46 Principles of Sustainable Energy Systems
© 2008 Taylor & Francis Group, LLC
than 200 ft below the surface are normally extracted in open cast mines. The coal is blasted
and fractured in strips, thus the term strip-mining. In the United States, more than 65% of
coal is surface mined. Underground coal mines have been developed up to 1500 ft deep,
but some mines in China have developed to depths of 3900 ft. Spoil refers to the overbur-
den, top soil, and sedimentary layers between coal seams that must be placed and stored.
The overburden rate refers to the depth of overburden compared to the coal seam depth.
Typical overburden rates exceed 10:1 (10 ft of soil for 1 ft of coal). The spoil materials are
acidic, and the spoil is placed in engi