2008-01-16_APPLICATION CORRESPONDENCE - C2008086 (3)

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DRAFT • Tilt and strain caused by subsidence may reactivate movement in a currently stable or dormant landslide and rockfall areas where slope movements would be expected to eventually naturally reoccur due to natural causes. In the case of unstable natural slopes they are most likely to develop, reoccur and grow on steeper slopes during periods of increased precipitation. If a dormant landslide or rockfall area starts moving during a dry period and within approximately 0.7 times the depth distant from an advancing longwall face, the movement has very likely to have been triggered by the mining. • Large tilt and horizontal strain values caused by longwall mining under the shallower overburden, close to the coal outcrop or on the lower sections of steep canyon walls on the southwest side of the Project Area, could potentially cause the greatest mining impacts on areas that are already unstable. 1. Tilt values greater than about 5 percent, with approximately 500-foot overburden depth or less, may impact areas that are already prone to landslides or rockfalls, particularly where the tilt direction parallels the downslope direction, and, therefore, increases the overall slope angle by roughly the maximum predicted tilt amount. See Figure 9 C and Figure 18. 2. The stability of geologic hazard areas may also be increased by subsidence, where the subsidence induced tilt direction is opposite to the topographic slope direction. In this instance, the overall slope angle would be decreased by as much as the maximum subsidence-induced tilt change. See Figures 9B. Horizontal tensile strain values generally greater than approximately 1 percent (10000 uE) at 500-foot overburden depths and less also may accelerate natural landslide movement or rockfall, particularly during periods of high or increased precipitation. Figure 18. Ribside Tension Cracks in Road Fill and Cliff Face, York Canyon Mine shows a location where a sandstone cliff face failed after some of the shale underlying a sandstone cliff face had been removed for the pioneer road and then the headgate end of the longwall panel was mined past at approximately 360 feet below but over 50 feet to the right of the cliff. The estimated non-maximum tensile strain acting on the cliff face was about 0.5 percent (5000 uE). 8.2 Mining Beneath Stream Courses The only permanent stream courses indicated on the Garvey Canyon Quadrangle and Howard Canyon Quadrangle for the Project Area are Big Salt Wash and East Salt Creek. Big Salt Wash is the only permanent stream that overlies planned Red Cliff Mine workings in the Cameo Seam. East Salt Creek does not cross over any part of the Cameo Seam within the Existing Coal Lease or Coal Lease Application area. The Cameo Seam outcrop crosses Big Salt Wash approximately 7,800 feet upstream from the southern boundary of the Coal Lease Application area, as shown on Figure 13. Overburden and Outcrop Map for the Project Area. Within the Existing Coal Lease area, the Cameo Seam outcrop crosses the intermittent stream course in Stove Canyon, Section 2, T. 8 S., R102 W. northwest of Big Salt Wash and the intermittent stream courses in Munger Canyon and its southeast tributary, Sections 22 and 27, S. 7 S., R. 102 W. These Cameo Seam outcrops are within the Project Area. Page 42 of 57