Evaluation of a deformation-controlled rock bolt under various tunnelling conditions considering radial-confining-pressure-dependent bond strength

Abstract

Significant rock mass deformation exemplified by squeezing and/or swelling during tunnelling under adverse geological and stress conditions can lead to the failure of rock bolts. To address this issue, a deformation-controlled rock bolt (DC bolt) was developed in a previous study. The DC bolt was designed to accommodate large ground deformations while resisting applied load, thereby controlling the shear displacement at the mortar–bolt interface and ensuring tunnel stability. This study experimentally and numerically investigated the effectiveness of a DC bolt under various rock mass and stress conditions. It considers the evolution of the radial confining pressure acting on the DC bolts during tunnel excavation to evaluate its impact on the bond strength and mechanical behaviour of the anchor–mortar interface. To achieve this, a three-dimensional numerical model that explicitly considers the mechanical behaviour of boreholes for rock bolts was constructed. After evaluating the mechanical parameters of the DC bolt based on a laboratory experiment, a tunnelling simulation was performed considering the deformation of the boreholes and their influence on the bolt–mortar interface strength. The results indicate that the radial confining pressure between the bolt and the mortar increased to more than 5 MPa with the tunnelling depth and progression of excavation, increasing the bond strength and therefore influencing the axial force of the bolts. Furthermore, the degree of influence of the confining pressure on the axial force depended on the stiffness of the rock mass. For rock masses with very low stiffness, the effect of the confining pressure was minimal because the large deformation of the rock mass caused bond failure immediately after installation of the bolt, prior to the increase in the bond strength. Conversely, the effect is pronounced for rock masses with relatively large stiffness values, increasing the axial force by a maximum of 40 % compared to the case without the radial-confining-pressure-dependent shear strength model. This study provides fundamental insights into the design of rock bolts, the mechanical behaviour of which is dictated by the anchor–mortar shear behaviour and failure, under various stress and ground conditions.