Civil Engineering Reference
In-Depth Information
2.6 Determination of permeability in the field
2.6.1 The pumping out test
The pumping out test can be used to measure the average k value of a stratum of soil below the water
table and is effective up to depths of about 45 m.
A casing of about 400 mm diameter is driven to bedrock or to impervious stratum. Observation wells
of at least 35 mm diameter are put down on radial lines from the casing, and both the casing and the
observation wells are perforated to allow easy entrance of water. The test consists of pumping water out
from the central casing at a measured rate (q), and observing the resulting drawdown in groundwater
level by means of the observation wells.
At least four observation wells, arranged in two rows at right angles to each other should be used
although it may be necessary to install extra wells if the initial ones give irregular results. If there is a risk
of fine soil particles clogging the observation wells then the wells should be surrounded by a suitably
graded filter material (the design of filters is discussed later in this chapter).
It may be that the site boundary conditions, e.g. a river, canal or a steep sloping surface of imperme-
able subsurface rock, a fault or a dyke, do not allow the two rows of observation wells to be placed at
right angles. In such circumstances the two rows of wells should be placed parallel to each other and at
right angles to the offending boundary.
The minimum distance between the observation wells and the pumping well should be ten times the
radius of the pumping well and at least one of the observation wells in each row should be at a radial
distance greater than twice the thickness of the ground being tested.
In addition to the observation wells an additional standpipe inside the pumping well is desirable so that
a reliable record of the drawdown of the well itself can be obtained.
Figure
2.5
illustrates conditions during pumping.
Consider an intermediate distance r from the centre of the pumping well and let the height of the GWL
above the impermeable layer during pumping be h.
The hydraulic gradient, i, is equal to the slope of the
=
∂
∂
h
r
h r curve
−
where 2
π
rh
=
area of the walls of an imaginary cylinder of radius r and height h. Now
∂
∂
h
r
q Aki
=
=
2
π
rhk
i.e.
∂
=
r
q
k
2
π
h h
∂
r
Fig. 2.5
The pumping out test.
