2021-09-09_REVISION - M2004044 (10)

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aquifer. Soil-bentonite slurry walls are a very common type of liner constructed in Colorado. Construction of a soil-bentonite slurry wall liner is planned at the Tucson South Resource East and West cells. Once the liners are constructed and meet an SEO required 90-day minimum leak test, lined water storage reservoirs located north of State Highway 7 will be available for use. Draft Technical Specifications dated July 2004 for the construction of soil bentonite slurry wall liners at Tucson South in Adams County Colorado were prepared by Tetra Tech RMC and submitted to the Division for review. These specifications are a "placeholder" and are on record with the Division as part of the existing permit for this property. Aggregate Industries has contracted with Civil Resources, LLC. to provide a final design of a soil-bentonite slurry wall liner for Tucson South. The applicant will submit a Technical Revision with the updated final design. The slurry wall will be constructed with a long reach excavator or other suitable trenching equipment. A trench approximately 3 feet wide and 100 to 150 feet in length will be excavated through the existing alluvium and several feet into the underlying claystone and sandstone bedrock. The total slurry wall depth varies from top to bottom based on the existing geology but will average 30 feet deep for this project. The trench will be flooded with fresh bentonite slurry to stabilize the excavation during construction of the slurry wall. A mixture of alluvium excavated from the trench, dry bentonite, and clayey supplemental fines will be mixed at the surface and placed into the trench in a semi-fluid state typically with a bulldozer and/or second excavator. The slurry in the trench is displaced by the soil-bentonite backfill as the excavation of the trench is advanced. Once the excavation of the trench followed by the backfill operation is complete, the soil-bentonite backfill consolidates and behaves as a soft clayey soil. The top of the slurry wall is typically capped with a few feet of clayey soils to speed up the primary consolidation, identify the approximate location of the slurry wall, and provide clayey soils to fill depressions due to the consolidation of the slurry wall. Other considerations required for successful construction of a soil-bentonite slurry wall include a stable, relatively flat gently sloping (i.e. less than 1 percent parallel to the wall) construction platform along the proposed slurry wall alignment. Construction of the construction platform along the slurry wall alignment is primarily on stable, native soils found at the site and should not present a challenge for construction equipment. There are narrow areas proposed for the construction platform at this site. However, the narrow areas are relatively short and soil-bentonite slurry wall construction activities should not be greatly affected by these sections. Once soil-bentonite slurry wall construction is complete, the construction platform will be final graded for stability and an operations and maintenance access route will be left in place. The route will be reclaimed and stabilized as a low traffic dirt road. Gravel Pit Lining Criteria Gravel pit liners are designed with the goal of meeting the leakage requirements outlined in the State of Colorado's document entitled "State Engineer Guidelines for Lining Criteria of Gravel Pits,"August 1999. As described in the guidelines, the intent of the soil-bentonite slurry wall design is to reduce the groundwater inflow (leakage rate) into the lined below-grade mining cell to at or below the Design Standard. The Design Standard is defined in the guidelines as the leakage rate that is not greater than 0.03 ft/day multiplied by the curtain area of the soil-bentonite slurry wall (i.e. the length of the perimeter wall in feet multiplied by the average vertical depth of the wall as measured from the ground surface to the mine cell floor along the toe of the cell side slope), plus 0.0015 ft/day multiplied by the area of the floor encompassed by the soil-bentonite liner. The guidelines allow for a liner to meet a performance standard Aggregate Industries—Tucson South Amendment—MLRB 112 Permit Application Exhibit E-5