Problem 6. (12 points) The earth dam cross-section with flow net shown in below

Question

Problem 6. (12 points) The earth dam cross-section with flow net shown in below figure is to be built on isotropic gravelly sand with silt and cobbles. This dam will extend a distance of 260 meters perpendicular to the cross-section. To reduce the flow rate through these soils, a concrete cutoff wall will be built as shown. Compute the total flow beneath the dam (Q in m'/s). Elevation 214 m Earth dam (assume impervious) RESERVOIR Elevation 189 m Elevation 188 m 25x 16m conerete cutofwall . Gravelly sand with siltand cobbles Elevation 154i. :: Stiff clay (assume impervious)

Explanation / Answer

flow under the dam is nothing but the seppage loss rate (Q) = K*H*(Nf /Nd )*L

here

K = coefficent of permiability

H = driving head

Nf  = no. of flow channels

Nd = no. of potential drops

L = length of dam

H = head loss = 214 - 189 = 25 m

Nf = 4

Nd = 20

K = 6*10-4 m/ sec

discharge (Q) = 6*10-4 *25*(4/20)*260

= 0.78 m3/sec

  

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