With reference to the strata data given in the sketch and Table 6.4, seam depth is 1,670

Question:

With reference to the strata data given in the sketch and Table 6.4, seam depth is 1,670 ft, strata dip is negligible, and entries are planned 24 ft wide. Mining height from the sandstone floor is 12 ft. If crosscuts are as wide as entries and pillar length is three times pillar width:

1. Determine the extraction ratio possible with a pillar safety factor of 1.4 and no size effects.

2. Determine the pillar dimensions, entry spacing (center to center), and crosscut spacing (c-c) at the FS and R determined previously.

3. If strata dip were 25◦, estimate pillar stresses at a depth of 1,670 ft, assuming an extraction ratio of 20%.

4. A joint set is present, strata are flat-lying and extraction is 20%; determine the range of joint dips that may lead to joint slip, assuming Mohr–Coulomb criteria for strata and joints with joint friction angle at 28◦ and negligible joint cohesion.

Co 103 psi (MPa) Rock type E 106 psi (GPa) 103 y pef (kNim?) To psi (MPa) h ft (m) 1.2 (8.3) 4.3 (29.7) 0.25 1.7 (I1.7)