock stiffness conditions. Sleeves of different specifications were used to simulate the stiffness environments of hard rock, medium-hard rock, and soft rock. Bolt pullout tests were conducted under these conditions, with synchronous monitoring using acoustic emission(AE), strain gauges, and digital image correlation(DIC) technology. The effects of surrounding rock stiffness on bolt anchoring strength, stiffness, and failure modes were analyzed, revealing the AE characteristics, shear stress distribution at the anchoring interface, and the evolution of circumferential and axial strains in the sleeves during the pullout process. The results show that as surrounding rock stiffness decreases, both the anchoring strength and stiffness of the bolts diminish, with their values under low-stiffness conditions being only 65% and 44% of those under high-stiffness conditions, respectively. Under high and medium-stiffness conditions, anchoring failure is primarily characterized by slip at the bolt-grout interface, whereas in low-stiffness conditions, slip predominantly occurs at the grout-rock interface. During the pullout loading process, specimens exhibit four distinct stages: elasticity, pre-peak plastic yielding, post-peak softening, and friction-sliding. Lower surrounding rock stiffness results in a more pronounced pre-peak plastic yielding stage. Interface debonding initiates near the peak shear stress and is located slightly below the bolt's midpoint. The lower the surrounding rock stiffness, the closer the debonding initiation point is to the midpoint of the bolt. As pullout displacement increases, the circumferential strain of the sleeve initially rises, then decreases, and finally stabilizes. Axial strain patterns in the sleeve show compression at the bottom and tension at the top, with peak strains located at both ends. The upper tensile region extends downward as pullout displacement increases, and axial strain magnitudes grow as surrounding rock stiffness decreases. In the friction-sliding stage, both anchoring interfaces are fully debonded, and the pullout force stabilizes, sustained primarily by frictional forces. These findings offer valuable insights for the design of rock bolt support systems under varying surrounding rock stiffness conditions.