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of this rockslide in Area MV) where a 25-foot thick slab failed that weighed some 500K tons and <br />slid 400-500 feet down the slope. The permit text presumed that these failures occurred on a 6- <br />inch clay seam that was believed to be a bedding plane. This clay seam was probably given too <br />much emphasis as a major contributor of slope instability for the quarry wall. <br />From our understanding, potential instability of the quarry wall drove the application by <br />the mine operator for a special use of forest lands above the quarry. In a decision notice of <br />2001, the USFS authorized the mine operator to lay back the granite slope above the steep <br />quarry walls on forest lands. The notice cites the previous mined areas as extremely steep and <br />poses serious stability concerns. It stated, "The layback of the top cut is proposed as a possible <br />solution to stabilizing the high wall and softening the visual impacts of the current steep slopes." <br />CGS has not had the opportunity to review any monitoring data resulting from these <br />historic failures, although a monitoring plan is referenced in the mine permit. <br />Possible Rockslide Failure Mechanisms and Contributing Factors <br />. During the site inspection and review of the mine materials received, we consider the <br />following to be the contributing factors for the failure of the west quarry wall, enumerated in <br />order from higher to lower significance. <br />1. The mine permit mentioned high bedding dips of the Sawatch near the upper fault of 70 <br />- 80 degrees with dips flattening toward the base of the slope. The MSHA report states <br />dips ranging from 47 to 53 degrees, which are similar to the dip reported in Morgan and <br />others (2003). Dips in Area H (the Hole) are approximately 35-45 degrees judging by <br />photos (Figure 3). Such steep beds, possible curvature of bedding planes, and <br />continued daylighting of bedding planes as the excavation of the hole deepened <br />certainly contributed to slope instability. This point was made quite clear in the MSHA <br />1986 report. An additional destabilizing factor was the mine plan to steepen the quarry <br />wall from an average slope gradient of 1.5:1 to 1.2:1 within the excavation of the Area H <br />hole. <br />2. Poor rock-mass quality in the area because of brittle deformation (e.g., slickensides, <br />altered zones, fractures, and fault gouge) from proximity to the Rampart Range fault <br />zone. This poor rockmass quality allowed the rockslide to behave more like a soil and <br />6