Bolt profile and grout strength influence on shear characteristics of the bolt–grout interface

The bolt–grout interface is a weak link in the anchoring system. Therefore, understanding the load transfer and failure mechanisms of the bolt–grout interface is essential for the design of stability control in rock engineering. This study analyzes the macroscopic and microscopic shear behaviors of two bolt profiles under different grout strengths using laboratory testing and finite-discrete coupling numerical simulations. Experimental results show that grout strength is a key factor influencing the shear behavior of the bolt–grout interface. As grout strength decreased from 42.4 to 12.5 MPa, the initial peak shear strengths of bolt profile 1 (BP1) and bolt profile 2 (BP2) decreased by 40.71% and 56.62%, respectively, while their ultimate shear strengths decreased by 40.49% and 57.19%, respectively. The numerical reproduced the shear process, showing that differences in grout damage around various ribs lead to different grout–rib contact conditions, which in turn result in varying levels of stress concentration at each rib. Under high-strength grout, cracks mainly concentrate within 2 mm behind the ribs, while in low-strength grout, the grout between the ribs is completely damaged. Compared to BP1, BP2 generates fewer cracks in the initial shear stage. For BP2, the smaller rib angle causes the grout to undergo dilational slip along the ribs, resulting in stick-slip behavior under higher grout strength. In BP1, the larger rib angle causes the grout to compress against the ribs, providing greater shear strength.