A simplified model to study the behavior of pre-tensioned fully grouted bolts around tunnels and to analyze the more important influencing parameters

This paper develops an analytical method to obtain distribution of the axial force in the pretensioned fully grouted rockbolts around a circular tunnel. For his purpose, the axial force along the bolt is calculated by the assumption of rigid connection between the bolt and rock mass. To include the effect of relative shear displacement, the condition of end boundaries of bolt is focused on i.e. the reduction of the force on the bolt head is calculated through modeling of the flexibilities of its component (washer, plate …) and tunnel surface. Then, this force is simulated by applying a fictitious compressive force on the bolt head. The distribution of this fictitious force along the bolt is obtained to subtract from that of rigid connection. According to a case study, the theoretical prediction of axial force in the passive cases is comparable to the in-situ measured data. The magnitude of pre-tensioning force, delay in installing of bolts, stiffness of bolt head, and rockbolt density are considered as parameters to perform the sensitive analysis.