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• III IIIIIIIIIIIII III • 999 ~ r ~,g~ ~~H ~,. November 8, 1996 Owens Brothers Concrete 5550 Sheridan Boulevard Arvada, Colorado 80004 ~~ AEE Protect No. 8-317-000015 Attention: Mr. Mike Owens RE: Geotechnical Engineering Opinion and Response to Comments Post-Mining Stability of the Sinclair Pipeline Easement Owens Brothers Concrete Company Del Camino Sand and Gravel Mine Weld County, Colorado Dear Mr. Owens: We have received comments dated October 18, 1996 from the State of Colorado Division of Minerals and Geology (DMG), through Gary Curtiss and prepared by Allen Sorenson. Their letter relates to the amount of setback that should be required to keep the Sindair Pipeline safe from slope stability issues in worst-case and extreme worst-case mining scenarios. They have no problem with the application from the standpoint of post-mining, redaimed configuration. The concern expressed by DMG is with temporary vertical or near vertical cut slopes that might be made and left un-redaimed under various conditions. We agree with their analysis and condusions that vertical slopes are indeed unstable under unsaturated, saturated or rapid draw- down conditions. If the unlikely event of DMG's Case B were to occur with Failure Surface #3 (FS=1.25), the Sindair Pipeline would be within the failed area under rapid drawdown conditions, because the pipeline is located 5 feet doser to the east easement line than the west line. We do not recommend vertical slopes in granular materials, even temporarily, because it cannot be shown that such a slope will stand. The issue is to recommend the slope that will allow the operator to recover the maximum amount of permitted resource safely, with a minimum of risk to personnel and equipment safety as well as future exposure to repair work outside of mining limits and easement lines.