2001-05-29_PERMIT FILE - M2001023

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DATE: -<ay 29, 2001 PTO: Tom Schreiner vTROM: Allen Sorenson RE: L--'Mining Setbacks, Camilletti and Sons,Inc., Camilletti Milner Pit No.2, File No. M1200_1-W — The application for the Camilletti Milner Pit No. 2 commits the pit operator to certain setbacks from Yampa Valley Electric Association transmission line that transects the permit area, and the Public Service Company gas pipeline that skirts the permit area. These setbacks are included as enforceable components of the mining and reclamation plans in order to assure that the structures are not damaged by the operation of the pit. Based on the review of the information and analysis included in the application, the Division has determined that the proposed setbacks are adequate to prevent damage to the structures as a result of pit slope failure. However, the enforceability of the setbacks will be enhanced through modification of the permitted location from which the setbacks will be measured, as discussed below. Also, certain slope movement monitoring is required to assure the integrity of the gas pipeline under relatively more subtle stress relief type earth movements. In the case of the gas pipeline, the Applicant commits to setting back 20 feet from the edge of the pipeline easement to the top edge of the excavated pit, and further commits to mining the pit slope at no steeper than 1.25:1 (horizontal:vertical). The attached figure 1 illustrates the pit slope configuration for compliance with this application commitment. Figure 1 further shows that the setback to the easement line will be 70 feet measured as a horizontal distance from the toe of the excavated slope under the assumption of a 40-foot total pit depth. Please note that review of the exploratory boring logs included in the permit application indicate that 40 foot pit depths in the areas of both the gas pipeline at the south pit perimeter and the power line in the pit center, are suitable pit depth assumptions for design purposes. It has been the Division's direct experience that pit operators find it difficult to maintain precise control of pit slope angles along a working face of a sand and gravel mine. For this reason, the Division determines compliance with setbacks based on measurements taken from the toe of the excavated slope to the easement line or the structure to be protected. In the case of the proposed Camilletti Milner Pit No. 2, the Applicant should commit to a setback of 70 feet from the toe of the excavated pit slope to the gas pipeline easement boundary. As figure 1 and 2 attached to this memo illustrate, the Division's proposed setback measurement results in no practical difference in the amount of material that may be mined if the pit operator excavates at a 1.25:1 as proposed in the application. The Division's proposal has the advantage of providing enhanced ease-of-compliance in the event that the pit is mined at a steeper angle.