Land subsidence above underground mines in the main bituminous coal fields is a significant problem. Apparently, there is no depth for a mine at which the surface can be considered safe. More insidious is the fact that land subsidence might not occur until more than 100 years after mining has ceased. The possibility of subsidence above an abandoned mine must be anticipated except where total extraction has been achieved, permitting subsidence concurrent with mining or where large pillars adequate for long term support remain.
Precisely when collapse might take place in the absence of stabilization is not predictable. Based on available information (see Exhibit 2) a dominant portion of Fayette County west of Chestnut Ridge has been undermined and needs careful consideration prior to any development. In some areas, controls to limit construction and development may be justified to protect the public.
Sinkholes & Troughs
Subsidence features over mines are classified as sinkholes or troughs. A sinkhole is a depression in the ground surface that occurs from collapse of the overburden into a mine opening (a room or entry). A study of subsidence in the Pittsburgh area revealed that 95% of the sinkholes occurred on sites located less than 60 feet above mine level.
A trough is a shallow, commonly broad, dish shaped depression that develops when the overburden sags downward into a mine opening in response to extraction, crushing of mine pillars, or punching of mine pillars into the mine floor. There appears to be no safe depth of mining that prevents trough development.