2008-04-22_PERMIT FILE - C1981017 (4)

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• • The final slope, not exceeding 2(H) 1(V), should be strictly maintained. Terraces should be included if present in the original design. The surface of final slopes should be com- pacted to prevent erosion. The construction of the outer shell of the pile should be performed under continuous supervision of an experienced engineer of Mid-Continent's staff. Periodically, the compaction of the outer shell should be verified by in situ density measurement using the sand cone method. The surface of the pile should be vegetated as specified in the Permit Application. 5.2 F,OCK TUNNEL DISPOSAL FACILITIY The rock tunnel waste was designed to be deposited in a facility with outer slopes of 2(H) 1(V), with two intermediate berm, 10 feet wide. The maximum height of the facility was 50 feet.~The reasons for the intermediate benches are not clear. They apparently are not required for stability purposes as in- dicated by the high factor of safety, estimated by all stability analyses. There appears to be no reason to include these benches (terraces) because of erosion control, as the slopes are rela- tively very low. The original design recommended to place the wash systematically in lifts and to eliminate end-dumping. The t'r,ickness of individual lifts is not to exceed 12 inches and com- pnction by the placing equipment were recommended for the outer she 1 1 of the pi 1 e. No specific construction and compaction methods were recom- mended for the inner part of the waste pile, except that the thickness of lifts do not exceed 24 inches was recommended. The original design of the rock tunnel waste disposal facility does not distinguish the stability conditions of temporary and final slopes. In our opinion, the difference be- tween temporary and permanent slopes is important. There is no reason to maintain slopes to 2(K) 1(V) or flatter on the tem- porary slopes. Temporary slopes are well stable at slopes equal to the angle of repose of the waste, or some 30 to 35 degrees. Our inspection and testing shows thGt the waste has been deposited in horizontal lifts, as specifiec, and it was compacted to an acceptable degree. The only exceptions are locations where high moisture content of the fill, probably prompted by recent snowfall, results in less sa*_isfactory compaction. In view of the good quality of the rockfill and the extremely conservative 20 .:~Y .., -., .. . . _. G ..5!DR000~15„~JlittlCs.iNC.