Dynamic support systems helps reduce stope threats in deep mine
Vale’s Copper Cliff Mine is located within the Copper Cliff Offset in Sudbury and extends about 8km south from the Sudbury Igneous Complex into the footwall rocks. Of all the major geological structures present at Copper Cliff Mine, two structures are known to be seismically active; The 900 orebody (OB) cross fault, which strikes east–west and dips at about 55° towards north and The Quartz Diabase Dyke (trap) located between 100 and 900 OBs striking east–west and dipping steeply towards north.
Rockburst history
A review of the rockburst/seismic event history over the past 13 years at Copper Cliff Mine revealed that there were approximately 40 rockburst/significant seismic event incidents in total that occurred in four different orebodies. Of all these incidents, 35 of them (roughly 87%) occurred within the 100/900 OBs, and the remaining five incidents (almost 13%) took place in the 120 and 880 OBs.
Of all the rockbursts, the 3.8 Mn event that occurred on September 11, 2008 in the 100/900 OBs following a crown blast was considered to be the most significant. Although the location of the major event was on 3050 L in the 100 OB, the damage was extended across nearly a 300m vertical block starting from 2700 to 3710 L.
Approximately 3000 tonnes of material were displaced at five locations on different levels. The damage was mostly associated with either the trap dykes and/or 900 X-fault.
The support system at the damage locations mainly consisted of resin grouted rebars, and mechanically anchored bolts in the back, and anchored mechanical bolts on the walls to 1.5m above the floor installed through #6 gauge welded wire mesh.
At some locations, shotcrete and cable bolts were used as a secondary support system. The installed ground support system was too stiff in nature and it did not provide much yielding capability.
Accordingly, the support system that was employed at the damaged locations was incapable of taking the impact of dynamic loading caused by the 3.8 Mn event.
It should be noted that a central blasting system was used and the Copper Cliff Mine re-entry protocol after major seismic events was followed. No personnel injuries occurred due to these events.
It has been concluded that the trap dyke and the 900 OB X-fault are major contributing factors for elevated seismicity in the 100 and 900 OBs. Since all the stopes along the 900 OB X-fault were mined out on the mining front between 3500 and 3050 L, the natural confinement that the orebody provided to the fault plane was taken out.
As a result, a major displacement might have occurred along the fault-plane and caused the 3.8 Mn event after taking the crown blast in the 94561 stope between 3050 to 3200 L on 11 September 2008. By all means, the crown blast could have triggered the slip and caused the large magnitude event. The 3.8 Mn event was considered to be a result of mining the 94561 stope between 3050 and 3200 L.
Since large magnitude events are associated with damage to underground excavations and the installed ground support systems, mining in the burst-prone ground conditions pose a greater challenge both in terms of safety and production.
The 3.8 Mn rockburst triggered a series of rockbursts within the limits of the 100/900 OBs and caused damage at multiple locations on different levels. In order to rehabilitate all the damaged areas, considerable time and resources were spent, and production was significantly impacted.
Support system
A system was introduced in all the burst-prone areas at Copper Cliff Mine, with a view to minimize or completely eliminate the damage to the installed ground support and/or the underground excavations in the event of future occurrences.
Support elements
Based on the guidelines outlined in the ‘Canadian Rockburst Support Handbook,’ the following ground support elements were identified and used in the burst-prone ground conditions at Copper Cliff Mine.
For walls: 1.95m long FS-46 split sets on a 1.2 × 0.75 m pattern with #4 gauge welded wire mesh, followed by a minimum 76mm thick pass of plain shotcrete, and then 2.3m long modified cone bolts on a 1.2 × 1.8m pattern with #0 gauge mesh straps. The wall bolting was usually extended to the floor level.
For the back: 2.4m resin rebars on a 1.2 × 0.75m pattern with #4 gauge welded wire mesh, followed by a minimum 76mm thick pass of plain shotcrete, and then 2.3m long modified cone bolts on a 1.2 × 1.8m pattern with #0 gauge mesh straps. In addition, 6.3m long twin cable bolts were used in a ramp, where the depth of failure was almost 5.1m from the seismic events. The purpose of the cable bolts was to reinforce the rock mass as well as hold the broken rock mass by anchoring them in the solid ground.
Performance
After introducing the burst-resistant system at Copper Cliff Mine, mining in the 100/900 OB was resumed. Four stopes were mined out successfully without any significant damage.
With the resumption of mining in the 100/900 OB, Copper Cliff Mine once again started to experience elevated seismic activity, particularly while mining the stopes surrounding the trap dyke. Several seismic events/rockbursts, ranging from 1.2 to 2.9 Mn, occurred while mining the 9551 and 9281 stopes.
It was interesting to observe that there was no damage, after the 2.9 Mn event that occurred on February 18, 2009, while mining the 9551 stope.
In fact, the event was located within 20–30m from the top and bottom sills, respectively. This has demonstrated that the rockburst resistant support system that was installed after the large 3.8 Mn event had sufficient energy absorption capacity to withstand the impact of a 2.9 Mn event.
While mining the 9281 stope, the installed burst-resistant support system was repeatedly subjected to seismic event impacts and showed some signs of negligible damage. Although it is difficult to assess the impact of previous seismic events in a quantitative manner, the ground control engineer identifies whether there are signs of support yielding based on their observations, and/or field instrumentation monitoring, if any. If so, it may be prudent to install extra support in an effort to compensate for any potential loss in the safety margin.
Conclusions
Even though many seismic events occurred in the 100/900 OBs while mining in the burst-prone ground conditions, no significant damage was associated with such events after introducing the burst-resistant support system at Copper Cliff Mine. It was evident from the underground observations that a well-designed dynamic support system will cope very well in the event of large and repeated seismic events, by sustaining the impact of dynamic loading with no, or negligible damage to the underground excavations and/or the installed ground support system. Four stopes were mined out successfully without any significant damage after introducing the burst-resistant support system in the areas at Copper Cliff Mine.
Information for this article provided by D. Reddy Chinnasane, Dr. Mike Yao, David Landry and P. Paradis-Sokoloski, of Vale Canada Ltd., Canada.
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