2001-12-11_PERMIT FILE - C1981019 (6)

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a • slide may have occurred regardless of the pond and topsoil pile. Several other slides of various sizes are present in the general area, and the slide directions vary. We believe the slides are caused by the adverse orientation of the bedrock, coupled with seepage during spring runoff and precipitation. North to northeast facing slopes are considered to be mast likely to have unfavorable bedrock orientations, although exceptions can occur. General Grading Recommendations Our conclusions and recommendations are based upon the observations and subsurface information described above, The risks of slides occurring in the natural slopes adjacent to the proposed pond cannot he completely eliminated considering the evidence of several slides in natural slopes in the vicinity. Based upon our observations and experience, the following recommendations are provided to reduce the risk as much as practical, and to mitigate potential that the pond construction contributes to instability of the native slopes. Theoretically, any excavation or modification of the existing slopes could be considered to increase the risk of instability. The purpose of our recommendations is to characterize the risk in a general manner. This will allow development of plans which consider both operational requirements and prudent risk management. • 1. In our opinion the west slope (Fig. 1) is judged to be more susceptible to developing instability due to the effects of road or pond excavation than the east slope. 2. Initial consideration should be given to reducing or eliminating slope excavation to create the pond/embankment. This will limit volume and likely require a longer, higher embankment. The higher embankment presents slight additional risks of a different manner and may not be practical. 3. If excavation within the valley below the east and west slopes is required, the depth and slope of the excavations should be minimized. Slopes on the order of 3:1 (H:~ to 2:1 are considered feasible without excessive risk of inducing instability. In our opinion, using 2:1 excavation slopes does not result in an unacceptably high level of risk. If the required volume/embankment height combination can be achieved with 3:1 slopes (or a combination of 3:1 slopes below the western slope and 2:1 below the eastern slope) then this would be preferable. 4. The road section is planned to be all cut. We judge road construction or grading has lower risk on the eastern slope. COLOWI'O COAL CO.LP. cnlr za,oes 3