A comparison of seismic response to conventional and face destress blasting during deep tunnel development

Abstract

Abstract A novel design of development face destress blasting was implemented during the construction of an experimental tunnel at great depth. A second tunnel was developed nearby using conventional blasting as a control. The tunnels were developed parallel to one another and perpendicular to a high sub-horizontal stress. High resolution seismic monitoring was used to record and compare the seismic response generated by each excavation. Analysis of the seismic data from the conventionally blasted tunnel indicated that the seismogenic zone of stress-driven instability extended up to 3.6\xa0m ahead of the face. Destress blasting within the corresponding zone of the adjacent tunnel had the effect of reducing the rock mass stiffness, primarily due to weakening of the pre-existing natural discontinuities. The reduction in rock mass stiffness was inferred from the spatial broadening of the seismogenic zone and associated reduction in the measured spatial density of events, radiated energy and seismic potency ahead of the face. High strain gradients around the unsupported portion of the conventionally blasted excavation were implied by the rate at which the spatial density of seismicity changed with respect to the tunnel face position. In contrast, the change in the spatial density of seismicity around the destressed development face was much more gradual. This was indicative of lower strain gradients in the rock there. A reduction in rock mass stiffness following destress blasting was also indicated by the much wider variety of seismic source mechanisms recorded adjacent to the destressed tunnel. Seismic source mechanisms associated with destress blasting were also more clearly characteristic of compressive overstressing with fracture closure. The source mechanism data also indicated that destress blasting induced instability on all natural joint sets. When compared to conventional development blasting, destress blasting typically reduced violent strain energy release from the rock mass and the associated seismicity, but not always.