Civil Engineering Reference
In-Depth Information
Figure 14.10
Flownets for anisotropic soil.
is approximately 20 kN/m
3
and
i
c
is approximately unity. Piping
or boiling will generally only occur for upward seepage towards the ground surface,
as shown in Fig. 14.9. You can create piping in the apparatus shown in Fig. 6.7 by
extending the standpipe and filling it to a height above ground level that is about twice
the depth of the model.
For many soils
γ
14.7 Seepage through anisotropic soils
Many soils are layered either because they were naturally deposited in changing depo-
sitional environments or because they were compacted in layers, with the result that
the permeability for horizontal flow
k
h
is often considerably greater than the perme-
ability for vertical flow
k
z
. In this case the flownet is not square and flowlines and
equipotentials do not intersect orthogonally, as shown in Fig. 14.10(a).
The flownet can, however, be made to be square by transforming the horizontal axis
to
H
and the mean coefficient of permeability to
k
, where
k
z
k
h
H
H
=
(14.14)
k
z
k
h
k
=
(14.15)
as shown in Fig. 14.10(b). The theoretical derivations for these transformations
are beyond the scope of this topic and are given in textbooks on hydraulics and
groundwater flow.
14.8 Summary
1. For steady state seepage pore water pressures
u
at point are given by the
potential
P
:
u
γ
P
=
h
w
+
z
=
w
+
z
(14.1)
where
z
is the elevation of the point above an arbitrary datum.
