Civil Engineering Reference
In-Depth Information
dry for a considerable time. The reason for this is that if the clay has low permeabil-
ity it will take a very long time for sufficient water to flow from the ground to fill
the borehole. This means you can only determine pore pressures, and groundwater
conditions, from observation in boreholes in soils with relatively high permeability.
For clays and soils with low permeability you will need to use special piezometers (i.e.
instruments to measure pore pressures). In the final analysis the groundwater condi-
tions must be reasonable and self-consistent and compatible with the soils and the
regional hydrogeology.
Values for the coefficient of permeability
k
can be found from the results of
in situ
pumping tests. For coarse-grained soils steady state conditions will be reached quickly.
Figure 17.6(a) illustrates steady state flow towards a pumped well. The potential at a
radius
r
is
P
and, from Darcy's law (see Chapter 14), the rate of flow
q
is
rPk
d
P
d
r
q
=
Aki
=
2
π
(17.2)
or
d
r
r
2
π
k
=
P
d
P
(17.3)
q
Figure 17.6 In situ
permeability tests.
