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contents on one tested sample were 119.0 pounds per cubic foot(pcf)and 13.0 percent <br /> respectively. <br /> • Unit 2 Sand and Gravel—This unit consists of the sand and gravel with local cobbles. This <br /> unit is actively mined for aggregate products. The unit is highly permeable and contains <br /> groundwater where not dewatered during the mining process. Based on the boring logs,the sand <br /> and gravel thickness ranges from approximately 13 to 20 feet. Local"mud lenses"may also be <br /> present within this unit. <br /> • Unit 3 Bedrock—Colton(1978)maps the bedrock in the area as the Upper Transition Member of <br /> the Pierre Shale and the Fox Hills Sandstone. Colton describes the Pierre Shale Member as <br /> friable sandstone to shaly sandstone containing sandy shale. He describes the Fox Hills <br /> Sandstone as fine grained silty sandstone interbedded with grey fissle shale. The boring logs <br /> refer to the bedrock as siltstone. Laboratory analysis on one bedrock sample classified the sample <br /> as sandy claystone. The packer permeability testing indicates the bedrock permeabilities are too <br /> low to measure. The maximum depth of bedrock encountered in the borings was 24 feet. <br /> Slope Liner Design <br /> Data from the field investigations and the project design parameters were utilized to design the slope liner <br /> and below grade reservoir as shown in the construction plans. The below grade reservoir shown on the <br /> plans is very close to the reservoir layout on file with the Colorado Division of Mining,Reclamation,and <br /> Safety(DRMS). Minor changes were made on the south side to account for a liner of sufficient width. In <br /> addition,a 150 foot setback is required near the gas well north of Phase I and minor curves were made in <br /> the alignment to make the slope liner more constructible. <br /> The slope liner is designed to meet State Engineer seepage requirements. The slope liner extends to the <br /> top of the pit highwall commonly resulting in the construction of slope liner above the reservoir high <br /> waterline. Constructing the slope liner above the high water line is necessary to minimize groundwater <br /> seepage into the pit. <br /> The slope liner is designed with 10-foot wide crest transitioning into a 3:1 (H:V)slope extending to <br /> bedrock. Stability and seepage analyses indicate this slope configuration and crest width will be stable <br /> under our interpretation of the existing site conditions. Overburden obtained from site stripping meeting <br /> project specifications will be utilized for Zone la and lb embankment construction. A cutoff key is <br /> designed at the base of the liner. The cutoff will extend four feet into the bedrock(after stripping <br /> approximately one foot of weathered bedrock)and will be 10 feet wide at the base. Compaction to 95% <br /> of Standard Proctor density using a sheepsfoot compactor will be required on all liner fill. <br /> The mine limits commonly extend beyond the slope liner crest. In these areas miscellaneous fill will be <br /> placed returning grades to near the original grades. <br /> Local more permeable areas of bedrock may be encountered during construction. These areas may <br /> require remediation efforts such as the construction of a deeper keyway or a compacted clay blanket. Our <br /> analyses are for a maximum liner height of 24 feet. This depth coincides with the maximum depth to <br /> bedrock at the site. If during mining deeper depths are encountered,Tetra Tech should be contacted to <br /> reevaluate the design. <br /> Page 3of5 <br />