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• <br />-26- <br />of 16 feet wide and 16 feet high. Slake durability tests on <br />sandstone samples from a 10-foot thick sandstone bed above <br />the Lennox seam in the mine area indicate that this material <br />will be suitable for drain rockfill. Based on gradation analysis <br />of the existing spoil, we recommend that the drain rock be sized <br />according to the following requirements. <br />(a) 100 percent - 3 feet or smaller <br />(b) 35 percent - 2 feet in size <br />(c) 10 percent - 1.5 feet in size <br />We anticipate that this material will be developed by selec- <br />tive excavation of sandstone from the mine area. <br />5. It will be necessary to provide a filter zone between the re- <br />quired underdrains and the spoil. The filter zone was designed <br />using the design method of the U.S. Bureau of Relcamation pub- <br />lished in "Design of Small Dams". This method is based on pro- <br />viding transition zones to prevent infiltration of fines from <br />one zone to the adjacent zone. For the anticipated gradation <br />• of spoil, we recommend that the filter have a maximum size of <br />24 inches. At least 90 percent should pass the 18-inch size, <br />approximately 45 percent should be minus 8-inch in size and <br />about 10 percent should be minus 3-inch size. A maximum 3 per- <br />cent should pass the No. 200 sieve. In order to develop the <br />filter material, we anticipate that it will be necessary to <br />quarry and crush sandstone. <br />6. In our opinion, the overall stability is strongly influenced by <br />natural containment. Recommended spoil Area No. I is confined <br />on three sides by natural features and on the fourth side by <br />an existing haul road. Implications of a failure in this area <br />are minimal. If a major failure occurred, it could affect op- <br />erational problems on the haul road, however, there would be <br />little or no danger as far as human habitation is concerned. <br />The area is removed from your proposed mining area and tiie sur- <br />ficial slumps on the intermediate benches could have minimal <br />impact. <br />7. Two alternative fill configurations were developed for Area II. <br />In our opinion, Configuration II-B is substantially more stable <br />because of natural confinement, than Configuration II-A. In <br />our opinion, Configuration II-B should be used. Risk of failure <br />of II-B is primarily attributed to a toe-spreading failure and <br />can be controlled by construction of the recommended buttress <br />toe. Slope failure in the spoil slopes at Area II could cause <br />• problems with mine haul roads. <br />I~~ <br />v <br />