Load Distribution Along Fully Grouted Bolts, With Emphasis on Cable Bolt Reinforcement

Abstract

SUMMARYExplanation for the widely reported observation that fully grouted reinforcement is more effective in hard rock that behaves as a discontinuum than in soft rock is presented. Analytical solutions are presented for the distribution of displacement and load along an untensioned fully grouted elastic bolt, of specified bond stiffness, which is activated during excavation by either a continuous or discontinuous distribution of rock displacement. The results indicate that significantly higher axial loads are developed for the discontinuous case.Since the mechanics of bond failure depend on the type of bolt and grout used, in the second part of the paper a finite difference formulation is introduced and combined with a non-linear model for the bond behaviour of a cement grouted seven-wire strand cable bolt. The results of a parametric study indicate that, because the bond is frictional and depends on confinement at the borehole wall, for the same profile of rock mass displacement lower loads are developed in soft rock. Furthermore, in soft rock, excavation induced stress changes can cause a dramatic reduction in bond strength, so that, even after significant rock mass displacement, the axial load developed is significantly less than the tensile strength of the cable. A combination of these effects can explain why failures of cable bolted ground involve debonding at the cable-grout interface in soft rock, and why instances of cable rupture are confined to hard, blocky rock masses.

Bolt-Grout Interactions in Elastoplastic Rock Mass Using Coupled FEM-FDM Techniques

“…This relationship is a linear function (presumed) of the relative movement between rock and bolt [2,20]. The relationship is expressed as…”

“…For this purpose, bolt is discretized along its length for implementation of finite difference scheme as shown in Figure 4. Hyett et al [2] provided the basic formulation of (14) using the finite difference method. In the following, the significant part of that concept is mentioned for implementation of (14) in FEM procedure.…”

“…The Newton-Raphson method is applied to obtain solution for displacements. In this procedure, for each load step and for every iteration within a load step, Hoek-Brown yield criterion (see (2)) is evaluated at each Gauss point of a rock element with the updated trial stress, as follows.…”

“…The bolt length between the neutral point and the other free end of the bolt (inside the rock mass) is designated as anchor length and the developed shear stress drags the bolt towards the rock mass or, in other words, anchors the bolt into the rock mass. Based on these concepts, shear stresses and axial loads developed in a bolt rod are analytically formulated by many researchers who consider the effect of faceplate at the free end of the bolt [1,2]. Bolt grout interactions around a circular tunnel in Hoek and Brown medium have been formulated analytically considering a bolt density factor [3].…”

“…Bolt grout interactions around a circular tunnel in Hoek and Brown medium have been formulated analytically considering a bolt density factor [3]. In their study, Indraratna and Kaiser [3] demonstrated that due to the installation of bolts, the Hoek and Brown parameters m b , s, and uniaxial compressive strength of intact rock, σ ci , would change to new values m 2 Advances in Civil Engineering used for solving differential equation, which governs the interactions between rock bolt and grout material [2].…”