A CONSTITUTIVE LAW FOR BOND FAILURE OF FULLY-GROUTED CABLE BOLTS USING A MODIFIED HOEK CELL

Abstract A series of pull tests were performed using fully-grouted seven-wire strand cable, in which the confining pressure at the outside of the cement annulus was maintained constant using a modified Hoek cell. The bond strength was shown to increase with confining pressure. The associated radial displacements at the outside of the cement annulus were measured by two sets of diametrically opposed strain gauge arms located at the midpoint of the test section. The radial dilations decreased with confining pressure, so that at pressures corresponding to high bond strengths, less than 20 μm of radial deformation was generated after 50 mm of axial displacement. Observations under the scanning electron microscope revealed that the low dilations can be explained by the occurrence of an “unscrewing” failure mechanism along the majority of the test section. The data were used to develop a frictional-dilational model for cable bolt failure in both graphical and mathematical form. The latter is amemable to implementation in numerical programs, and presents an opportunity to incorporate realistic cable bolt behaviour into numerical analyses. From an operational perspective, the miniscule radial dilations induced by bond failure, which are responsible for the generation of radial pressure at the cable-grout interface and the development of frictional bond strength, explain the sensitivity of the bond strength to (i) the grout quality, (ii) the radial stiffness of the borehole wall and (iii) mining-induced destressing.