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<br /> <br />side Mine. An angle of internal friction of 38.8 degrees and a compression <br />strength at pillar ribside of 698 psi were used to provide a conservative <br />estimate of coal strength. A confined core pillar loading analysis adapted <br />from A. H. Wilson was used to estimate the factors of safety for various <br />retreat pillar sizes at various overburden depths. <br />The results of the pillar design analyses are presented in Figure 2.05.6A <br />and Table 2.05.66. Example calculations and information to support these <br />results are provided in Appendix G. <br />