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Subsitlence, ft <br />15 <br />-5 <br />8 <br />.7 <br />-9 <br />.g <br />• <br />• <br />The goodness of fit, as measured by the correlation <br />coefficient between measured and calculated values, <br />significantly improved only after varying both the <br />subsidence factor and angle of draw with depth. <br />Figures 8 and 9 compare the calculated and <br />measured results for the monuments over the D7, <br />D8, and D9 panels. Regression coefficient is 87%, <br />indicating a good overall linear correlation between <br />measured and calculated values at the monitoring <br />locations. <br />From this modeling exercise, it was apparent that (1) <br />subsidence parameters are depth dependent and (2) <br />there are nonlinear ground movements locally. <br />These nonlinear effects are minor based on rather <br />high regression coefficient of 87%. The calibrated <br />version of the model has been used for estimating <br />surface movements above both single and multiple <br />seam reserves on a routine basis. <br />5. CONCLUSIONS <br />Based on geotechnical drilling, examination of core, <br />analyses of mechanical properties, and numerical <br />modeling, the authors have characterized over- <br />burdenstrength and subsidence characteristics atthe <br />Bowie mines. Using Bieniawski's classification of <br />rocks, the authors consider most of overburden units <br />as "fair." Rock quality improves at the massive C <br />Sandstone horizon, improving this unit's ability to <br />distribute loads and reduce differential movements <br />in future workings in the Upper B Seam. <br />~ ~---~ -- .2 •-/-- <br />i - T - - - - <br />i• ~ • <br />- •_ <br />V • • • <br />- i • Y=Q9h -0.4719 <br />- - - - ~=ne71a- - <br />• <br />Meaa~retl, ft <br />Fig. 9. Correlation between calculated and measured <br />subsidence values <br />Subsidence monitoring over the longwall panels in <br />the Upper D Seam indicates adeptlydependent <br />angle of draw and subsidence factor. The <br />subsidence is locally higher in one area under <br />topographic highs and lower in adjacent areas in <br />topographic lows. The subsidence factor varies from <br />0.6 to 0.75 over the panels and is generally less than <br />0.4 over the gate pillars. Topography also influences <br />horizontal movements and the location of fractures, <br />creating fracturing at the topographic highs. Overall, <br />under such steep topographies, ground movements <br />are always downhill. <br />Unit conversion constants <br />Len Meter m 3.28 ft <br />Stress Me a ascal MPa 145 si <br />A -9 <br />Fig. 8. Compared calculated and measured subsidence, vaziable cover <br />