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

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DRAFT gob until the sandstone suddenly breaks. This is particularly troublesome when the longwall face roughly parallels awidely-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 surtace. 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 uniformly if all the gateroad pillars yield when the first panel passes. However, when the adjacent panel passes the yielded pillars major overburden loading must be carried by the coal at the tailgate corner of the face, see Figure 2. Keeping the tailgate entry open to ventilate the longwall become a serious problem. 4.0 GEOLOGIC FACTORS INFLUENCING SUBSIDENCE It is extremely difficult to quantify the impact of geology on the extraction of coal and the resulting subsidence of the ground surface. There are some obvious generalities that can be stated with complete confidence, but predicting what will happen and where is fraught with risk. The overall geology of the coal bearing Mesaverde Group is generally known, but the site specific geologic conditions aren't fully understood because it is only possible to see outcrops, the immediate roof and floor and the coal seam and the overburden lithology is changing. Page 13 of 57