A thin ?low? metallurgical coal seam 3 ft (1 m) thick is mined at a depth of 980 ft (300 m). The main entries are mined 4 ft into the R1 roof stratum to give sufficient clearance for track haulage. With reference to the strati graphic column in the sketch and the rock properties given in Table 6.12, assume rectangular pillars that are twice as long as they are wide and crosscuts are the same width as the entries. Find:?

1 the maximum possible extraction ratio;

2 pillar width (at a safety factor of 1.5 with respect to compression), and

3 the crosscut spacing (at Fc = 1.5). Show by sketch.

Table 6.12 Strata data Rock type E 106 psi (GPa) Co psi (MPa) To psi (MPa) y pef (kN/m?) h ft (m) 21.5 (6.55) 15.2 (4.63) 1.0 (0.30) 1.5 (0.46) 6.0 (1.83) 3.0 (0.91) 10.3 (3.14) 13.0 (3.96) 12.6 (86.9) 12.7 (87.6) 5.4 (37.2) 6.3 (43.4) 12.6 (86.9) 0.75 (5.2) 12.3 (84.8) 8.1 (55.9) 160 (25.3) 158 (25.0) 152 (23.1) 146 (24.7) 156 (24.7) 92 (14.6) 155 (24.5) 160 (25.3) R5 0.19 0.21 12,700 (87.6) 14,200 (97.9) 7,980 (55.0) 8,450 (58.3) 12,700 (87.6) 3,750 (25.9) 11,500 (79.3) 13,700 (94.5) 1,270 (8.8) 1,34 (9.2) 610 (4.2) 760 (5.2) 1,270 (8.8) 425 (2.9) 1,050 (7.2) 1.260 (8.7) R4 R3 0.23 0.31 R2 RI 0.19 0.27 SI FI 0.18 0.33 F2 Surface (Not to scale) Overburden RS mudstone R4 sandstone R3 laminated shale R2 laminated sandstone RI (roof) sandstone SI (seam - coal) FI (floor) shale F2 limescone