Mechanism of bond failure and load distribution along fully grouted cable-bolts

Abstract

The mechanism of bond failure in cables of conventional and of modified geometry has been investigated through laboratory tests under constant confining pressure. The axial load and radial dilatation due to axial cable displacement were determined from the tests. Bond failure in conventional cables is 'cable untwisting', whereas 'grout shearing' is the dominant mode in modified geometries. Higher axial load and radial dilatation are generated in the modified cables than in conventional cables as a consequence of the bulge structure present in the cable. The greater dilatation generated produces higher radial pressure, depending on the degree of confinement provided by the rock mass. Since the load in the cable is strongly dependent on the magnitude of this pressure, such behaviour makes modified cable an excellent choice in mining situations where a considerable amount of mining-induced stress relaxation may occur.On the basis of the laboratory results models are introduced to simulate properly the bond failure mechanism in the cable. These are integrated, with the use of a finite-difference algorithm, to calculate the load distribution along a long bolt—the primary interest of the mining engineer—which is determined for continuous and discontinuous rock-mass displacement cases. Under identical conditions higher axial load will be generated in the bolt if the rock displacement is caused by the opening of few cracks. This suggests that the behaviour of a fully grouted bolt is a characteristic not only of the bolt and the grout but also of the rock-mass deformation.