Code of Federal Regulations Title 43. Public Lands:  Interior § 43.3931.100 Boundary pillars and buffer zones

(a) For underground mining operations, all boundary pillars must be at least 50 feet thick, unless otherwise specified in writing by the BLM. Boundary and other main pillars may be mined only with the BLM's prior written consent or on the BLM's order. For in-situ operations, a 50–foot buffer zone from the Federal lease line is required.

(b) If the oil shale on adjacent Federal lands has been worked out beyond any boundary pillar and no hazards exist, the operator or lessee must, on the BLM's written order, mine out and remove all available oil shale in such boundary pillar, both in the lands covered by the lease and in the adjacent Federal lands, when the BLM determines that such oil shale can be mined safely without undue hardship to the operator or lessee.

(c) If the mining rights in adjacent lands are privately owned or controlled, the lessee must have an agreement with the owners of such interests for the extraction of the oil shale in the boundary pillars.