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STATE OF COLORADO DIVtStON OF MINERALS AND GEOLOGY Department of Natural Resources 1313 Sherman Sc, Room 2i5 Deaver, Colorado 80203 Phone: (303) 866-3567 FAX: {303) 832-8106 DATE: May 24, 2002 TO: Gregg Squire FROM: Allen Sorenson ~- J ~OiQurrc-1 RE: Slurry Wall Des' n, Specifications, and~uality Assurance Pr~ram, City of Greele , 25`t' Avenue Gravel Pit, File No. M-2002-020 BiII Owens Governor Greg E-Walchee Executive Director Michael R. Long Division Director I have completed a review of the package of information submitted by the Applicant on May 13, 2002. This package includes design information and drawings, technical specifications with a quality control plan, a geotechnical investigation report, and a post construction performance testing plan. The construction of the slurry wall in accordance with these plans and specifications should result in a reservoir that is sealed from the surrounding alluvial aquifer. The one aspect of the proposal that is somewhat unique in relation to the other on-going or completed grave] pit sealing projects along the Front Range, is the use of use of Soil-Cement-Bentonite backfill in the construction of the subsurface barrier wall. The addition of cement in the backfill mix tends to result in higher permeability compared to Soil-Bentonite barrier walls. The design summary included in the May 13, 2002 submittal states that the addition cement to the backfill is for the purpose of strengthening the barrier wall to resist pressure from differential head on either side of the wall. The design summary states that a 3-foot wide Soil-Cement-Bentonite wall will resist maximum anticipated differential head, whereas a 5-foot wide wall would be required if Soil-Bentonite backfill were to be used. The Applicant's submittal includes a backfill mix design report describing the preparation and testing of a laboratory generated Soil-Cement-Bentonite backfill sample. Based on the results of one triaxial permeability test, the report concludes that the design backfill will be less permeable than the target hydraulic conductivity for the project. The Division of Minerals and Geology (DMG} accepts the design, specifications, and quality assurance program presented by the Applicant, despite some concern with the potential overall leakage rate of the proposed Soil-Cement-Bentonite barrier wall. This acceptance is based on three factors: I. The design has been accepted by the Office of the State Engineer (OSE) as being suitable to meet their standards for construction materials and methods, and their overall leakage standards (see letter from OSE to Haley and Aldrich, Inc. that is attached to this memo). 2. The ultimate performance of the sealed reservoir is assured by the conduct of 90-day liner test to be completed following construction, as well as on-going water budget accounting requirements overseen by the OSE. There will also be a minimum of six permeability tests conducted on the Soil-Cement-Bentonite backfill during construction of the barrier wall. DIVlStON OF MINERALS (GEOLOGY Qrr Ulb~ RECLAMATION MINING•SAFETY