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27 <br /> gradient is unity and that the vertical discharge per unit area is equal to <br /> Ka. <br /> Computation of Model Parameters <br /> J <br /> The parameters required to calculate the mine inflows from the above <br /> formulas are the transmissivity of the coal seam, T; the mean vertical <br /> permeability of the overburden, Ka; the effective thickness of overburden, <br /> b a; the drawdown at the face, so; the length of perimeter, and the area of <br /> the exposed roof. For the purposes of determining the model parameters and <br /> computing mine inflows it is assumed that the piezometric surface is parallel <br /> to the surface of the coal seam so that so is a constant. <br /> Current mining activities in the Maxwell seam have advanced to within <br /> ' 1100 ft. of well LA 218A and the piezometric surface elevation at that lo-cation cation is being affected by the drawdown in the mine. Figure 7 shows the <br /> 1 distribution of piezometric surface along a line joining LA 221A, LA 218A, <br /> and a point in the mine southwest of LA 218A. This figure indicates that <br /> the piezometric surface elevation in LA 221A is a valid representation <br /> of the undisturbed surface. The elevation of the coal seam in LA 221A is <br /> i 6748, yielding a value of 225 ft. for so. It is assumed that ba is also <br /> 225 ft. which is tantamount to regarding the entire 225 ft. above the coal <br /> as being saturated and capable of transmitting water vertically. <br /> The data in Figure 7 also provide the opportunity to compute the leak- <br /> ance B from which Ka may be estimated. The drawdown in LA 218A is 55 ft. <br /> and is located 1100 ft. from the mine face. From the foregoing formulas <br /> i <br /> _ x _ 1100 <br /> B In so In 225 - 781 ft. <br /> 5 55 <br /> j The definition of B is rearranged so that Ka can be computed: <br /> Ka - Tb2 _ (0.56) (225) = 2.lx10-4 ft/d. <br /> B (781 )2 <br /> The reliability of this value of vertical permeability of the overburden can <br /> be checked by computing Ka from the measured vertical permeabilities of the <br /> 1 <br />