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• 2.05.6(6) <br />the ground surface. This gradual failure of pillars and surface <br />movement which is planned at the Bear No. 3 Mine will be measurable <br />but probably not noticeable at the ground surface. <br />• <br />Prediction of chimney collapse above mine openings in the Bear No. <br />3 Mine is important because a chimney collapse has the potential to <br />breach the surface, possibly reactivating an old land slide or <br />capturing streams or springs if it should occur directly underneath <br />such a surface feature. Prediction of trough subsidence is <br />important because the tensile strains in the overlying strata that <br />would follow pillar collapse will lower the surface topography <br />possibly reactivating landslides and capturing streams or springs <br />on the surface. <br />Chimney Collapse Prediction <br />Piggott and Eynon (1977) presented a mathematical method of <br />predicting the height of chimney development based on the mining <br />height and the percent free swell of the collapsing roof rock. The <br />Bear No. 3 Mine mains are presently being mined at a eight to ten <br />foot mining height. This prediction will assume a maximum possible <br />twelve foot mining height. If twelve feet of coal is extracted <br />from the B -seam and the overburden is entirely sandstone, 67 <br />percent swell, the maximum height of potential conical, worst -case, <br />chimney collapse would be approximately 54 feet. If twelve feet of <br />B -Seam coal is extracted and the entire overburden is shale, 33 <br />percent swell, the maximum height of conical, worst -case, chimney <br />collapse is approximately 109 feet. <br />Conservative application of Piggot and Eynon's (1977) worst -case <br />conical chimney height prediction method indicates that even a <br />worst -case conical chimney collapse should not breach the 110 foot <br />minimum overburden limit adopted by the Operator for the Bear No. <br />3 Mine. <br />Conical chimney collapse height (H) Piggot and Eynon (1977) <br />Sandstone overburden <br />H = 3h(100) /%S h - mining height (12 feet) <br />H = 3(12)(100)/67 %S - percent free swell (67 %) <br />H = 54 feet <br />2.05 -59 Rev. 11 -30 -93 <br />