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

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• Ground stresses and mining induced stress concentrations increase with increasing overburden above a coal seam. Room-and-pillar mining becomes significantly more difficult in overburden more than 1,500 to 2,000 feet thick, because the mine roofs and pillars are already more highly stressed, before coal extraction transfers additional overburden stress. Miners can be forced out of an area by roof falls, pillar slabbing, rib sloughing and floor bumps before planned pillar robbing can be completed. The longwall method overcomes some of the room-and-pillar stability problems. There are no highly stressed pillars present that are split during pillar robbing on the retreat from a panel. Abutment stresses are generally lower and more uniform than in coal mined by the room-and-pillar method. There is also a major body of solid confined load carrying coal immediately in front of the longwall face. • More frequent and larger magnitude bumps and related seismic activity may occur where a large incompletely caved and consolidated gob area develops behind the longwall face supports. The presence of a thick sandstone bed, such as the Rollins sandstone, in the near roof can be progressively cantilevered further out over the gob until the sandstone suddenly breaks. This is particularly troublesome when the longwall face roughly parallels a widely-spaced and persistent joint set. When the shearer undercuts such a joint, the face supports can be subject to a sudden load increase, i.e. a long line of joint blocks can suddenly be released. When a moderately large rigid gateroad pillar is loaded by the abutment arch from mining of the longwall panel on one side, considerable strain energy can be stored in the pillar. The loading of the pillar will be rapidly doubled when the adjacent panel is mined past the pillar on the other side. If the strength of the gateroad pillar is only marginally strong enough to carry the arched load, the stored strain energy can be suddenly released as a rib bump or outburst. It is necessary to achieve a balance between a rigid gateroad pillar when the first panel passes and a pillar that will yield, but not fail until the second panel has been mined well past the location. A barrier pillar can be left between every set of two longwall panels. This practice can waste part of the coal resource. A rigid barrier pillar between adjacent longwall panels can induce higher tensile strains in the overlying ground surface. Rigid barrier pillars are normally designed to isolate panel groups and protect mains and submains and bleeder rooms. Rigid barrier pillars can locally concentrate stresses in closely overlying and underlying coal seams hindering their future mining. • For a given point of observation on the surface, the compression arch will have dissipated when subsidence and surface strains have ceased. This, however, takes time, potentially years for the differential stresses to decrease to a stable and permanently supportable level. Active measurable surface subsidence will temporarily decrease significantly when the given point is over a gateroad and between 0.5 to 0.7 times the depth horizontally from any adjacent active longwall panel face. If none of the gateroad pillars are rigid to the load applied when the first adjacent panel passes, less subsidence will occur on the surface over the gateroad when the adjacent longwall panel is mined. When the gateroad pillars yield, the excess load they were unable to support will be transferred to the unmined coal in the adjacent panel. The adjacent gob (collapsed immediate roof rock) will be more uniform if all the gateroad pillars yield when the first panel passes. However, when the C-10 DBMS 302