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Chapter IV <br /> A study accomplished by Dr. Bruce A. Collins for Mid-Continent in 1985 based on four <br /> years of subsidence monitoring data concludes that the maximum vertical displacement <br /> under worst case conditions would not exceed 7.2 feet. See Collins' study at Appendix <br /> S IV-F-4. <br /> The surface expression of the amount of vertical subsidence, even under worst case <br /> conditions, should appear in a relatively uniform pattern. This would be due to the <br /> predominantly weak, highly-stratified rock (alternating strong brittle sandstones and <br /> plastic shales) throughout the stratigraphic column (and particularly those at and near <br /> the surface)-and the magnitude of the depth to the coal bed over which subsidence <br /> stresses are distributed. <br /> These conditons are conducive to "plastic" deformation of the surface as opposed to <br /> development of widespread random fractures and hummocked profiles. <br /> The resultant horizontal dimension of subsidence is determined by the angle of draw. <br /> The angle of draw is influenced by rock strength and structural features within the rock <br /> sequence, such as joints and faults. Topographic features such as extensive <br /> (compared to mine depth) abrupt slope changes also affect angle of draw. <br /> The angle of draw has been measured in rocks similar to those in Coal Basin as 12 <br /> degrees to 20 degrees. It is believed that an average of 15 degrees would include the <br /> worst case situation. Therefore, the horizontal distance from the rib line of the mine <br /> panel to the edge of disturbance would be: <br /> ix=h tan 150 cost G <br /> x'=horizontal projection fo the limit of draw corrected <br /> for topographic slope <br /> h=vertical depth from coal to the surface <br /> 0=the angle of slope taken from the horizontal. <br /> Depth from the surface to the B Bed ranges from 2200 feet to 2800 feet throughout <br /> most of the planned mining area during the five year permit period. Therefore, <br /> subsidence could occur on the surface from 600 to 750 feet from the vertical extension <br /> of the edge of the coal to be mined. <br /> The Dr. Collins study at Appendix IV-F-4, based on four years monitoring data, <br /> determines that the worst case angle of draw would not exceed 18 degrees and that <br /> subsidence could occur on the surface from 400 to 815 feet from the vertical extension <br /> of the edge of mined out areas. <br /> In summary, this analysis of worst case surface subsidence concludes that the <br /> maximum vertical disturbance above the Coal Basin underground workings would be <br /> 30 <br />