Geoscience Reference
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h A
h B
h C
Q v
D
A
C
Q h
B
H
L 1
L 2
Figure 4.11 Groundwater flow in a coastal or riverbank zone
In the aquifer, in zone AB, the groundwater flow is horizontal and in the zone
BC nearly horizontal (the deviation is in the order of k v / k h < 1%); see the dashed
flowline in Fig 4.11, which breaks at the interface. In zone AB the groundwater-
pressure gradient is i = ( h B - h A )/ L 1 = -0.04, the corresponding seepage velocity is
q h = - k h i = 4x10 -6 m/s, and therefore the total seepage under the dike becomes Q h =
q h H = 4x10 -5 m 3 /sm (about 3.5 m 3 per day per meter dike length).
In the zone BC, the groundwater leaks slowly vertically through the top clay
layer. Since after location C no effect is supposed, all the seepage Q h must vanish
by leakage in the zone BC. In average the potential head in zone BC is h v = ( h B -
h C )/2 = 1 m. The average (vertical) drop over the top clay layer is therefore 1 m,
the corresponding gradient is i v = - h v / D = -0.4, and the average vertical seepage in
the clay becomes q v = - i v k v = 0.4x10 -6 m/s. Therefore, the total leakage becomes Q v
= q v L 2 , which must be equal to Q h . Thus L 2 = Q h / q v = 100 m. 18
The pore pressure at the sand-clay interface, at location B, applying (4.7), is u B =
( D+h B )
w = 45 kPa. It exceeds the local vertical soil stress
vB , which is equal to
the local soil weight per m 2 , which, if for the clay
= 16 kN/m 3 is adopted, leads to
D = 40 kPa, at the interface. Hence, the clay layer will be lifted up (heave)
and a thin gap filled with water will occur at the interface, its length in the order of
25 m, in the present case. Since the pore pressure in this gap is limited by the local
soil weight, the potential head reduces to h B = (
vB =
w -D ) = 1.5 m (this is the value
one would measure). The occurrence of uplift affects the dike stability, since local
shear resistance will be significantly reduced.
Which river height would just not cause any uplift at the leeside of the dike? So,
is the stability of the dike affected by this river level?
vB /
application 4.5
A cofferdam has been constructed (see Fig 14.12) with two anchored walls AB
and CD and a fill with coarse clean sand. Estimate the permeability and the
variance (see Table 2.6). Determine from the sketched groundwater flow pattern
18 The leakage factor
( k h HD/k v ) is for the present case 54 m. A typical leakage zone
would be 3 = 162 m. The value L 2 of 100 m differs because of the simplifications.
=
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