Effects of encapsulation length and grout stiffness on the load-bearing mechanics of rock bolts

Rock bolts are widely adopted in slope stabilization and underground construction due to their cost-effectiveness and capacity to satisfy demanding requirements for load-bearing and deformation control. Nevertheless, a comprehensive understanding of their mechanical behavior under varying encapsulation lengths and grout compositions remains lacking, primarily owing to the scarcity of systematic experimental investigations and precise internal measurements. To address this gap, this study conducts large-scale pull-out tests following a full factorial experimental design, integrating both external load and internal strain measurements to systematically evaluate the effects of encapsulation length and grout properties on bolt performance. The results indicate that both factors significantly influence the load-bearing response under pull-out conditions. Specifically, longer bolts and high-strength, low-stiffness grouts exhibited greater overall stiffness, thereby enhancing resistance to deformation. Increasing encapsulation length improved ultimate load capacity, reduced unit displacement per unit length, and alleviated stress concentration along the bolt shaft. The influence of grout type was primarily governed by its compressive strength, with softer, lower-strength grouts promoting more uniform internal force distributions along the bolt. Although no significant interaction effect was observed between the two factors, their individual contributions were distinct: encapsulation length exerted a stronger influence on ultimate load capacity, whereas grout type predominantly affected ultimate displacement and shear strength.