Coal is mined in two seams that are shown in the sketch. Mining is full seam height in both seams. Entries are 24 ft (7.3 m) wide. Crosscuts associated with driving main entries are 18 ft (5.5 m) wide and driven on 120 ft (36.6 m) centers. Associated pillar length is three times pillar width. With reference to the stratigraphic column in the sketch and the rock properties tabulated there, determine the minimum pillar dimensions possible. Show solution is a carefully labeled plan view sketch. Note: No size effects arepresent.

E(10 psi (GPa) Rock -pcf (KNim) Colpsi) (MPa) Tolpsi) (MPa) hift) (m) type 50 (0.34) I Over 144.0 1.70 0.25 1,000 1,694 burden (22.8) (11.7) (6.9) (516) 2 Massive 148.2 3.00 0.20 19,000 850 1,714 (5.86) (522) sandstone (23.4) (20.7) (131.9) 3 Coal 0.30 1,719 (524) 75.0 0.35 3,500 130 (11.9) (24.1) (0.90) (24) 1.50 (10.3) 0.10 4 Layered sandstone 149.5 17,500 (120.7) (4.83) 700 1,741 (23.6) (531) 5 Massive 3.00 (20.7) 148.2 0.20 19,000 850 1,744 sandstone (23.4) (131.9) (5.86) (532) 6 Sandy 4.50 170.0 0.10 15,000 1.000 1,747 (6.90) shale (26.9) (31.0) (103.4) (533) 7 Coal 75.0 0.35 0.30 3,500 1.300 1.754 (8.97) (11.9) (2.4) (24.1) (535) 8 Massive sandstone 1,000 (134.5) (6.90) (542) 148.2 3.50 0.20 19,500 1,778 (23 4) (24.1) 9 Layered sandstone 0.10 149.5 1.50 17,500 700 2.250 (4.83) (23.6) (10.3) (120.7) (658) Sketch and geologic column, straca depths and rock properties