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.. ~a --~ <br />.,r ~' <br />eventually by using junior water rights in priority. For these reasons, the Division will not <br />require bonding to fill reservoirs for operations alone the Front Ranee. However. the Dtvision <br />will require a s[atement from the applican[ describing their conceptual plan for filling the <br />reservoir. <br />Slurrv Wall Costs <br />Slurry wall installation costs include geotechnical investigation and testing, design and quality <br />control, mobilization and setup, excavation of regolith. excavation of the bedrock key, delivery <br />of bentonite and water, mixine of slurry, mixing of soil/bentonite, backfilling of soil/bentonite. <br />clean up and demobilization, and testing and quality assurance. Specialized equipment is <br />required to excavate deep slurry trenches, and the cost of excavation increases dramaticallyfor <br />trenches deeper than 35 feet and/or if the bedrock into which the slurry wall will key is hazd or <br />otherwise difficult to excavate. However slurry wall costs may or may not be substantially <br />increased by depth to bedrock or by excessive difficulty in excavating the key trench, depending <br />on mine site location. <br />The following slurry wail cost references are illustrative: <br />• The U.S. Army Corps of Engineers installed slurry walls in an extensive levee improvement <br />project in the Pocket azea neaz Sacramento, California. These slurry walls were 30 feet deep <br />with a reported unit installation cos[ of $5.00 per square foot. <br />• The Federal Remediation Technologies Roundtable "Remediation Technologies Screenin; <br />Matrix and Reference Guide" reporu slurry wall installation costs of between $5.00 and <br />57.00 per squaze foot. <br />• Environmental Protection Agency document EPA 542-R-98-005, "Evaluation of Subsurface <br />Engineered Barriers at Waste Sites," August 1998 reporu slurry wall installation costs of <br />between $5.00 and $15.00 per square foot. <br />The costs to install slung walls at waste containment sites aze higher than the costs to line <br />clean water reservoirs using a slurry wall. This is partially due to the need to conduct <br />chemical compatibility testing and the higher degree of quality conaol used at waste <br />containment sites. <br />In a recent case, the applicant proposed a unit cost of $3.00 per sq. h for slurry wall installation, <br />and based on our review of recent bids for scurry walls around ~ avel pits on the Front Range, <br />this is believed to be appropriate at this point in time. The slurry wail bond, whether it is for <br />installation azound the entire reservoir perimeter or is a 20 percent contingency bond as <br />discussed above, should not be released until DMG is provided writ[en confirmation from the <br />OSE that the specified leakage criteria has been met. <br />As stated above, i[ is pertinent to [he bond amount that the depth to bedrock used in estimating <br />the extent of the slurry wail required is accurately de[ermined and that nature of the bedrock is <br />investigated. The applicant should provide information from bore holes at the si[e showing the <br />depth to gravel and the [ype of bedrock presen[, any variability in the bedrock encountered. and <br />the depth of weathering in the bedrock. I[ is implicit to the proposed slurry wail plan tha[ the <br />bedrock is a competent seepage barrier, and this is most likely [o be the case. However, if <br />Fractured zones, sandy lenses or layers, or deeply weathered bedrock aze present, pit floor lining <br />or a deeper bedrock keyway for [he slurry wall may be required. Either solution would increase <br />