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Brighton Water Storage Project ' Slope Stability Calculations Adams County, Colorado is well below the maximum hydraulic conductivity of 1 x 10.6 cm/s allowed by the State Engineer's office for such material (Division of Mines and Geology, 2000). Therefore, "' GROUND recommends that site claystone material, free of deleterious materials, be used to construct the proposed pond liner. In accordance with the Division of Mines and Geology (2000), GROi1ND recommends that the proposed liner be keyed into the underlying bedrock. The keyway should be at least 4 feet in depth and 4 or more feet in width. - - The excavated claystone materials should be thoroughly mixed to achieve a uniform moisture content,=placed in uniform lifts not exceeding 8 inches in loose thickness, and compacted to at least 95 percent of the maximum standard Proctor density at moisture contents from 1 percent • below to 3 percent above the optimum as determined by ASTM D-698. We anticipate that these soils may exhibit significant pumping, rutting, and deflection at moisture contents near optimum and above. The Contractor should be prepared to work with these highly plastic clay materials. On the~innei (excavated slope) side, the liner materials should be benched into the excavation back cut slopes with benches no, more than 2 feet in height. It maybe beneficial to over-build the clay liner slope and trim it back in order to achieve adequate compaction in the materials near the outer slope face, - Caze should be taken by the Contractor not to allow desiccation of the liner material between the time of its placement and placement of the overlying~`miscellaneous ftll.' _ The Geotechnica] Engineer should observe the exposed excavation surface prior to placement of fill, and observe earthwork operations and test the soils. Additional hydraulic conductivity tests should be performed of the material. as placed. , Slope Stability The analyses performed for this study indicate that the excavation back cut, constructed at an arigle of 1 %:1 (horizontal :vertical) will be sufficiently stable against a deep seated failure for a temporary condition with a factor of safety on the order' of 1.2 to 1.3, depending largely on the local in-place density of the material and the depth to bedrock. This factor of safety assumes a de-watered condition whether by natural drainage or by active, construction de-watering. Significantly steeper back cut slopes, such as the 1:1 (horizontal vertical) slope originally contemplated, .do not have adequate factors of safety in the nearly - cohesionless alluvia] sand and gravel. The excavation back slopes will be subject to erosion during significant rainfall events as well as local; surficial slope failures. Job No. 02-0065 ~ Ground Engineering Consultants, Inc. ~ ~ Page 6