2008-04-15_REVISION - M1980244 (197)

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• WHEXSIopeEvaluation AdrianBrown 7. BENCH DESIGN AND CONSTRUCTION 7.1 Single Benching The bench design for the Whitehorse Extension Mine is based on single benching. This method has the following stability, safety, and engineering benefits: 1. It creates a larger number of benches, increasing the probability that catch capacity will be retained in the wall during development. 2. It limits the mean distance a rock can fall to a maximum of two benches (70 feet). This is because there is a finite probability that a bench will be lost at any location, estimated at 15% for the WHEX Mine, because of the evidence of jointsets on foliation (in the granite) and in the rockmass (in volcanics). If a triple bench approach were used, there would be a 15% chance that a rock could fall unimpeded 210 feet, creating significant safety concerns when it Lands. With single benching, there is a 15% chance a rock could fall 70 feet, and a 0.01 % chance it could fall 210 feet unimpeded. This makes single benching a much safer mine wall with respect to preventing falling rock hazazds. 3. The drilling for the single bench system never requires personnel or equipment closer than • approximately 25 feet from the toe of the wall after excavation. This moves surveying, drilling, and blasting personnel away from any location where falling rock can impact operations. 4. The design for excavation of the wall materials is the same for all benches in each wall zone, simplifying design reducing the opportunity for error, and maximizing design flexibility. 7.2 Vertical Bench Faces Vertical inter-bench faces are proposed for the WHEX mine. The reason for this is for engineering and safety: 1. Vertical interbench faces maximize the catch capacity on the bench below the face, as the face does not intrude into the face. 2. Vertical faces do not deflect displaced rocks outwazds as they fall. The vertical bench allows any falling rock to land on the bench essentially vertically, minimizing the probability that the rock will be propelled over the bench and down the slope. 3. Vertical faces are easiest to pre-split or trim blast, as the blast holes are most readily drilled vertically. 7.3 Bench Design Details The bench designs to achieve the proposed slopes are shown in Table 8. • These designs are illustrated in Plate 3. It should be noted that this design are somewhat generic, in that the actual bench geometry will be locally influenced by geology, structure, wall geometry, haul road Report 1385L.200309017 18