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<br />ROCKY MOUNTAIN CONSULTANTS, INC. <br />November 18, 1999 <br />Ms. Lori Potter <br />Kelly/Haglund/Garnsey & Kahn, LLC <br />1441 Eighteenth Street, Suite 300 <br />Denver, Colorado 80202-1255 <br />r~~~~~~, Inc <br />" G 4 ~ggg Premiere Builtling <br />hVISr0f10f 825 Delaware Ave.. Suite 500 <br />Mirterabg Gea Longmont, CO 80501 <br />~y (303) 772-5282 <br />Metro (303) 665-6283 <br />FAX (303) 665-6959 <br />(first Initial, last name)@long.rmcco.com <br />Re: Review of "West Pit Hydraulic Barrier Preliminary Design Report," Prepared by <br />Shepherd Miller, Inc.; RMC .:ob No. 1S-3754.001.30 <br />Dear Lori: <br />Rocky Mountain Consultants, Inc. (RMC) has reviewed the November 15, 1999 jraft document <br />entitled, "West Pit Hydraulic Barrier Preliminary Design Report," prepared by Shepherd Miller, <br />Inc. (SMI) for the BMRI San Luis Mine. Our comments on this document are provided below. <br />Section 3.0 Hydraulic Barrier Design <br />The slurry wall dimensions listed herein are 1,670 feet long by 50 feet deep, for ;3 surface area of <br />83,500 square feet. Using the unit cost of $6.40 per square-foot presented in the October 31, <br />1999 "Preliminary West Pit Cost Estimate" document, a slurry wall of the dimensions presented <br />herein would cost an estimated $534,400 to install. This figure is more than twice' the dollar amount <br />of $256,000 presented in the October 31 cost estimate. <br />Mix design testing should be conducted to determine the suitability of processed (screened) soils <br />and the quantity of dry bentonite required. Mixed samples representative of soil-bentonite backfill <br />should be tested for permeability using flexible wall cells in accordance with ASTM D-5084, Method <br />D, with effective pressure of five to 10 psi. The tests should be performed using both clean and <br />contaminated groundwater. <br />Section 3.1 Hydrology <br />SMI states that lowering the water level in the West Pit behind the slurry wall would provide long- <br />term hydrologic assurance that there would not be outflow of groundwater from the West Pit. <br />However, BMRI does not commit to operating along-term pump and treat system to maintain a <br />depressed water table within the West Pit, which is essential to the success of tha slurry wall. <br />Section 4.0 Construction Considerations <br />The topography at the eastern end on the proposed slurry wall alignment is steep. The slurry wall <br />contractor may have to construct a series of steps along the slurry wall alignment, using cut and fill <br />techniques, to facilitate installation of the slurry wall. <br />CIVIL AND ENVIRONMENTAL ENGINEERING PLANNING <br />