2012-02-16_REVISION - M2002020 (9)

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Slurry Wall Construction As mentioned previously, slurry wall technology has been used successfully in many situations to provide a vertical, relatively impermeable barrier between alluvial groundwater and below -grade reservoirs. Slurry walls (also known as slurry cut -off walls or slurry trenches) are non - structural barriers constructed underground to stop the flow of groundwater. Soil - bentonite slurry walls are the most common type of slurry wall used in Colorado and the United States. For the 25 Avenue Site slurry wall, cement may be added to the slurry and backfill for added stability and strength. The 25 Avenue Site slurry wall will be constructed with a long reach excavator, clamshell, or other suitable trenching equipment. A trench approximately 3 to 5 feet wide and 100 to 150 feet in length is established by excavating through the entire length of the alluvium and several feet into the shale bedrock below. The total slurry wall depth from top to bottom will be approximately 60 feet. The trench is filled with slurry that stabilizes the excavation and begins to seal the sides of the trench. A blend of soil excavated from the trench, dry bentonite, and borrow soils are mixed at the surface and placed into the trench in a semi -fluid state by a bulldozer or second excavator, displacing the bentonite slurry in the trench. As backfill is added, additional trench is excavated and the displaced bentonite slurry fills the new length of trench. Once the backfill operation is complete, the soil - bentonite backfill sets up slightly and acts like a soft clayey soil. The upper few feet of the trench are backfilled with clean soils to stabilize and protect the top of the slurry wall. Other considerations for slurry wall construction include a stable and sizable working area along the trench alignment. The slurry wall alignment on the outside of the existing SW Ponds is primarily on stable, native soils and should not present a challenge for construction equipment. The working area becomes relatively narrow in some areas, but construction activities should not be hampered even in these sections. Along portions of the slurry wall alignment, the working area will need to be located on previously reclaimed 3H:1 V slopes above the existing pond water surface. Preparation of the slurry wall alignment in these locations will include grading a 50 to 100 foot wide, relatively flat working space into the slope. When slurry wall construction is complete, the working space will be final graded for stability and an operations and maintenance access route will be left in place. The route will be reclaimed with topsoil and grass seeding as a low traffic dirt road. The SW Ponds already provide a partially excavated reservoir cavity. Excavation of the mining cell will further define the reservoir cavity and necessary alignment for the slurry wall. In addition to the slurry wall surrounding the mining cell, the slurry wall may be constructed to connect with the existing 25 Avenue Site reservoir slurry wall to minimize the slurry wall construction length. Reservoir Performance Standards According to the State Engineer's Office (SEO), the intent of the reservoir lining design (design standard) is to achieve g roundwater inflow (leakage rate) into the reservoir that is not greater than 0.03 ft/day (1 x 10 cm/sec) multiplied by the length of the perimeter wall in feet multiplied by the average vertical depth of the perimeter wall as measured from the ground surface to the Greeley 25'' Avenue Site DRMS 112 Permit Amendment Application Page 11