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[~ <br />zo <br />i6 <br />w 12 <br />a <br />S <br />8 <br />E <br />~ B <br />40 <br />a9 <br />9 <br />8 <br />8 20 <br />E <br />i <br />All holes <br />Average=2,000,000 psi <br />Stantlartl tleviation=1,800,000 psi <br />t0 <br />1 3000 6000 9000 12000 15000 18000 21000 24000 0 1 2 3 4 5 6 7 8 9 10 11 12 <br />u,tiuul canpresaive s4enpk, pai Younp•a ma4uWa, mpsi <br />Fig. 3. Histograms for tmiaxial compressive strength (left) and Young's modulus (right) for three holes <br />1z <br />ig <br />6 <br />~ J <br />za 3z 3s ap N se n s6 s9 ae sa n rs ea <br />Rack Mue Retiry <br />Fig. 4. Calculated RMR histogram frequency diagram <br />along a continuous corehole <br />3. SUBSIDENCE CHARACTERISTICS <br />Subsidence characteristics for any coal field depend <br />on site-specific geologic conditions and mining <br />practices, including strata competence, geologic <br />structure, topography, extraction height, extraction <br />speed, and mine designs. Rapid changes in <br />topographic conditions, a factor of interest to this <br />study, are known to influence both vertical and <br />horizontal movements, although detailed measure- <br />ments are lacking in the western U.S. coal fields <br />except in one case. Studies by the Colorado School <br />of Mines over the Raton Basin, New Mexico, first <br />addressed the influence of steep topographic <br />conditions [7]. This study indicated high subsidence <br />under topographic highs and low subsidence under <br />topographic lows, i.e., the "pile-up" effect. <br />3.1 Subsidence Parameters <br />Subsidence engineering parameters were estimated <br />using the results of both site-specific measurements <br />and other regional measurements by the U.S. <br />Geological Survey (USGS) and mining companies <br />within the North Fork valley. During the 1970s, <br />USGS completed subsidence monitoring over the <br />historic U.S. Steel mine near the BRL No. 2 opera- <br />tions (figure 1). <br />Figure 1 presents the location of subsidence monu- <br />ments over the Upper D Seam longwall block in the <br />study area. The monument grid was gradually <br />expanded by as mining continued to the north in the <br />D1 to D9 panels. In the gateroads of these longwall <br />panels, BRL used yield abutment pillars while grad- <br />ually increasing abutment pillar width from 30 m <br />(98 ft) in the D3 headgate to 35 m (114 ft) in the DS <br />headgate and beyond. Surface topography influence- <br />ed both the location of the monuments and the <br />frequency ofmonitoring some monuments. Because <br />of the ease of access, a great many monuments were <br />positioned to the west on the top of the hill. Some <br />A -4 <br />