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• indicated. The vertical displacement plot (Figure 53c) shows very little tendency for roof <br />spalling or floor heave. / <br />Two separate cases of seam interaction between the Sage Creek and Wolf Creek <br />in Seneca IIW were analyzed. Openings in the two seams were offset so that the edge of <br />a pillar in the upper seam coincides with the mid-span of an opening in the lower seam, <br />therefore simulating aworst-case for shear stress. The first case, shown in Figure 54 <br />incorporates a 21-ft interburden. This is twice the mining height of the lower (Wolf <br />Creek) seam, and is the minimum recommended by NSA. Should isolated roof failures <br />occur, it is likely that an interburden of twice the mining height is sufficient to forestal] <br />migration to the ]eve] of the upper seam. Although there is some interaction between the <br />seams. stability is maintained, with safety factors of not less than 3.0 in the pillazs (Figure <br />54a). With an increased interburden thickness of 30.5 ft (21 ft of rock in addition to 9.5 <br />ft of roof coal. see Figure 55}. there is definitely less interaction between the seams, <br />although the improvement in conditions is mazginal, and does not appear to justify a <br />higher criterion for interburden thickness. <br />Single seam mining cases for the Wadge in Seneca IIW and Yoast, and the Wolf <br />Creek in Yoast are shown in Figures 56 through 58. Good stability is indicated <br />throughout, with the only cause for concern being the thick coal roof of the Wolf Creek <br />Seam. which may be the source of nuisance falls. If this proves to be the case, it may be <br />advisable to move the mining horizon up higher in the seam, still leaving roof coal, but <br />• limiting its thickness. <br />Both the LAMODEL and UDEC modeling efforts support the validity of the web <br />and barrier pillar design curves. and suggest that the roof, floor, and interburden will <br />remain stable. ]t should be noted, however. that the models were not calibrated to field <br />experience. but rather input parameters ~a-ere based on physical property data, experience, <br />and engineering judgment, Until the designs are validated in the field, caution should be <br />exercised in their application. <br />6.0 OPERATIONAL CONSIDERATIONS <br />Operational considerations have been touched on in the previous sections, and are <br />listed here for clarity, and for more detailed discussion. <br />Limitations Caused by Seam Dip - By far the most serious obstacle to highwall mining <br />in both the Seneca ]IW and Yoast areas is seam dip. As indicated in Tables 5 and 6, and <br />on Figures 29 through 33, several portions of the highwall in each area appear to be <br />unminable due to downdips or sidedips that exceed the capabilities of the PM. Turning <br />the miner at an angle off of perpendicular to the highwall can help this situation, but <br />comes at the price of some operational inefficiencies. Retrofitting the machine with 60-in <br />jacks helps this situation considerably. but does not completely eliminate it. <br />• ,4nother issue related to the dip of the Sage Creek Seam is the minimum mining <br />height of 50 in. ]n order to mine with a sidedip. minim height is sacrificed to level the <br />Seneca Coal Compam ~Q NSA Engineering, ]nc. <br />Hiehu~all Mine Design Repon June 200. <br />