2006-05-22_REVISION - M1982015 (3)

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writing (and specify on the revised maps) the minimum setback distance that the slurry wall will be installed from the permit boundary. It is also necessary for the applicant to clearly define in writing (and specify on the revised maps) the minimum setback distance the slurry wall will be from the top of north bank of St Vrain Creek. Please respond. 5. The "Supplemental Geotechnical Engineering Report" prepared by Terracon indicates that the slurry cut-off walls should be "keyed" or "socketed" into the relatively impermeable competent/intact claystone or sandstone bedrock to reduce groundwater seepage. The report further states that supplemental coring and packer permeability testing in the azea of Boring Numbers 45, 47, & 48 indicated that the upper 5 to 10 feet of the sandstone layer was moderately to severely fractured and that "keying" or "socketing" into this type of sandstone bedrock could result in excessive seepage losses, and that deeper keyways for the slurry cut-off walls will likely be required in this area. Further, the upper 1 to 2 feet of claystone exhibited moderate weathering/fracturing and should not be considered competent/intact bedrock. In response to these observations, the Division requests that the applicant submit a plan or proposal of construction specifications that provides specific measures to be implemented during varying site geologic conditions to assure that the slurry wall will be adequately keyed into competent bedrock throughout the entire perimeter of the proposed installation, and minimize the potential for excessive seepage losses once construction of the slurry wall is completed. Please provide. 6. The "Supplemental Geotechnical Engineering Report" prepared by Terracon indicates that the possibility of groundwater fluctuations should be considered when developing design and construction plans for the slurry wall installation, and that fluctuations in groundwater levels can best be determined by implementation of a groundwater monitoring plan. Please state affirmatively whether such a groundwater monitoring plan will be developed and implemented as part of the slurry wall installation project. If yes, please provide the details of the groundwater monitoring plan, such as location of monitoring stations, frequency of monitoring, design of monitoring stations, length of proposed monitoring, and purpose of groundwater data collected. Please respond. 7. In conformance with Rule 6.4.5, please provide engineering details and construction specifications for the sheetpile wall shown in the proposed swimming area on the Terracon reclamation plan maps. 8. Please provide engineering designs and supporting calculations for the sizing of spillways, culverts, and/or any other hydraulic structure that will be installed in the reclaimed pond. Please note that any permanent hydraulic feature will need to be adequately sized to withstand the velocity and volume of flows expected from a 100-year flood event. (We suggest you refer to the attached copy of the Division's "In-stream Aggregate Extraction & Reclamation Guidance Document" that provides specific design criteria for sizing of hydraulic structures). Please provide. 9. Please quantify the velocity and volume of flows expected on site from a 100-year flood event, as well as the elevation of the 100-year base flood event and its relation to the elevation of the spillways and lake shore embankment. Also, please quantify or characterize the expected