Safely re-opening a collapsed extraction level drive in a resource-limited environment

Abstract

In May 2016, Henderson Mine was operating at a reduced rate when one of the extraction level drives experienced accelerating convergence in a 30 m section of drift. The converging area contained four drawpoints and an orepass opening. Attempts to address the convergence were unsuccessful and the area was abandoned by constructing bulkheads on each end of the converging zone. In June of that year, accelerating convergence was again seen in the pair of drawpoints just south of the closure area. Given that the drift was closed to the north, the most practical method of securing the area was to stack the drawpoint intersection to the next pillar with muck and pump it full to the back with concrete. Following this, no further convergence of note occurred in the area. Following closure of the area, the mechanism behind the convergence was investigated. A combination of factors including undercutting strategy, draw rates, and geology is believed to have contributed. Following understanding the mechanisms of the closure, time was allowed for the cave to mature while normal draw continued around the area. Only when the cave was believed to be fully developed around the area and the loading mechanism removed, was re-entry considered. In April 2017, work began on re-opening the collapsed portion of the drive. In the past, these projects have been challenging efforts involving large quantities of chemical grout and heavy steel set work. With the operating environment and resources available, this methodology was no longer attractive. A simpler approach needed to be developed. The methodology used was based on short advances with in-cycle support using the tools already on hand for normal repair and support operations. A combination of friction bolts, shotcrete, and inflatable bolts was used, supplemented with cable bolts and cement grout as needed. Work was done as resources could be made available and was stopped any time there was a question as to how to best approach the next advance. This slow and steady approach allowed time for confirmation of the support effectiveness with each advance, as well as allowing other work to continue as required in the mine. The result has been successful re-mining of the drift, including re-opening of the orepass, with no safety incidents, no major stability issues, and with a minimum of impact to the rest of the operation.