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

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single panel may range from supercritical under shallow overburden to subcritical under deeper overburden. • Gate road yield pillars will tend to yield more with increasing overburden depth, such that two or more adjacent panels begin to approach the theoretical behavior of a single super-panel at overburden depths greater than 1,000 to 1,500 feet. At these depths, gateroad yield pillars may be loaded beyond the minimum loading and will begin to crush. Even yield pillars are extremely unlikely to yield to the level of the adjacent caved, broken and compacted gob behind the shield canopies at the face of the longwall panel. Figure 3 Estimated Gateroad Pillar Loads From Mining First Adjacent Panel indicates the minimum load the planned 30-foot by 80-foot gateroad yield pillar must support. The 80-foot by 180-foot gateroad pillars are designed to support the load arched from over the gob when the first adjacent panel passes as the result of the yielding of the 30-foot by 80-foot pillar. Figure 3 also shows the estimated maximum rigid pillar load transferred onto the 80-foot by 180-foot gateroad pillar after the first adjacent panel has passed. 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 a width/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 C-20 DBMS 312