Bond strength of grouted cable bolts

Abstract

A conceptual model for fully-grouted cable bolts is presented explaining the development of the bond strength. According to the theory, the bond strengths of bolts is primarily frictional and depends on the pressure at the bolt-grout interface. The pressure increase at this interface is a function of the grout dilation or radial movement caused by the rough surfaces of twisted cables. The formation of this dilational pressure during axial loading is explained using models of elasticity, brittle cracking of grout and dilative bolt-grout interface. The study shows that the mechanical and elastic properties of rock and grout, the geometry of the borehole and the bolt are all relevant parameters in the development of the bolt-grout interface pressure. The theory explains load drop often observed during short bond length pull-out tests as the consequence of unstable crack propagation. It is found that these load drops should disappear if the ratio of rock to grout stiffness exceeds 0.68. It is concluded that the bond strength increases with the stiffness ratio (rock-to-grout), the strength of grout, the friction coefficient between bolt and grout, and decreases with the diameter of the borehole. The results confirmed that grouted cables can develop more bond strength in stiff than in soft rocks and therefore conventional cable bolts may not be effective in soft rocks or disturbed rock masses. The theory presented in this paper constitutes the background for a companion paper on the effect of field stress changes on bond strength.