Use of cable bolts to reinforce the hanging pillars and improve the ore recovery when stopes are mined using double top sills at Vale’s Copper Cliff Mine

Abstract

There is variability in the geometry of the ore outline, particularly between the footwall (FW) and hanging wall (HW), across the strike length of the various orebodies at Copper Cliff Mine. This geometry creates a requirement to plan transverse sills on one side of the orebody and longitudinal sills on the other side of the orebody on certain levels. In fact, there have been instances where some stopes have been mined using two sills (either transverse or longitudinal) on the top cut. With Vale's Vertical Retreat Mining Method (VRM), the drilling must be done from both of the top sills using an in-the-hole (ITH) drill. Once the crown part of the stope has been blasted, an undercut pillar or hanging pillar, i.e. a pillar without any support beneath it, is created. Without any additional reinforcement installed into the hanging pillar, there is a high probability that the hanging pillar itself will fail simply by falling into the open stope as a result of the lack of confinement. The sudden failure of a hanging pillar can lead to additional instability as a result of the excessive span across the open stope. The failed material can also lead to operational issues such as a choked drawpoint; in this case, large pieces of ore inhibit mucking in the bottom sill, thus making it difficult to recover all the ore from the stope. Cable bolts have been proven to be one of the most effective reinforcing ground support elements for different applications in underground hard rock mining. Specifically at Copper Cliff Mine, cable bolts are used to pre-support the pillars between the top sills, prior to crown blasting and prior to creating the hanging pillar geometry. Later in the mining cycle, the hanging pillars are further supported by backfill; the fill is placed tight underneath the hanging pillars for long-term, regional support. To date, 10 stopes have been successfully mined in various orebodies at Copper Cliff Mine using cable bolts to support hanging pillars during mining. This paper describes case studies where pillars were undercut, and includes an outline of the methods that were used to mitigate the risks of ground failure. A discussion of the cable bolt design methodology for hanging pillar geometries, and a summary of the benefits achieved in terms of safety, stability and additional ore recovery as also included.

Reduction of ore dilution when mining low-thickness ore bodies by means of artificial maintenance of the mined-out area

“…D.R. Chinnasane, M. Knutson and A. Watt in their studies [36] state that at the Copper Cliff Mine (Canada), the cable bolts are only used to temporarily maintain the minedout space, and then, during mining, the hanging walls should be additionally supported by backfilling. Foreign researchers Hutchinson and Diederichs [37] provide an overview of how and where cable bolts can be used to support, strengthen or hold rock mass around the majority of mine workings in an underground mine.…”