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Mr. Lee Sampson <br />September 22, 2022 <br />Page 6 <br /> <br />Agapito Associates, Inc. <br /> <br />Figure 5. Photograph Taken during January 27, 2021, by Colowyo Showing the <br />Constructed Underdrain and the Natural Sorting of Spoil Material Following End <br />Dumping <br />THE ADEQUACY OF THE ROCK DRAIN IN LITTLE COLLOM DRAW <br />Waste dump construction experts3 suggest that there are two basic methods of constructing rock <br />drains for permanent valley dump fills. Depending on the availability of sufficient quantities of <br />suitable rock to convey the design flow, the ideal method involves good-quality spoil end dumped <br />from the top of the valley slopes, where the underdrain is formed by segregation of coarse particles <br />at the toe of the dump. Furthermore, it is generally accepted that adequate segregation will typically <br />occur for repose dump heights of greater than 65 ft4. Where the source rock material is generally <br />coarse and competent (as in this case), rock drain development via natural segregation is generally <br />the preferred method of construction in permanent valley fills. <br /> <br />Where waste rock is coarse and competent (as in this case), a single end dumped lift will generally <br />create a superior rock drain than pre-construction of a drain with selected material. The total dump <br />height should include the depth required for segregation plus the required depth of drain material4. <br />For the Little Collom Draw valley fill, this would equate to 65 ft plus 16 ft (the required height of <br />a rock drain for fills greater than 1,000,000 cubic yards). Therefore, the minimum height of the <br />dump pile required to promote adequate segregation for a rock drain is 81 ft. <br /> <br /> <br />3 Lighthall, P.C., C.D. Sellers and W.D. Burton (1985), “Design of Waste Dumps with Flow Through Rock Drains,” <br />Proceedings of the 9th Annual British Columbia Mine Reclamation Symposium, Kamloops, BC, The Technical <br />and Research Committee on Reclamation. <br />4 Hawley M. and Cunning J. (2017), “Guidelines for Mine Waste Dump and Stockpile Design,” CSIRO Publishing.