2015-12-03_GENERAL DOCUMENTS - M1974070 (5)

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eventually by using junior water rights in priority. For these reasons, the Division will not require bonding to fill reservoirs for operations along the Front Range. However, the Division will require a statement from the applicant describing their conceptual plan for filling the reservoir. Slurry Wall Costs Slurry wall installation costs include geotechnical investigation and testing, design and quality control, mobilization and setup, excavation of regolith, excavation of the bedrock key, delivery of bentonite and water, mixing of slurry, mixing of soil/bentonite, backfilling of soil/bentonite, clean up and demobilization, and testing and quality assurance. Specialized equipment is required to excavate deep slurry trenches, and the cost of excavation increases dramatically for trenches deeper than 35 feet and/or if the bedrock into which the slurry wall will key is hard or otherwise difficult to excavate. However slurry wall costs may or may not be substantially increased by depth to bedrock or by excessive difficulty in excavating the key trench, depending on mine site location. The following slurry wall cost references are illustrative: • The U.S. Army Corps of Engineers installed slurry walls in an extensive levee improvement project in the Pocket area near Sacramento, California. These slurry walls were 30 feet deep with a reported unit installation cost of$5.00 per square foot. • The Federal Remediation Technologies Roundtable "Remediation Technologies Screening Matrix and Reference Guide" reports slurry wall installation costs of between$5.00 and $7.00 per square foot. • Environmental Protection Agency document EPA 542-R-98-005, "Evaluation of Subsurface Engineered Barriers at Waste Sites,"August 1998 reports slurry wall installation costs of between$5.00 and$15.00 per square foot. The costs to install slurry walls at waste containment sites are higher than the costs to line clean water reservoirs using a slurry wall. This is partially due to the need to conduct chemical compatibility testing and the higher degree of quality control used at waste containment sites. In a recent case, the applicant proposed a unit cost of$3.00 per sq. ft for slurry wall installation, and based on our review of recent bids for slurry walls around gravel pits on the Front Range, this is believed to be appropriate at this point in time. The slurry wall bond, whether it is for installation around the entire reservoir perimeter or is a 20 percent contingency bond as discussed above, should not be released until DMG is provided written confirmation from the OSE that the specified leakage criteria has beemm€t. As stated above, it is pertinent to the bond amount that the depth to bedrock used in estimating the extent of the slurry wall required is accurately determined and that nature of the bedrock is investigated. The applicant should provide information from bore holes at the site showing the depth to gravel and the type of bedrock present, any variability in the bedrock encountered, and the depth of weathering in the bedrock. It is implicit to the proposed slurry wall plan that the bedrock is a competent seepage barrier, and this is most likely to be the case. However, if fractured zones, sandy lenses or layers, or deeply weathered bedrock are present, pit floor lining `^ or a deeper bedrock keyway for the slurry wall may be required. Either solution would increase J