Shear behavior of bolt-grout interfaces under constant normal stiffness: Influence of grout strength and thickness

The strength and thickness of the grout significantly influence the behavior of the bolt-grout interface (BGI). Understanding how these factors affect load transfer and failure mechanisms is crucial for designing stable and effective rock mass reinforcement systems. This study systematically analyzed the effects of grout strength and thickness on the shear behavior of BGI through shear tests under constant normal stiffness conditions. The results showed that grout strength is a crucial factor affecting the shear strength of BGI. When the grout strength decreased from 42.4 to 12.5 MPa, the peak shear stress of BGI decreased by up to 63.29%, and the ultimate shear stress decreased by 65.56%. Additionally, changes in grout strength directly affected the shear failure modes of BGI. Under high-strength grout conditions, dilational slip failure (DSF) was predominant, while under low-strength conditions, sheared-crush failure, and sheared-off failure (SOF) were more common, indicating that lower-strength grouts are more prone to shear failure and interface separation. In contrast, grout thickness had a relatively minor impact on the shear strength and failure mode of BGI. Increasing the thickness from 3 to 12 mm resulted in minimal changes in shear strength, suggesting that thickness is not a dominant factor in the shear performance of BGI. The failure modes of the BGI can be profoundly characterized by acoustic emission (AE) parameters. As the failure mode transitions from DSF to SOF, the pronounced increase in AE hits and energy, coupled with the upward trends in the singularity index Delta alpha and fractal dimension change Delta f(alpha), suggests a more intricate failure process under low-strength grout conditions.