Main entries are driven in sets of seven at a depth of 1,530 ft in flat-lying strata. Entries are 20 ft wide (Wo); pillars are 80 ft wide (Wp). In vertical section showing entries and pillar widths, the two-dimensional extraction ratio R' = Wo/(Wo + Wp). Three proposals are presented for increasing this extraction ratio to 33% from the existing 20%. (1) Decrease pillar width to 40 ft while keeping entry width the same; (2) increase entry width while keeping pillar width the same, and (3) decrease pillar width while increasing entry width to 33-1/3 ft, while keeping the center to center spacing of pillars and entries the same. Which plan is preferable from the rock mechanics viewpoint? Explain.