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-7- <br /> <br />Observed Angles of Repose <br />Our firm and others have made numerous observations of angle of <br />repose for various materials found at the Colowyo Mine and other nearby <br />mines. The overburden rock exhibits angles of repose ranging from about 34 <br />to 40 degrees for loosely dumped spoil. The Canadian Pit Slope Manual <br />reports typical values of the angle of internal friction for shale spoil <br />materials on the order of 44 degrees, and for sandstones friction ranges <br />between 35 and 45 degrees. Values of 35, 37 and 39 degrees were used during <br />our stability analyses to judge The impact of spoil internal angle friction upon <br />slope stability. <br />Gradational Characteristics <br />. Our observations at the site indicate spoil generally breaks down to <br />materials on the order of 3 to 5 feet or smaller. Our firm has performed <br />tests on materials from a similar nearby mining operation. These tests have <br />indicated approximately I percent silt and clay sized particles (passing the <br />No. 200 U.S. standard sieve). It is our opinion compaction which occurs <br />during placement and additional filling further breaks down the spoil <br />materials, although we doubt more than 5 percent -200 materials develop <br />within The spoil over time. <br />Our laboratory tests included two-cycle slake-durability tests of the <br />potential drain materials. These tests indicated that available sandstones <br />from surface outcrops near the spoil area exhibited slake durability indices in <br />the range of 92 to 99 percent. Those materials sampled from the pit <br />generally exhibited slake durability indices in The range of 96 to 98 percent, <br /> <br />