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• effects on the RDA are limited to times when underground mining is occurring near <br />the RDA. Current mining plans are to complete Panels 8 and 9 by the end of 1996. <br />After this time underground mining will be located far from the RDA and possible <br />~ effects are considered negligible. During 1995 and 1996 Twentymile Coal Company <br />estimates about 460,000 cubic yards of coal waste will be processed, with <br />approximately 35 percent (161,000 cy) going to the waste pile. For this volume we <br />~ estimate the first segment of the waste pile will be less than 20 feet high. We have <br />discussed the potential ground motions with Mr. Swanson and reviewed the <br />preliminary information he has compiled. In our opinion, the potential ground <br />~ ~ motions due to underground mining should not have a significant detrimental effect <br />on the pile stabilky. Given the low pile height during the time period of concern, we <br />• do not believe sufficient risk exists to merit further analysis. <br />Internal Drainage <br />The coal waste is considered fairly permeable, test results form the previous <br />~ investigation indicated permeability values on the order of 4 to 8 x 10'' centimeter <br /> <br />per second (cm/sec). Tests were performed on samples of simulated processed and <br />unprocessed coal waste with approximately 15 to 20 percent passing the No. 30 <br />sieve and 2 to 9 percent passing the No. 200 sieve. <br />The RDA will be covered with 4 feet of sail cover. Recent studies of spoil <br />~ piles wkh similar cover (Reference 18) indicate that some infiltration through the <br />cover layer occurs during periods of snowbelt. The typical value was estimated to <br />be 3 inches of water infiltration per year. The footprint of the revised RDA is <br />~ approximately 46 acres. If the water infiltrates over a period of 1 month, and then <br />• <br />17 <br /> <br />