Development entries 16 ft (4.9 m) wide are driven in a steeply dipping anthracite coal seam at a depth of 760 ft (232 m). Crosscuts are not a factor with distance between raises on the dip at 300 ft (91 m). (The extraction ratio in cross section is thus R'.) Strata dip is 55◦; seam thickness is 28 ft (8.5 m). The premining stress field is considered to be attributable to gravity alone, although no measurements are available. Laboratory tests for unconfined compressive strength show a size effect given by Cp = C1 [0.78 + 0.22(D/L)]. Specifically, C1 = 5, 250 psi (36.2 MPa). Triaxial test show data that fit a Mohr–Coulomb criterion reasonably well with φ = 35◦ and c = 1, 210 psi (8.34 MPa). A safety factor with respect to pillar compression of 2.75 is desired. Estimate the maximum safe extraction ratio and corresponding pillar width?