Civil Engineering Reference
In-Depth Information
Steady state seepage
14.1 Groundwater conditions
You know that water flows downhill and you have probably studied the flow of water
in pipes and open channels in courses on hydraulics. Water also flows through soils
in much the same way but the flow is retarded as it flows past the grains. Theories for
groundwater flow are covered in courses in hydraulics and all I will do here is consider
the topics essential for geotechnical engineering. There are essentially three separate
conditions for groundwater in geotechnical engineering and simple examples of these
are illustrated in Fig. 14.1.
(a) Hydrostatic states
This condition, illustrated in Fig. 14.1(a), was discussed in Sec. 6.3. If the water table,
or phreatic surface, is level there is no flow. Pore pressures are hydrostatic, are given
by
u
=
γ
w
h
w
and this is the same whether there are soil grains or not.
(b) Steady state seepage
If the phreatic surface is not level, as in Fig. 14.1(b), water will flow along flowlines
such as ABC. At any point, such as at A, the pore pressures will be
u
=
γ
w
h
w
, where
h
w
is the height of water in a standpipe. Note that the level of water in the pipe does not
necessarily define the phreatic surface (see Sec. 14.5). Notice also that in Fig. 14.1(b)
the flow is apparently uphill from A to B and that the pore pressure at C is greater
than that at B.
The basic rule for the flow of water through a single element of soil is Darcy's law,
which was introduced in Sec. 6.10 in connection with relative rates of loading and
drainage. In this chapter we will extend Darcy's law to cover seepage through a whole
region of soil. The essential feature of steady state seepage is that neither the pore pres-
sures nor the rates of flow change with time. Since effective stresses remain constant,
the soil grains can be taken to be stationary as water flows through the pore channels.
(c) Consolidating soil
When pore pressures change with time effective stresses and soil volumes also change
with time. This process, which couples Darcy's seepage theory with soil compression
