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<br />27 <br />• ~ gradient is unity and that the vertical discharge per unit area is equal to <br />Ka <br />Computation of Model Parameters <br />The parameters required to calculate the mine inflo~as 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 />i ba; the drawdown at the face, so; the length of perimeter, and the area of <br />1 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- <br />cation is being affected by the drawdown in the mine. Figure 7 shows the <br />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 />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 ~dhich Ka may be estimated. The drawdown in L.4 218A is 55 ft. <br />and is located 1100 ft. from the mine face. From the foregoing formulas <br />x 1100 <br />8 = In so = In 22~ = 781 ft. <br />S 55 <br />The definition of 6 is rearranged so that Ka can he computed: <br />K __ Tba __ (0.56} (225} = 2.1x10-4 ft/d. <br />a 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 penneabilities of the <br />