2008-01-16_APPLICATION CORRESPONDENCE - C2008086 (3)

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DRAFT The 80-foot by 180-foot gateroad pillar could be allowed to yield after the first adjacent panel has passed. In that case, as the second panel is retreated a major arched load could be transferred onto the tailgate corner of the second adjacent longwall panel from both gob areas shown on Figure 2,. Rigid gateroad pillars, such as the 80-foot by 180-foot pillars, are designed to help protect the tailgate corner during longwall mining. Rigid gateroad pillars, such as the 80-foot wide by 180-foot long gateroad pillars, shown on Figure 5. Estimated Gateroad Pillar Loads From Mining Second Adjacent Panel, must support arched loads from over both adjacent panels or they will yield and very likely crush a short distance after the second panel has passed, as indicated by the arrow showing the "Panel Face Retreat Direction" on Figure 2. Plan View of Planned Gateroad Pillars. The estimated rigid pillar loading shown on Figure 5 is for 1500 feet, but individual Red Cliff Mine panels may have as much as 2000 feet of overburden in the Coal Lease Application area. At 1500 feet, the maximum estimated rigid pillar load on the 80-foot by 180-foot resulted in an estimated stress of 6930 psi. At the planned maximum depth of 2000 feet, the estimated rigid pillar stress is 10760 psi, approximately a 55% increase. Both rigid pillar stresses exceed the 4760 psi uniaxial compressive strength of specimens from the Cameo "B" Seam at the Roadside Mine near Palisade, Colorado. However, an 80-foot wide by 11-foot high pillar should be stronger than the ASTM Standard 2-inch diameter by 4-inch long core test sample specified by American Society for Testing and Materials (ASTM), in the method for unconfined compressive strength of intact rock core specimens D2938. The rigid pillar has awidth/height ratio of 7.3 versus 0.5 for the core specimens. The central part of the rigid pillars will be capable of carrying much greater stresses because of the central core of the pillar is confined by the coal around the core. Pillar ribsides of rigid pillars at the Roadside Mine rapidly sloughed into the adjacent entries and crosscuts at 1800 feet of depth. When the coal sloughed off such a pillar ribside was removed, the entry width had increased. The shape on the exposed pillar ribsides is commonly referred to as "hour glassed". After such a cleanup, the pillars sloughed again, and repeated until the pillar ribsides were supported and restrained. 6.0 SUBSIDENCE ESTIMATION OVER CAMEO SEAM LONGWALL PANELS, RED CLIFF MINE PROJECT AREA The primarily graphical subsidence estimation method developed by the British National Coal Board (NCB, 1975) for estimating trough subsidence over longwalls was used for the Red Cliff Mine Project Area. The method was based on 177 profiles measured over named longwall panels and 10 over unnamed longwall panels. The provides a means of making aworst-case estimate of the maximum vertical subsidence (Smax), tensile strain (+E), compressive strain (-E) and slope change or tilt (G) of the ground surface anywhere over a longwall panel, provided the mining height (m), mining depth (h) and panel dimensions are known. Graphs provide a means of constructing a subsidence profile from the center of a longwall panel across the sides or ends of the panel to the limit of subsidence. A graph also provides a method of constructing a horizontal strain profile from the center of a longwall panel across the sides or ends of the panel to the limit of subsidence. Page 23 of 57