The performance of mesh reinforced shotcrete in dynamic loading and bursting conditions at Inco's Creighton Mine.
C.C. Langille Supervisor of Rock Mechanics and Ground Control INCO Limited, Ontario Division, Copper Cliff, Ontario, Canada
S. Espley Rock Mechanics Engineer INCO Ltd, Mines Research, Copper Cliff, Ontario, Canada
D. Thibodeau Rock Mechanics Specialist INCO Ltd, Creighton Mine, Copper Cliff, Ontario, Canada
B.W. Buss Engineering Supervisor INCO Limited, Coleman Mine, Copper Cliff, Ontario, Canada
ABSTRACT: The use of shotcrete as ground support and reinforcement has become firmly established at INCO, Ontario Division Mines. One area of support performance for which there has been little published is the effectiveness of mesh reinforced shotcrete under dynamic loading or bursting conditions. This paper will summarize the authors' observations of shotcrete performance in bursting and dynamic conditions at Inco's Creighton Mine.
Two areas are the focus of observations and field trials. The first are the bottomsill extraction horizons located on the 7200 and 7400 levels. Development of the 7200 sills was completed in 1993. Development on the 7400 extraction sills was initiated in 1994 and continued through 1997, using a similar approach to the 7200 Level development, using the experience gained from the mining of the 7200 Sills. The pillars were designed as yielding pillars with a 1:1 pillar to drift cross-section. The pillars were heavily destress blasted to prevent future "loading-up" of the pillars during progression of VRM mining. The resulting yielded pillars were contained by a support system consisting of Split Set bolts and #4 gauge welded wire mesh. The effect of low to moderate energy release on the current support system maintaining these broken pillars is accentuated by the broken nature of the pillars. Shotcrete has been used successfully in both a reconditioning application and a brow stability application. In this case the support mechanism of the shotcrete is its ability to prevent dilation of the rock joints and stress fractures, creating a reinforced rock arch with very tough, energy absorbing characteristics.
The second area is in the topsill drill horizons on 7000 and 7220 levels. Developing on 7000 Level presented some unique challenges as a result of convergence of the rib pillars in the back of the 14' x 14' sill drifts and bursting associated with high mining induced stress in the hangingwall and footwall abutments while approaching the mining zones from the abutment and during VRM production mining. The initial goals of this shotcrete application were to investigate the replacement of timber square sets, in areas of little or no cohesive strength, with a mesh reinforced shotcrete liner. The drifts are driven under the remnant rib pillars of the 7000 level Mechanized Cut-and-fill unit, with sandfill exposed in the shoulders and in some cases in the back. What little support pressure offered by the timber was removed during VRM blasting when the stope was broken through and resulted in failures of the back above the timber. The shotcrete acts as a "supermesh" to retain and hold loose material and prevent unravelling of the sandfill from around the rib pillar. In the case of the 7220 topsill development, a thin skin of remnant ore was left above the back, and support of the sills was achieved through the use of mesh reinforced shotcrete to reinforce the remnant ore "beam".
Several examples of burst or blast damage being halted or contained at the shotcrete are observed and reported. In both applications, bottomsills and topsills, several goals were achieved and additional challenges identified. The introduction of shotcrete in both applications was successful from a ground support and from an operational perspective and the system of mesh reinforced shotcrete for both applications is now being adopted on a wider scale throughout the Ontario Division.