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~ ..I~ .''g, I lt'.i the surface. Aa the pillars are mined And the mine roof is allowed to cave, the broken and fallen roof rock expands or swells up eventually reaching the dome of the arch created by caving. Once this happens, ground movement is greatly reduced in magnitude. Movement will continue to occur until compaction is complete. The maximum limit of the caving may occur in the Cliff House sandstone becuase this formation is most likely to create the equilibrium arch. A second consideration is the thickness of the overburden. Generally, the thicker the overburden, the less likely ground substances will reach the sur- face. Overburden at the Blue Flame Coal Mine is shallow (maximum of 300 feet) compared to some operations on the western slope of Colorado which have over- burden depths of ],000 to 2,000 feet. The shallow overburden depths at mine would increase the probability of surface subsidence. However, the shallow overburden has resulted in a much reduced stress field around the mine working which enables much higher recovery of coal. This is the third consideration - the completeness of coal recovery. • Where 908 or more of the coal can be successfully recovered from a given mining section, the ground disturbances are more likely to cause a uniform setting of surface strata rather than the creation of cracks and joints. A gradual settling (trough shape) of the surface 1s preferable to cracks and joints. Based on experience in extracting pillars of the King Coal Mine, the condition of the old (abandoned) workings, dry mining environment, drill hole information, the known local geologic structure, rock strengths, overburden depths, and visual field inspections of the surface, it is our best determina- tion (without the aid of a traverse survey) that no surface subsidence ;has yet occurred and that the probability of its occurrence is thirty to forty percent as additional panels or mining sections are developed and pillared. If ground subsidence does reach the surface, we expect the surface subsidence to take the form of a trough resulting from gentle settling strata. The impact will be minimal to moderate with a surface traverse study required to determine its maximum depth of settlement or subsidence. Severe tension cracks or joints of widths greater than fractions of an inch or lengths greater than five to ten feet are not expected.