Civil Engineering Reference
In-Depth Information
2.
Seepage of water through soil is governed by Darcy's law
V
=
ki
(14.4)
where
V
is the seepage velocity and
i
is the hydraulic gradient given by
=−
δ
P
δ
i
(14.2)
s
3.
Steady state seepage through a region of soil is described by a square flownet
consisting of an orthogonal net of flowlines and equipotential. Pore pressures can
be calculated from equipotentials. The total rate of flow through a flow through
a flownet is given by
k
N
f
q
=−
N
d
P
(14.9)
where
P
is the change of potential across the whole flownet. Flownets can be
obtained by sketching orthogonal nets that satisfy the boundary conditions.
4.
Seepage towards the ground surface or towards a slope may cause instabilities due
to piping or erosion.
For upward seepage towards the ground surface the critical hydraulic gradient
when
σ
v
=
0 is given by
=
γ
γ
i
c
w
−
1
(14.13)
Worked examples
Example 14.1: Confined flow
Figure 14.11 illustrates flow towards a long (out of the
page) land drain through a layer of soil with permeability
k
10
−
6
m/s sandwiched
between clay and rock, both of which may be considered to be impermeable. The water
level in the drain is 1 m below the water table which is at ground level 9 m away.
The phreatic surface is above the top of the soil and the flow is confined by the
impermeable clay. A simple square flownet is shown in Fig. 14.11 in which
N
f
=
=
3
Figure 14.11
Confined flow towards a drain - Example 14.1.
