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STATE OF COLORADO <br /> MINED LAND RECLAMATION DIVISION of Lazo <br /> Department of N;..tural Resources N0 <br /> 1313 S-rrman St., Room 215 c <br /> Denver,CO 80203 <br /> 303 866-3567 +1876+ <br /> FAX 303 83�-8106 <br /> Roy Romer, <br /> Governor <br /> DATE: October i991 Fred Banta, <br /> Division Director <br /> Tr" _r-ry Ranney <br /> FROM: Jim Pendleton <br /> RE: National K Coal,Company s TR-01 - Coal Waste Bank Redesign <br /> --� (Permit No. C-81-035) <br /> I have reviewed the supplemental submittal made by NKC, Inc. This submittal <br /> presents an amended pile design for the waste bank proposed to be built at the <br /> NKC Mine. <br /> Mr. Don R. May, P. E. , engineering consultant for the operator, prepared the <br /> attached report update. Mr. May accomplished the soil material strength <br /> testing requested in our earlier adequacy memo. The result of the triaxial <br /> laboratory testing determined in-situ soil material strength stronger than <br /> assumed in the earlier stability analysis. Therefore, Mr. May' s earlier <br /> stability analysis has been corroborated. <br /> Mr. May also obtained undisturbed samples of the existing waste and tested <br /> their density. He also performed standard Proctor tests of that waste <br /> material . By comparison, the in-situ waste pile samples had relative <br /> densities of 91% and 94% of their optimum Proctor density of 91 pcf at 12.0`yo <br /> moisture content. Therefore, I concur with Mr. May' s opinion that the <br /> existing waste pile exceeds the required 90% relative density requirement. <br /> Mr. May proposes to amend the pile design to completely fill the small <br /> drainage adjoining the east end of the existing waste pile. Mr. May proposes <br /> to bury a culvert beneath the pile to accommodate the flow which occasionally <br /> occurs in this drainage. Unfortunately, this is unacceptable. The culvert <br /> pipe requires maintenance which will not be available following the complete <br /> reclamation and bond release by NKC, Inc. Construction of a cross-valley <br /> waste bank cannot be approved by the Division. If the culvert were to clog or <br /> a runoff event exceeded its capacity, the waste would be breached and eroded. <br /> I recommend that Mr. May consider reconfiguring the amended fill to <br /> accommodate an overland drainage channel paralleling the groin of the eastern <br /> boundary of the pile . This channel could be excavated into undisturbed rock <br /> and soil , avoiding infiltration of waters and erosion of the waste pile. <br /> With an appropriate reconfiguration and the design of a stable channel , the <br /> fill could be acceptable . Proper hydrologic calculations will be required to <br /> facilitate the design of that channel , if these calculations have not been <br /> previously completed for this drainage . <br /> cc : Dave Berry <br /> JAP/ern <br /> 9193F <br />