Laserfiche WebLink
0 4.0 MAINS PILLAR STABILITY <br />Because pillar stability is important for controlling overburden deformation, pillar <br />stresses and factors of safety were calculated for the Third North mains while modeling <br />the influence of overlying workings in the Upper D Seam and neighboring works in the <br />historic U.S. Steel mine. Figure 1 presents the planned location of mains in the Upper B <br />Seam relative to old workings in the Upper D Seam and in the Upper B Seam. <br />Pillar stability was evaluated using an estimate of pillar strength and stress. Pillar <br />strength was estimated using a method developed by NIOSH researchers (Mark and <br />Chase 1997). Pillar stress was calculated using a numerical model. The model represents <br />vertical stress distribution on the Upper B seam, including variations in topography, and <br />mining geometry. Pillar factor of safety was calculated during the development work <br />beneath the stream valley by dividing pillar strength by stress. <br />The pseudo-three-dimensional boundary-element code MULSIMTI was used for <br />calculating stress distributions over the area of interest. This proprietary program <br />incorporates elastic, strain-softening material and is suitable for multiple-seam <br />excavations in dipping seams and variable topographies (Maleki 2002). Model input is <br />shown in table 5 for this elastic analysis. <br />Figure 1 presents the planned mining geometries and the location of the fine mesh. <br />Mining geometry includes a set of mains being driven under the drainage. The total <br />modeled area is three times larger than shown in figure 1 including longwall panels in the <br />Upper D Seam and room-and-pillar panels in the US Steel Mine. <br />In figure 6, square elements are 9- by 9- by 10-ft coal elements, the color of which <br />depends on the stresses acting on them. In this figure, we have also included the factor <br />of safety for each element while individual pillar dimensions and mining height are <br />considered (9.5 ft). Pillar strength is calculated using the Mark-Bieniaswki formula <br />• (Mark and Chase 1997), which depends on pillar dimensions and excavation height. <br />Maleki Technologies, Inc. Page 16