1. Barczak TM. 1989. Safety evaluation of longwall roof supports, United States Bureau of Mines, IC 9221, Pittsburgh Research Centre, Pittsburgh, PA, USA.
2. Barczak TM, Schwemmer DE. 1988. Stiffness characteristics of longwall shields, US Department of Interior, Bureau of Mines, RI 9154, Pittsburgh Research Centre, Pittsburgh, PA, USA.
3. BP Statistical Energy Survey. 2010. http://www.bp.com/subsection.do?categoryId=9023763&contentId=7044548. (accessed 31 January 2011).
4. Interaction of powered supports and the strata in coal seams under a heavy roof