2008-04-15_REVISION - M1980244 (192)

Home Browse Search

• GHMXSIopeEvaluation First, only light blasting will be required in the "Very Poor" rock zone (Plate 12). This material is sufficiently friable in core samples to suggest that it could largely be free-dug. Hole loading should be minimized to reduce wall disturbance by bench shots forming the wall. Second, blasting should be performed to minimize disturbance of the final wall, and the final benches. A suggested blast design for final wall and bench locations at GHMX is presented in Plate 13. This design draws on experience in blast testing in the Main Cresson Mine (Itasca, 1999; ABC, 2003), and has been found to be effective in this mining district in highly fractured environments, as well as in intact rock. Features of the blast approach are: 1. Light trim or pre-split blasting against the final wall, with close-spaced holes (maximum 5 feet) to ensure that atrim/split line will develop in all rock types. 2. Two foot sub-drilling at the toe of the slope to prevent the formation of a hard toe, which results in a "wedge" at the base of the toe that can result in any falling rock being propelled laterally out onto the working level. 3. Avoidance of disturbing the next bench crest by: a. Aligning blast hole lines so that the next crest falls equidistant from two blast hole lines. b. Not sub-drilling blast holes on the two lines that straddle the future crest location. • 4. Loading the holes in the vicinity of the final wall and bench as lightly as is consistent with required breakage. Third, the final walls can be formed using trim or pre-split blasting: 1. Pre-split. If pre-split blasting to form the wall is chosen, this row should be shot prior to mining on the level from which it is drilled. Bench mining on the same level as the pre-split that is conducted prior to forming the pre-split should maintain a minimum of three parallel rows from the presplit line to ensure good containment of the pre-split blast. The pre-split detonation order should be such as to sweep from one end of the blasthole line to the other. 2. Trim. If trim blasting against the wall is chosen, the row should be shot last in the bench round, with delays to result in a sweep blast from one end to the other. 4.3 Alluvium/Colluvium Walls Walls in alluvium/colluvium should be excavated so as to obtain an average overall slope of 36 degrees. The method of achieving this design can be determined in the mine. The material could be free-dug with interbench slopes at 40 degrees, 1-foot (nominal) intermediate benches, and a 17.5-foot catch bench every three benches. The slope that will result will be 36°. In the alternative, the alluvium/colluvium could be lightly blasted, with vertical 35-foot drill holes. Upon excavation, the material will likely ravel to an approximately 40° interbench slope, and the above geometry can be created in this fashion. • Both methods of excavation can be achieved safely, and will produce a safe overall slope. Report1385E.20071126