Friction bolt anchored wire rope for rock support in burst-prone ground

CIM Bulletin, Vol. 88, No. 988, 1995

D.D. Tannant and P.K. Kaiser, Geomechanics Research Centre, Laurentian University, Sudbury, Ontario

A support system for large ground convergence conditions or burst-prone ground must contain holding elements (rockbolts, cable bolts, etc.) that are strong yet capable of sustaining large deformations, i.e., they must absorb as much energy as possible. Steel wire rope anchored in a borehole by a friction bolt provides these desired yield characteristics if the factional anchorage is placed deep enough into the rockmass, i.e., in non-yielding rock or beyond the expected depth of rock ejected during a rockburst. Pull-out tests were conducted on wire ropes anchored by inflated Swellex bolts to assess the feasibility of this support concept. The test results demonstrate that a Swellex bolt generates a factional anchorage that allows the wire rope to slide at high loads and to absorb significantly more energy than either a Swellex bolt alone, or a cement-grouted cable. The rope/friction bolt combination can be engineered to provide the desired energy absorption and allowable deformation by varying the installed lengths of the friction bolt and the rope. For yielding rope support, the contact length between the rope and Swellex bolt must be short enough to ensure that the rope can slip well before its tensile capacity is reached. A safe maximum overlap length is about one metre for the wire rope used in this testing. When used as support in the backs of drifts, a swaged button on the ropes end must be used to ultimately mobilize the full rope capacity thereby protecting against gravity-driven falls of ground.
Keywords: Rock mechanics, Frictional anchorages, Rockbursts, Swellex bolts, Cables, Wire ropes, Pull-out tests, Energy absorption.