Experimental and Numerical Investigations on Fully Grouted Instrumented Rock Bolts Subjected to Pull Load

Rebar type rock bolts of 22 mm diameter and length of 600 mm were instrumented with four strain gauges along the length. The embedded length of the bolt was 450 mm grouted with resin inside a cement mortar cylindrical sample of diameter 250 mm. The diameter of the drill hole was varied as 32, 36 and 42 mm. The load-deformation relationships and variation in axial strains was measured with the pull load until debonding occurred. The bond strength and stiffness data revealed that hole diameter should be kept as low as possible for better performance of bolt. The axial strains at different locations along the bolt rod are analyzed and a modified equation based on Farmer (1975) is presented. Numerical studies on the axisymmetric models of the above experiments reveal the debonding phenomena between the bolt-grout and grout-rock interfaces which are evident from the experiments. A cohesion reducing constitutive model in the interfaces is applied to simulate the debonding with applied load and are explained in the paper. 1. INTRODUCTION Rock bolt is the key reinforcement member used to provide support to rock strata in tunnels, underground mine drives/galleries and other excavations. A bolt enhances the mechanical properties of rock masses and reduces deformation and support time (Pinazzi et al., 2020). A fully grouted rock bolt generates shear stress along the interfaces of grout-bolt and grout-rock and resists the movement of rock by increasing the overall stiffness of the bolted zone (Cai et al., 2004; Deb & Das, 2010). Every year millions of rock bolts are installed worldwide to support and stabilize engineering structures (Benmokrane et al., 1995). On other hand, the mechanics of shearing along the interfaces of bolt-grout-rock due to pull load was investigated by a few researchers. These studies showed that in the absence of rock movement, shear strain along the bolt rod varies non-linearly having the maximum value at the load side and near zero value at the other end (Farmer, 1975). Li & Stillborg, 1999 showed a similar distribution of strain development on the bolt rod considering the deformation of rock. Since then, several researchers attempted to simulate the shearing mechanism of bolt-grout and grout-rock interfaces using experimental and numerical models. A significant contribution was made to understand and evaluate the mechanism of bolt-grout interfaces (Hyett et al., 1996). Moreover, the extended finite element method (XFEM) was also proposed for analyzing the behavior of grouted rock bolts and applied for simulating reinforcement around tunnels and slopes (Deb and Gujjala, 2018). This experimental study evaluated the effects of grout, hole diameter and rate of loading on bond strength, stiffness and distribution of axial strains along the bolt rod.