Interaction of grouted rock bolts and hard rock masses at variable loading in a test drift of the Kiirunavaara Mine, Sweden

This paper describes the interaction between a reinforced drift and the surrounding rock. It attempts to explain the support mechanisms and characteristics of cement grouted rock bolts, as used in hard rock mining and underground construction. A 20 m long section of a test drift at the 514 m level in the Kiirunavaara Mine, was instrumented with 20 rock bolts mounted with strain gauges and linked to a data logger, and 50 bolts combined with single-anchor extensometer. Rock movements were measured with 23 borehole extensometers, 45 convergence measurement stations, and precise levelling. 14 stress monitors recorded changes in the tangential stresses in the surrounding rock mass. As mining with sublevel caving progressed past the test drift, local stresses in the surrounding rock mass changed. The variable loading imposed on the drift was monitored in the rock bolts, and the reaction of the rock mass was recorded simultaneously with the rock deformation instruments, for a period of 20 months. The bolts carried load at discrete points, which in most cases can be related to intersections with discontinuities in the rock mass. A number of rock bolts indicated minor displacements and strains. Displacements of the periphery of drift were about 5 mm, and changed direction while mining progressed. Tangential stresses in the rock varied between ± 10 MPa. Field measurements of changes in displacements and tangential stresses induced by progressive mining are in fairly good agreement with results of a finite element analysis.