2008-04-15_REVISION - M1980244 (193)

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L,~ • MainCressonSlopeEvaluation AdrianBrown 5.4 East Wall of South Mine 5.4.1 Joint Sets Structural features observed in the East Wall of the South Mine are presented in Figure 12, and the joint information is contoured in Figure 13. The east wall of the south mine is characterized by the following joint sets: 1. Dip direction variable/90° dip (Variable strike, 90° dip [vertical]). The principal joint set on the east wall is a vertical set whose strike is variable, with a somewhat greater frequency of joints striking east-west. This vertical set is partly the result of columnar jointing in the wall rocks, which causes strike directions to vary. 2. Dip direction 330°/10° dip (Northeast-Southwest strike, 10° dip [almost horizontal]). There is a minor, essentially horizontal joint set with a slight dip to the south-east. Due to the field recording method used, this set may be under-represented in the data; oriented core provides somewhat better representation, and identifies this joint set (Call and Nicholas, 1994). 5.4.2 Fracture Intensity • Based on observations in the field, the mean spacing between fractures in the same joint set is in the order of 1 foot, with major joints and faults (extending more than one bench) spaced in the order of 10-20 feet apart. Rock Quality Designator (RQD) values from core holes drilled into this wall as part of the Call and Nicholas study were about 92% for breccia, and 80% for porphyritic phonolite (Call and Nicholas, 1994). 5.4.3 Structural Stability The structural stability of the wall has been evaluated in Figure 14. This shows the polar domains in which structural failure could occur overlain on the structural information, by type. The structural failure conditions are: 1. Sliding Failure. Sliding failure is unlikely. There have been few observed fractures which have the potential to create a sliding failure on this wall in the upper benches. No such failures have been observed to occur, although the wall is still only approximately 450 feet high, and has not been developed to its full slope. However, the incidence of fractures dipping into the mine at 40 to 60 degrees appears to have increased as the mine has deepened (Figure 5), raising concern about the significance of these features at depth. 2. Toppling Failure. One potential toppling feature has been identified on the east wall of the South Mine. This "toppler" has a dip direction of 75° and a dip of 60° (i.e. dipping 60° with a strike of N15°W), dips into the wall, and contains clay and slickensides. This feature has been monitored since mid-1995, and some movement has been observed. However, there does not appear to be a family of these structures, and the one structure observed does not appear to • extend to depth. Accordingly, this feature is not considered significant for stability at this time. 1385D.980612 27