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- ' ~ • • III IIIIIIIIIIIII III <br />~~,~~ <br />.~ s. <br />r~° <br />November 8, 1998 <br />Owens Brothers Concrete AEE Protect No. 6-317-000015 <br />5550 Sheridan Boulevard <br />Arvada, Colorado 80004 <br />Attention: Mr. Mike Owens <br />RE: Geotechnical Engineering Opinion and Response to Comments <br />Post-Mining Stability of the Sinclair Pipeline Easement <br />Owens Brothers Concrete Company <br />Del Camino Sand and Gravel Mine <br />Weld County, Colorado <br />Dear Mr. Owens: <br />We have received comments dated October 18, 1998 from the State of Colorado Division of <br />Minerals and Geology (DMG), through Gary Curtiss and prepared by Allen Sorenson. Their letter <br />relates to the amount of setback that should be required to keep the Sincair Pipeline safe from <br />slope stability issues in worst-case and extreme worst-case mining scenarios. They have no <br />problem with the application from the standpoint of post-mining, recaimed configuration. <br />The concern expressed by DMG is with temporary vertical or near vertical cut slopes that might <br />be made and left un-reclaimed under various conditions. We agree with their analysis and <br />contusions that vertical slopes are indeed unstable under unsaturated, saturated or rapid draw- <br />down conditions. <br />If the unlikely event of DMG's Case B were to occur with Failure Surface #3 (FS=1.25), the <br />Sincair Pipeline would be within the failed area under rapid drewdown conditions, because the <br />pipeline is located 5 feet Goser to the east easement line than the west line. <br />We do not recommend vertical slopes in granular materials, even temporarily, because it cannot <br />be shown that such a slope will stand. The issue is to recommend the slope that will allow the <br />operator to recover the maximum amount of permitted resource safely, with a minimum of risk to <br />personnel and equipment safety as well as future exposure to repair work outside of mining limits <br />and easement lines. <br />