A pillar in a hard rock, strata-bound, room, and pillar mine is transected by a small fault that dips 75? due east and strikes due north. The ore horizon dips due west 15? and also strikes due north. Depth to the pillar of interest is 1,230 ft (375 m). Overburden specific weight averages 158pcf (25kN/m3). Rock and joint properties are given in Table 6.1. The preexcavation stress is caused by gravity alone; the horizontal stress is estimated to be one-fourth the vertical stress before mining. Determine whether the pillar is stable in consideration of a planned extraction ratio of 75%.

Property material Co psi (MPa) To psi (MPa) c psi (MPa) Rock Joint 15,000 (103) 66.6 (0.459) 2,372 (16.4) 54.9 28.0 24.0 (0.166) 20.0 (0.138) 1,500 (10.3)