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21 - ~) <br /> assumptions made. It would therefore appear that a ratio <br /> iof 0.30 future planning purposes would be sufficiently con- <br /> servative. <br /> Finally, while obviously measureable surface subsidence has <br /> occurred over mined-out areas of the Coal Basin mines, <br /> inspections of large areas of the surface by helicopter and <br /> ground traverse, by a variety of governmental agency personnel, <br /> consulting engineers and geologists, and company engineers <br /> and environmental specialists, have revealed no visual effects <br /> of this subsidence, such as surface cracks, bed separations, <br /> subsidence pits (hummocked profiles) , or subsidence-induced <br /> slumps or landslides, in spite of nearly 30 years of mining <br /> activities, including 8 years og longwall mining which results <br /> in complete extraction of the coal bed over fairly large areas. <br /> While there are only a few improvements, such as ventilation <br /> fans, power lines and light-duty access roads, over mined-out <br /> areas, no damage to any of these facilities has been noted. <br /> The total lack of observed subsidence effects to date is <br /> attributed to a combination of the strength of overburden rocks <br /> (particularly apparent in M bed workings over B bed subsidence <br /> zones; while stress effects have occurred marginal to these <br /> zones, no offsets or other breaks have been observed) ; to <br /> bulking of overburden into caved areas; to the generally great <br /> depth of cover; and to professionally designed and managed <br /> extraction techniques, which have left a minimum of pillars, <br /> barriers and so on over which compression arches and hummocked <br /> profiles could develop as adjacent mined-out areas subside. <br />