No pillar recovery shall be undertaken until such time as the plans are approved by the Department of Mines and Mineral Industries.¹ Application for pillar recovery shall be accompanied by four copies of the plan which shall include such information as shall be determined by the department. The approval or disapproval shall be based on the factors of depth, the thickness of the bed, the percentage of pillars proposed to be extracted and to be left, the effect on pillars remaining in overlying beds, and any other special features that may be deemed necessary by the department.