Main entries are driven in sets of seven at a depth of 1,466 mt in flat-lying strata. Entries are 6 m wide (Wo); pillars are 24 m 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 12 m 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 10 m, while keeping the center to center spacing of pillars and entries the same. Which plan is preferable from the rock mechanics viewpoint? Explain.