With reference to the sketch, Table 6.13 and the lower coal seam, if a factor of safety of 1.5 with respect to compression is required when entries and crosscuts are actually 18 ft (5.5 m) wide and pillars are twice as long as they are wide, find (with neglect of any size effect on strength):

1 width of pillars and length of pillars;

2 entry and crosscut spacing (center tocenter).

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Table 6.13 Strata properties h(ft) (m) E(1 0ʻ)psi (GPa) Co(psi) (MPa) Rock -pef (kN/m?) To(psi) (MPa) type 50 I Over 1,000 144.0 1.70 0.25 1,694 (516) burden (22.8) (I1.7) (6.9) (0.34) 3.00 2 Massive 148.2 0.20 19,000 850 1,714 (23.4) (20.7) (522) sandstone (131.9) (5.86) 0.30 3 Coal 75.0 0.35 3,500 130 1,719 (11.9) (2.4) (24.1) (0.90) (524) 4 Layered 17,500 (120.7) 1,741 149.5 1.50 0.10 700 (23.6) (4.83) sandstone (10.3) (531) 5 Massive 148.2 3.00 0.20 19,000 850 1,744 (20.7) sandstone (23.4) (131.0) (5.86) (532) 6 Sandy shale 15,000 1,000 170.0 4.50 0.10 1,747 (31.0) (26.9) (103.4) (6.90) (533) 7 Coal 75.0 0.30 0.35 3,500 1,300 1,754 (24.1) (I1.9) (2.4) (8.97) (535) 8 Massive 1,000 (6.90) 148.2 3.50 0.20 19,500 1,778 (542) sandstone (23.4) (24. 1) (134.5) 9 Layered 149.5 1.50 0.10 17,500 700 2,250 (658) sandstone (23.6) (10.3) (120.7) (4.83) Sketch and geologic column