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s. r- ' 4.20 SURFACE SUBSIDENCE SUBSIDENCE CONTROL Underground coal mining frequently causes Ground Subsidence whin coal i pillars are extracted. It is highly desirable for the mine roof to cave after pillars are removed to relieve stress concentrations in the active mine work- ings. Ground subsidence resulting from pillar extraction may rupture to the surface causing disruption of surface strata. Such disruptions are ommonly called surface subsidence. Surface subsidence may take the form of tension cracks, troughs, of planes. Surface strata may settle gently so that the troughs are not noticeable because no cracks or joints are visible. This type of (pre surface subsidence leaves no unattractive visual scars on the surface. ' may, nevertheless, occur to buildings and surface waterways, but it likely. • METHODS OF CONTROL r shear readily ferred) Damage is un- Where no surface subsidence can be tolerated, the coal pillars mush remain intact, and the size of the pillars must be such that they do not squeze or crush out over a long period of time but are able to stand indefinitelly. The size of the pillars that must remain is a function of the overburden tY(icknees and the strength of the pillars. Limited numbers of pillars may be (left Sn place to support overburden at some critical location where buildings or water- ways exit while the majority of the pillars are partially or fully extracted. I PROBABILITY OF SURFACE SUBSIDENCE AT THE BLUE FLAME COAL MINE Whether or not subsidence will reach the surface is difficult to, ascer- tain. In the case of Blue Flame Coal Mine's plan area, several factora~ should be considered that will effect the probability of surface subsidence. First, the thick Cliff House sandstone is above the Menefee coal seams. This comoe- • tent sandstone formation is about 20 feet above the mine workings. Its great strength and thickness could easily prevent subsidence from reaching upward to