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r <br />• • iii iiiiiiiiiiiiiiii <br />STATE OF COLORADO <br />DIVISION OF MINERALS AND GEOLOGY <br />Depanmenl n( Nalu ral Resuurces <br />1 J 13 Sherman 51 , Room 215 I`y! <br />Denver, Colorado 80203 Illy <br />Phnnc (30JI 866] 567 <br />FA%:110]1 8J? BIO6 <br />DEPARTMENT OF <br />NATURAL <br />RESOURCES <br />DATE: October 18, 1996 Roy Romer <br />Governor <br />TO: Gar Curt15S lames S. LOChhead <br />y Ececulive DireCror <br />M¢hael B. Long <br />FROM: Allen Sorenson ~ Division Dueclor <br />RE: Setback of Excavation from the Sinclair Pipeline, Owens <br />Brothers Concrete Company, Del Camino Pit, File No. M-96-052 <br />I have completed a review of the AGRA Earth and Environmental report <br />titled "Geotechnical Engineering Opinion for Post-Mining Stability of the <br />Sinclair Pipeline Easement". This report is dated August 28, 1996 and was <br />prepared in support of the Owens Brothers Concrete Company permit <br />application for the Del Camino Pit, File No. M-96-052. The <br />slope/pipeline/easement configuration considered in the AGRA report <br />consists of a 3H:1V outslope made up of sand and gravel with the crest of <br />the slope located a minimum of 55 feet from the pipeline alignment and 30 <br />feet from the pipeline easement boundary. This is the configuration <br />proposed in the application for the reclaimed pit. For this <br />slope/pipeline/easement configuration, I concur with the AGRA opinion that <br />no hazard is posed to the Sinclair pipeline. <br />Accepting that the reclaimed pit configuration is stable with respect to <br />the pipeline, the critical consideration becomes that various <br />slope/pipeline/easement configurations that may occur during excavation <br />of the pit. Typically in a dry pit (dewatering prior to excavation is a <br />component of the proposed plan), operators will excavate from the toe of <br />a near vertical cut. The following information is taken from the <br />application to facilitate the analysis of the potential hazard to the <br />pipeline that excavation at a vertical cut may pose: <br />Maximum depth of excavation = 30 ft. <br />Excavation set-back from easement boundary = 30 ft. <br />Depth to ground water = 8 ft. <br />Depth of cover over pipeline = 4-8 ft. <br />For purpose of this analysis it was assumed that the pipeline <br />runs through the center of the easement. THIS MAY NOT BE THE <br />CASE. <br />I have completed stability analyses of two possible situations: <br />Case A: A 30 foot high vertical cut slope is excavated to within <br />30 feet of the pipeline easement. The dewatering pumps <br />