2008-02-22_APPLICATION CORRESPONDENCE - C2008086 (43)

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to be from 800 to 1200 feet wide, and could range from 7,300 feet to 13,500 feet in length. The Main Cameo Seam, also called the Lower Cameo Seam, outcrops at the mine portals, Section 3, T. 8 S., R. 102 W., 6th P.M., the lower reaches of Big Salt Wash in Sections 12 and 1, T. 8 S., R. 102 W., 6th P.M., the lower reaches of Garvey Canyon in Section 12, the lower reaches of Buniger Canyon in Section 1 of the Coal Lease Application area. Therefore, the overburden depth (depth of cover above the Main Cameo ranges from zero in the extreme southwestern part of the Coal Lease Application to slightly more than 2,000 feet on the extreme eastern part of the proposed lease area. The planned minimum overburden depth for longwall mining is 200 feet in order to minimize 1) the potential for chimney caving to the ground surface, 2) the interception and diversion of ground water through the mine workings, 3) the loss of surface water to the fracture zone overlying completed longwall panels and 4) the potential development of up to 20-inch wide surface fractures along the sides of the panels. The planned coal mining height ranges from 8 to 11 feet. The 11-foot maximum planned mining height was used as a conservative maximum thickness in the subsidence analysis. 3.2 Gateroad Pillar Configuration and Design The currently planned gateroads will generally follow the example on Figure 2. Plan View of Three Adjacent Longwall Panels, where the gateroad pillars involve one row of yield pillars and one row of rigid pillars. The advantage of this design is that it should minimize stress levels at the headgate and tailgate ends of the longwall face. The centerline distance between the planned 20-foot wide gateroad entries will be 100 feet for the projected 80-foot wide rigid pillars. The centerline distance between the 20-foot wide gateroad crosscuts will be 200 feet for the 180-foot long rigid pillars. The centerline distance between the gateroad entries adjacent the 30-foot wide yield pillars will be 50 feet and 100 feet between the crosscuts adjacent to the 80-foot long yield pillars. Every other crosscut for the yield pillars will line up with a rigid pillar crosscut. Figure 3. Estimated Gateroad Pillar Loads From Mining First Adjacent Panel indicates the estimated minimum load and average stress that must be supported by the 30-foot wide by 80-foot long yield pillars, if the yield pillar is not to potentially crush. Figure 3 also indicates the estimated rigid pillar load that must be supported by the planned 80-foot wide rigid pillars, after the longwall face of the first adjacent panel has been advanced roughly one Load Transfer Distance, approximately 329 feet, past any location. The Load Transfer Distance is how far from active mining that deformation or loading in response is measurable or otherwise detectable, and shown on Figure 4. Load Transfer Distance Data (compiled by Abel, 1988). Figure 5. Estimated Gateroad Pillar Loads From Mining Second Adjacent Panel indicates the estimated minimum load and average stress that must be supported by the planned 80-foot wide by 180-foot long rigid pillars, if the rigid pillar is not to potentially crush, after the longwall face of the second adjacent panel has been advanced roughly one Load Transfer Distance past any gateroad location. It is not essential that the central gateroad entry remain open for ventilation through the gob to the bleeder entries. Two of the active panel tailgate entries will be open to the bleeder entries at all times during mining of the panel. See Figure 2. The disadvantage of a line of rigid gateroad pillars through the gob is the potential for higher horizontal tensile strain at the ground surface overlying the gateroad because the overburden initially bends toward the first adjacent panel as it is mined and then in the C-4 DBMS 296