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0 4.0 MAINS ORIENTATION AND PILLAR DESIGNS <br />To address the stability of the overburden, we have addressed both stability of near -seam <br />strata during development work and stability of other stratigraphic units above. In this <br />section, the stability of the near -seam strata is evaluated at the corridor for the proposed <br />layout developed by Bowie (figure 1). More detailed evaluation of the stability of <br />overlying units, including pore pressure effects, is presented in section 5. <br />Near -seam stability was addressed using both empirical and analytical techniques. An <br />empirical technique developed by National Institute for Occupational Health and Safety <br />(NIOSH) was used to confirm that the planned orientation is optimal for stability of the <br />rooms with respect to the horizontal stress field. In addition, because pillar stability is <br />important for controlling overburden deformation, factors -of- safety were calculated for <br />individual pillars by dividing pillar strength by stress. Pillar stability was calculated using <br />a pseudo- three - dimensional boundary- element technique while modeling variations in <br />4) overburden. <br />4.1 Mains Orientation <br />From a geotechnical point of view, orientation of the matins should be favorable with <br />respect to horizontal stress to reduce stress - induced damage to near -seam rocks, and <br />geologic structure to reduce joint - controlled instabilities (rib rolls and roof falls). <br />If horizontal stresses are high in comparison to rock strength, mines can experience <br />ground control problems. Under such circumstances, the operator needs to orient the <br />mine favorably with respect to horizontal stresses to reduce stress concentration (and thus <br />the potential for stress - induced damage). In general, the rule of thumb is that the mains <br />should be oriented near parallel to maximum horizontal stress to minimize roof stability <br />problems. <br />0 <br />Maleki Technologies, Inc. Page 26 <br />