2008-02-22_APPLICATION CORRESPONDENCE - C2008086 (43)

Home Browse Search

3. Maximum dynamic compressive strain decreased by an average of approximately 62 percent (Peng, 1992, Fig. 3.8). The scatter of the dynamic compressive strain data indicated on Fig. 3.8 is nearly as large as that for the dynamic tilt data. It appears statistically possible to state that the maximum dynamic compressive strain decreased with increasing face advance rate. 8.0 IMPACTS OF SUBSIDENCE ON STRUCTURALLY SENSITIVE AREAS 8.1 Longwall Mining in Geologic Hazard Areas of Landslides, Rockfalls, and Unstable Slopes These unstable areas occur naturally on steep canyon walls in the Mesaverde Group. Unstable slope features already present can be adversely impacted by longwall mining. • It is important to develop an inventory of baseline data on any landslide, rockfall, and generally unstable areas before mining begins, so that movements due to natural processes can be excluded from any potential mining impacts if they would create a hazard to the public. • It is also important to have an assessment plan to distinguish between mining-related impacts on existing unstable areas and other activities, such as road construction. The assessment plan should include a subsidence monitoring program which should indicate the maximum angle of draw to the maximum limit of subsidence effects for the Red Cliff Mine Project area. • 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. 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 C-39 DBMS 331