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<br />9s~ <br />Harry Ranney [~F~~~vEG <br />Reclarc,ation Sper_ialist <br />Mined Land Reclamation Division ,u~ `~,199~ <br />Colorado Dept. of Natural Resources <br />1313 Sherman St. Room 215 Dlvisior of MinBiais b ~eoiogq <br />Denver CO $0203 <br />May 29, 1993 <br />Dear Harry, <br />This letter is to propose a modification to the design size of <br />the ,underdrain for the waste pile at National King Coal Mine <br />{NKC). As stated in the waste bank design report, the underdrain <br />shall meet the minimum standards presented in Section 4.09.2, <br />page 291 of the state regulation. This regulation lists two <br />underdrain sizes, one for waste piles with a volume greater than <br />1,000,000 yd' and one for vnlUTne less than 1,000,000 yds. The <br />required drain is 4 ft, high by 10 ft, wide filled with rock. <br />Because of the urgency in completing the design report the <br />smaller :,ize was cited without evaluation of its appropriateness <br />for the NKC mine. <br />Witt, construction of this drain beginning irr the near future <br />it would seem appropriate to review the underdrain design. The <br />purpose of the drain is to intercept rain water which falls <br />directly on the pile, infiltrator; and flows througt, the material <br />to the base of the pile as well as any groundwater which might <br />flow from the adjacent natural ground into the waste pile. As yo,a <br />know the total expected design volume for the mine waste pile is <br />only 30,000 yd~, When compared to the 1,000,000 yd~ cited in the <br />regulations it is evident that the NKC waste pile is a very small <br />pile and subsequently wo,ald require a smaller underdrain. A 3 ft. <br />x 3 ft. or 4 ft. x 3 ft. drain wo,ald seem more reasonable. <br />Another design to be considered, which would meet the <br />objective of preventing any of the contaminated water from <br />entering the surface drainage ditch on the north side of the <br />valley, would be to allow a smaller drain but require that the <br />bottom of the drain be below the invert of the ditch. This would <br />ass,are interception of any flow through the waste pile and the <br />adjacent natural ground. <br />I know that changes like this should have been rr,ade earlier in <br />the process; but I think you'd agree tt,at ,uch a rnas;sive drain is <br />