The zone of disturbance around blasted tunnels at depth

Abstract

Two large-scale field tests in granite for identifying possible excavation-induced rock disturbance have been performed in Sweden as part of the on-going research for developing techniques for disposal of highly-radioactive waste from Swedish nuclear reactors. The tests involved determination of the effect of blasting and stress changes on the near-field rock and have led to a preliminary model of excavation damage. Blasting produces a disturbed zone with enhanced axial hydraulic conductivity from a few decimeters to more than 1 m from the tunnel periphery, depending on the change. The axial hydraulic conductivity is increased by 2–3 orders of magnitude in this zone, a net value being about 10 −8 m/sec in rock with a virgin conductivity of 10 −11−10 −10 m/sec. The increase in conductivity is partly due to stress changes, which raise the axial conductivity also further out in the rock. Such influence may take place to a distance from the tunnel periphery of up to 50% of the tunnel radius and the net axial conductivity of the zone is estimated to be about 10 times higher than the average virgin conductivity if the tunnel is suitably oriented with respect to the fracture sets. This change is associated with a reduction in radial conductivity of around 5 times.