43 CFR 3594.2 - Support pillars.
Sufficient pillars shall be left during first mining to ensure the ultimate maximum recovery of mineral deposits prior to abandonment. All boundary pillars shall be 50 feet thick unless otherwise specified in writing by the authorized officer. Boundary and other main pillars shall be mined only with the written consent or by order of the authorized officer.
Title 43 published on 2013-10-01
no entries appear in the Federal Register after this date.