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TETRA TECH <br />November 21, 2017 <br />Connie Davis, Land Advisor <br />Aggregate Industries—WCR, Inc. <br />1687 Cole Boulevard, Suite 300 <br />Golden, Colorado 80401 <br />RE: DRMS Permit M-2004-031, Hazeltine Mine, As-Constructed Certification; Adams <br />County, Colorado; Tetra Tech Job No. 133-23514-17002 <br />Dear Ms. Davis: <br />We have prepared this letter in response to the DRMS requirement to submit a certification by a <br />qualified engineer, certifying that elements of improvements have been constructed to the <br />required compaction and specifications. The constructed elements include the surface drainage <br />control structure, referred to as the Orr Property Drain, located on the east side of the mine. <br />In accordance of the contract terms between Aggregate Industries and the City of Thornton, an <br />outlet with an invert elevation of 5036.0 is planned to hydraulically connect the reclaimed <br />Hazeltine Mine to Bull Seep. The outlet is located on the northwest corner of the mine. The <br />outlet has not been constructed as of this date because the City of Thornton has not yet <br />obtained an easement from Denver Water, the adjacent landowner, to allow discharge into Bull <br />Seep. <br />Some modifications to the Orr Property Drain were made to these features, based upon field <br />conditions, to aid construction due to the presence of high groundwater. These modifications <br />were prepared under my direct supervision. The modifications included a sheet pile wall driven <br />into the existing slurry wall to provide an impermeable barrier downgradient to allow for <br />dewatering and construction of other drainage elements. <br />Additional modifications to the original design were incorporated to accommodate requests by <br />the City of Thornton. The first modification was a change from a piped design to an open <br />channel design. The second modification was the inclusion of a Parshall flume for flow <br />measurements. <br />Specified compaction requirements and concrete strength testing were reported to conform to <br />the technical specifications for the project to Tetra Tech by the Contractor and Aggregate <br />Industries. <br />Tetra Tech <br />1900 S.Sunset Street, Suite I-E, Longmont, CO 80501 <br />Tel 303-772-5282 Fax 303-772-7039 www.tetratech.com