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CTL/Thompson, Inc. performed an evaluation of the stability and density of the RDA and presented results in our report dated April 29, 1991 (Job No. 17,453). Our Mr. McOmber also visited the site April 28, 1993 and presented a letter describing our observations and discussions with Basin personnel (letter dated Mav 3, 1993). The scope of work for this evaluation included review of portions of the approved mine plan and results of previous compaction testing provided by Basin Resources, Inc. A representative of CTLJThompson, Inc. also visited the RDA facility May 5 through 7 to measure surface slopes on the reclaimed pile surface and to perform compaction tests in the uppermost zone of the refuse fill. The portions of the mine plan we were provided indicate the refuse till is planned with a series of 50-toot high slopes with a 2:1 (horizontal to vertical) outslope and separated by 20-foot wide benches. The benches are designed to drain at 3 to 5 percent to side ditches provided with rip-rap or "Reno Mattress" cover. The top of the refuse site is to be maintained with a crown in the center and slope to each side at 3 to 5%. An underdrain system is planned beneath the fill, extending in a "Y-shape" beneath two ephemeral drainages which converge beneath the site. A benched keyway cut is planned beneath the toe of the fill to stabilize the toe and allow for subdrainage. The fill materials are to be transported to the RDA site via conveyors, distributed with a front end loader and dozer in 2-foot layers, and compacted to achieve a minimum of 90% of the maximum standard Proctor dry density (ASTM D698-70). Our observations and tests regarding each of these design details are outlined below. Elevation data provided by Basin Resources indicates the shape of the refuse has been maintained as 2:1 (or flatter) slopes separated by intermediate benches. During our May 5-7 site visit, we measured the outslope to range between 43% (2.3:1) and 45% (2.2:1), or slightly flatter than the approved plan. This will enhance stability of the slope compared to the 2:1 slope planned. We measured the side slope of the intermediate benches to be about 1.3'6 to 3%, which is less than the approved slope of 3% to 5%. It is possible the slope was more steep and that settlement in the center of the benches has been greater than at the side, thus reducing the slope. It appears there is positive drainage off of the benches to the side drainage structures. The side drainage structures include grip-rap fined channel on the south side of the pile and a "Reno mattress" along the north side. The presence of these features is consistent with the portions of the plan provided. Historical photos and our inspections indicate refuse materials are being washed down the rip-rap channel on the east side of the fill, probably from the temporary stockpile at the conveyor discharge. We recommend the method of placement and/or surface slopes be altered to control this potential concern. It may be necessary to shape the fill surface to drain only to the west; this may require approved revision of the approved plan. If surface drainage is changed, the capacity of the Reno mattress should be checked. The standing water at the top of the pile had been drained using dewatering ditches (Item A above) prior to our April 28 site visit. The surface of the top of the pile was generally level. We recommend proper surface drainage be established as soon as practical after operations resume to rapidly remove water from the temporary pile surface. 2