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43 <br /> <br /> <br />• <br />to: 1) construct a compacted shale cutoff trench on the east and south <br />edges of the trench area that intercepts the old channel of Ennis Draw, <br />or 2) restrict mining operations to those areas that will not substantially <br />influence the subsurface water In the old channel of Ennis Draw will pro- <br />vide adequate protection. <br />6. Drawdown of Piezometric Surface Due to Pit Inflow <br />The theory leading to Eq. 1 also provides a means for estimating <br />the distance from the pit to points where the piezometric surface will <br />remain essentially undisturbed. The equation is <br />. L = ( STt X1/2 (2) ., <br />ya <br />where L is the distance from the pit to the point where the drawdown of <br />the piezometric surface is zero, and other symbols are as previously <br />defined. Using T=10 ft2/c and Sya=0.05, Eq. 2 yields the values of L <br />shown in Table 5.' Examples of the predicted shape -0f the piezometric <br />surface in the vicinity of the trench are shown in Figure 18. <br />Table 5. Distance From Pit to <br />Line of Zero Drawdown in the <br />The calculations shown in Table 5 Laramie Formation <br />indicate that the disturbance of the L t <br />ft years _ <br />piezometric surface in the Laramie 230 0.25 <br />330 0.50 <br />caused by pit inflow will extend less 400 0.75 _ <br />470 1.00 <br />than 0.5 miles on either side of the 660 2 <br />1050 5 <br />pit. Therefore, at no time during mining 1480 10 _ <br />2090 20 <br />should the piezometric surface in the <br />Laramie be disturbed beyond about 0.5,miles from the pit location. This <br />relatively small extent of the influence of the pit is due mainly to <br />the very small hydraulic conductivity of the Laramie overburden, causing 3 <br />the drawdown profile to be very steep in the vicinity of the pit. <br />