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40 <br /> <br />qo = natural flow In undisturbed aquifer per unit of trench • <br />length, (L3/LT) <br />ti = time at which segment i came into existence (T). <br />This equation was derived using a "succession of steady states" approach <br />which accounts for the variable saturated thickness of the aquifer in the <br />vicinity of the pit. The mean discharge (rate of inflow) over a time <br />period is computed by dividing the volume that has accumulated over the <br />period by the duration of the period. <br />Inflow estimates were. made by applying equation 1 to trench segments <br />500 ft in length. At a rate of advance equal to 50 ft/day, a new segment <br />comes into existence every 10 days until a trench length of 3000 ft is <br />obtained. After that time, no new segments are considered because the <br />trench will be advancing into previously drained overburden as explained <br />previously. The cumulative volume for the entire. length of open trench <br />at any time is computed by adding the volumes contributed up to the time <br />of interest by each segment. .. <br />The estimated cumulative volume and rates of inflow are shown in <br />.Figure 17 far the first 460 days of operations. The following values <br />were used for the parameters in equation 1: T = 10 ft2/d (average deter- <br />mined from aquifer tests),~Sya = 0.05 (typical value for clay and shale <br />materials), ho = 65 ft (average from Fig. 4), qo = 0.08 ft2/d (from <br />measured T and slope of piezometric surface), and L = 500 ft. The first <br />peak in discharge occurs at the time when the trench in Pit A reaches a <br />length of 3000 ft. The subsequent decline of inflow rate represents the <br />continued drainage to the 3000 ft trench under the influence of progres- <br />sively smaller hydraulic gradients. The second peak occurs as the result <br />•• <br />of opening a new trench in Pit B during the first part of the second year _ <br />