Practical method for the design of pretensioned fully grouted rockbolts in tunnels

This study develops an analytical approach to quantitatively model the efficiency of the pretensioning of grouted rockbolts in terms of reduction of tunnel convergence. In this study, the distribution of force along the pretensioned fully grouted bolt is calculated by the assumption of a rigid connection between the bolt and the rock mass. A compressive force is then applied to the bolt head on the tunnel surface to consider the shear relative displacement between the bolt and the rock mass. The magnitude of this compressive force is found by modeling the bolt boundaries’ stiffness. The theoretical proposed approach is simplified to be used for practical purposes. The results show that if the stiff end plate is tightened to the bolt head (complete planner contact), the grouting effect of the pretensioned fully grouted bolts on tunnel stability can be neglected.