ABSTRACT
An unusual mining situation required redesign due to regulatory and safety considerations. The mining configuration consisted of quartzite of economic interest, lying at a dip of 65° to 70°, overlain by a shale and underlain by highly-weathered shale, si1iceous and calcareous shale, and 1imestone. The overlying shale was stripped, exposing the quartzite, which was then mined by tunneling into the lowermost exposure, using large blasts to remove the toe. The blast undermined the quartzite unit and forced failure along a single stratigraphic parting. The resulting steep wall was over 60 m (200 ft) high in places, following this single stratigraphic parting with no benches. Because the highly-weathered shale (which behaved as a soil) was known to be unstable at these angles, a facing of quartzite was left in place to provide slope protection and buttress the toe. Over many decades of mining, problems arose when large blocks did not detach from the upper reaches of the wall, the resulting overhang making materials loading potentially unsafe. In other situations, the protective facing failed, leading to slumping of the under1ying shale with consequent production and ore di1ution problems.
The existence of numerous clearly-exposed failures allowed back-analyses to be used effectively. The area was mapped in detail, the geometry of the existing slopes determined, and the existing failures back-analyzed to determine the in-situ strength properties of the quartzite and the highly-weathered shale. Stability analyses of quartzite facing blocks tending to kick out from lateral forces from behind, single-plane slope analyses, and soil testing for Atterberg Limits and shear properties were also completed for the highly-weathered shale. A benching method of mining has been designed, in cooperation with regulatory agencies, to minimize the operational problems and provide stability.
