2002-01-28_REVISION - M1994093 - Laserfiche WebLink

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GEI Consultants, Inc. November I6, 2001 Project 00177 Mr. Andrew Legg, Project Manager City of Thornton Infrastructure and Maintenance Center 12450 Washington, Suite 100 Thornton, CO 80241 RECEIVED DEC 17 2001 Uivisi0a Of IldlOeral3 and Ge69 OYSouth Potomac Street Suite 200 Englewood, CO 80112 303 ~ 662 ~ 0100 303 ~ 662 • 8757 Fax Re: Release of Reclamation Responsibility Dahlia North Resources Pit (a.k.a., Dahlia North Pit) Permit No. M-1994-093, Technical Revision-02, Technical Revision-03, and Amendment-02 Dear Mr. Legg: The purpose of this letter is to summarize construction methods and our opinions regarding: i) the repair of the existing Dahlia North slurry wall near the northeast and southwest corners of the pit, and 2) our opinions regarding physical damage to nearby private wells during construction of the Dahlia North and Dahlia South, and the Dahlia North and East Sprat Platte Interconnection Pipeline Projects. GEI Consultants, Ina (GEI) was retained by the City of Thornton (Thornton) to observe and document construction of the interconnection pipeline projects, which were completed between March and August 2001, by BT Construction, Inc., Broomfield, Colorado. Existine Dahlia North Slurry Wall The existing Dahlia North slurry wall was penetrated and repaired at two locations during construction of the interconnection pipeline projects. During construction of the Dahlia North and Dahlia South interconnection pipeline, the existing slurry wall was penetrated and repaired near the southwest corner of the pit. During construction of the Dahlia North and East Sprat Platte interconnection pipeline, the existing slurry wall was penetrated and repaired near the northeast corner of the pit. Similar methods and procedures were used to repair the existing slurry wall at both locations. Seepage collars were constructed-around the interconnection pipelines at the locations where [he pipeline penetrated the existing Dahlia North slurry wall. The seepage collars are 8-inch-wide, 0.735-inch-thick steel flanges that are welded around the outside of the pipeline and wrapped with 80 mil protective tape. A 'h-inch-thick, 1-inch-wide strip of flexible bentonite-impregnated waterstop material was wrapped around the pipeline, adjacent to both sides of the seepage collars. The pipelines and seepage collars was then encased in cement flow-fill to provide a seepage barrier at the slurry wall penetration, and to protect the pipelines during reconstruction of the slurry wall above the pipelines. At the location where the existing Dahlia North slung wall intersects the Dahlia North and Dahlia South Interconnection Pipeline, near the southwest corner of the pit, cement flow-fill for the pipeline encasement extends about 2 feet into undisturbed portions of the existing slurry wall along the sides of the pipeline, about 2 feet into bedrock below the pipeline, and about 3 feet above the top of the pipeline. At the location where [he existing Dahlia North slurry wall intersects the Dahlia North and East Sprat Platte Interconnection Pipeline, near the northeast comer of the pit, cement flow-fill for the pipeline Offices Nationwide 01-11-Ib 001]] Dahlia Nonh Ralama[ion Complaion Itr