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Unformatted text preview: 39 where k, the permeability of the soil = 10-4 m/s; H, the overall head drop = 10 m; N F , the number of flowtubes = 4; and N H , the number of equipotential drops, = 4 The perimeter is 8 times the half-width of the excavation, = 160 m Hence s m m m s m q / 16 . 160 4 4 ) ( 10 ) / ( 10 3 4 = = or q = 160 litre/sec From the flownet, it may be seen that approximately 2 5 / 8 of the 4 flowtubes start in the medium sand. The proportion of the pumped groundwater that must therefore be recharged is approximately 2 5 / 8 4 66%. (Note that this estimate is on the high side, as some of this flow will enter the medium sand from the bedrock). The upward hydraulic gradient into the excavation is, scaling from the flownet, approximately 2.5 m 3.5 m or 0.71. While this is less than the critical value (i crit 1 for a soil with 2 w ), it is perhaps a little close for comfort particularly in the corners of the excavation, where flow from the two adjacent sides and the plane flownet calculation is not valid and should...
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- Fall '11
- Analytical Chemistry