Mechanism of bolt rib geometry in controlling the brittle-to-ductile failure transition of bolt-grout interfaces

Optimizing the mechanical behavior of the bolt-grout interface is paramount for the performance of fully-grouted rock bolts, with the bolt’s rib geometry being a critical design parameter. However, the mechanism by which rib geometry governs the interfacial failure mode remains unclear. To this end, this study employs a multi-scale approach, combining laboratory pull-out tests with Particle Flow Code (PFC) numerical simulations to systematically reveal the dominant role of bolt rib geometry on the interfacial failure mode from both macroscopic and microscopic scales. Experimental results reveal that as the bolt’s rib spacing-to-height ratio increases, the macroscopic failure mode of the interface undergoes a significant transition from brittle shear failure to highly ductile shear-dilation failure. The experimentally-calibrated PFC model reveals the underlying micro-mechanical mechanism of this transition: (1) in terms of force transmission, a larger rib spacing-to-height ratio causes the force chains to shift from a sub-horizontal orientation to a steeply inclined one, transferring the load more effectively into the deep-seated grout; and (2) in terms of damage mode, the crack propagation path correspondingly shifts from pure shear failure along the interface to a tensile-shear composite failure propagating into the grout. This study establishes that the rib spacing-to-height ratio is a key design parameter for regulating the ductility and post-peak mechanical behavior of the anchored system, offering a clear mechanical pathway for the optimal design of high-performance rock bolts intended for complex conditions such as large deformations or rock bursts.