A group of spaced apart in situ oil shale retorts is formed in a subterranean formation containing oil shale. At least one void is excavated in each retort site, and remaining formation within each retort site is explosively expanded toward such a void for forming a fragmented permeable mass of formation...http://www.google.de/patents/US4176882?utm_source=gb-gplus-sharePatent US4176882 - In situ oil shale retorts with gas barriers for maximizing product recovery
