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'~ • • III IIIIIIIIIIIII III <br />999 <br />Memo <br />To: Erica Crosby <br />Fm~re Allen Sorenson <br />CC: Cad Mount, Tom Schreiner <br />Date: 02120/01 <br />Re: Mining Setbacks, S 8 H Mine, Platte Valley Sand 8 Gravel, L.L.C., File No. M-2000-158 <br />I have reviewed the S & H Mine permit application and the proposed setbacks from the South Platte River and <br />from existing permanent and valuable manmade structures. The following discussion pertains to the setbacks <br />and reclaimed slopes that are proposed and describes the issues remaining to be addressed for the Division to <br />recommend approval of the setbacks. <br />The appligtion in Exhibits D and E describes setbacks from property boundaries and structures, particularly <br />gas/oil wells and pipelines. However, in Exhibit S the application states that no mining will occur within 200 feet <br />of any structures until the Applicant demonstrates that mining will not affect them or until an agreement is <br />reached with owners of structures to allow mining within 200 feet. If agreements are forged with persons having <br />an interest in the structures, and the agreements are drafted in the format required by Rule 6.4.19 and Section <br />34-32.5-115(4)(e), C.R.S., the Division's concerns will be satisfied and the setbacks specified in the agreements <br />will be approved. <br />If agreements cannot be reached with persons having an interest in the structures, Rule 6.4.19 provides the <br />option for the Applicant to prepare an engineering evaluation demonstrating that the proposed mine plan will be <br />protective of the structures. The application states that the depth of mining will be on the order of 45 feet and up <br />l0 60 feet. Where setbacks are discussed, a distance of 25 feet from gas/oil wells and pipelines is specified. If <br />actual dredge mining or conventional mining were to occur up to a 25 foot setback line with a near vertical 45 to <br />60 foot mine face, the integrity of the structure to be protected would be threatened. The Division will not accept <br />a 25 foot setback from the mining face to critical structures for a pit depth of 45 to 60 feet unless an acceptable <br />engineering demonstration of stability is provided. The application appears to address this issue in Exhibit D <br />where it is stated that: <br />When a dredge is used the native materials tend to sluff off the banks and assume an angle of repose <br />of 2% h to 1 v. This happens because the material in this an:a is unconsolidated and vertical slopes <br />can not be maintained as mining occurs. This means that to maintain a setback of 100 feet when the <br />gravel is 60 feet thick that mining line has to be no less than 75 feet inside the setback lines fo allow for <br />the sloughing. <br />The slope configuration and geometry discussed in the above quote are illustrated in the following diagram: <br />• Page 1 <br />