2004-10-28_REVISION - M1999021 - Laserfiche WebLink

Home Browse Search

CASTLE ROCK CONSULTTKG, LLC 536 EAST 2ND AVENUE CASTLE ROCK, CO 80108 VOICE (303) 66D-8603 FAX (303) 663-5117 EMAII. STANPETERSPEQAOL.COM October 7, 2004 Ms. Connie N. Davis Aggregate Industries, PO Box 337231 Greeley, CO 80633 Deaz Ms. Davis, As requested, I am summarizing the design basis for sizing the groundwater mitigation system pipelines for the Stegner mine in Fort Collins, Colorado. I understand that this information was requested by the Colorado Division of Mina;rals and Geology, and this letter will be included in your Response to Adequacy of the Technical Revision Request for the reclamation of this property. To determine the size of the upstream groundwater mitigation pipe, I first estimated the amount of groundwater that would have normally flowed though the sand and gravel, north to south across the property. With permeabilities of 11~ 1 to lE-2 cm/sec, the average fall of 5.5 feet over 1300LF, I estimated natural flows of 0.3 to 3 gpm. This assumes an interception depth of six inches (thus preventing any groundwater mounding) over approximately 1000 feet east to west. The six inch pipe size along the north alignment of 1200LF, falling O.Sft, should convey 44gpm. 7ihe excess capacity would convey additional groundwater, during times of heavy rainf~dl, etc. While the perforated pipe along the western alignment of the groundwater interceptor system will add water to convey, the efficiency of the same pipe diameter will be much greater, due to the steeper gradient (approximately four times steeper). The solid pipe along the southern alignment has an even greater slope. The invert elevations were chosen to intercept groundwater at elevations at historic pre-slurry wall conditions, and release this water along the south- eastern area, outside the slurry wall perimeter, where a depression of groundwater levels has occurred. These flow-rates do not even account for increased flow through the 1-1/2" gravel surrounding the perforated pipes. Please feel free to call if you have any questions, or need additional assistance. As ever`~~ / /~ Stan Peters, P.E.