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27 ~ `i gradient is unity and that the vertical discharge per unit area is equal to ~~ Ka. Computation of Model Parameters 1 The parameters required to calculate the mine inflows from the above 11 formulas are the transmissivity of the coal seam, T; the mean vertical permeability of the overburden, Ka; the effective thickness of overburden, I~ ba; the drawdown at the face, so; the length of perimeter, and the area of the exposed roof. For the purposes of determining the model parameters and computing mine inflows it is assumed that the piezometric surface is parallel 1~ to the surface of the coal seam so that so is a constant. Current mining activities in the Maxwell seam have advanced to within ~I 1100 ft. of well LA 21&4 and the piezometric surface elevation at that lo- cation is being affected by the drawdown in the mine. Figure 7 shows the I dtstribution of piezometric surface along aline joining LA 221A, L?. 218A, and a point in the mine southwest of lA 218A. This figure indicates that the piezometric surface elevation in LA 221A is a valid representation of the undisturbed surface. The elevation of the coal seam in lA 221A is 6748, yielding a value, of 225 ft. for so. It is assumed that ba is also 225 ft. which is tantamount to regarding the entire 225 ft. above the coal as being saturated and capable of transmitting water vertically. ~ The data in Figure 7 also provide the opportunity to compute the leak- , ante 8 from which Ka may be estimated. The drawdown in ~.4 218A is 55 ft. and is located 1100 ft. from the mine face. From the foregoing formulas ' x 1100 B~~ ~ ~ =781 ft. s The definition of B is rearranged so that Ka can be computed: Ka ~ ~0.56 225 2.1x10-4 ft/d. B (781)2 . +,~I. The reliability of this value of vertical permeability of the overburden can be checked by computing Ka from the measured vertical permeabilities of the