Civil Engineering Reference
In-Depth Information
Figure 7.11
Results of constant head permeability test - Example 7.1.
is correct. The coefficient of permeability given by the gradient of the line is
10
−
3
m/s
k
≈
1
×
Example 7.2: Interpretation of a falling head permeameter test
A falling head perme-
ameter has a diameter of 100 mm, the sample is 100 mm long and the area of the
standpipe is 70 mm
2
. Results of a test on a relatively fine-grained soil are given in
Table 7.2.
Table 7.2
Results of falling head permeability test - Example 7.2
Time (s)
Height of water in standpipe
above overflow (m)
(P
0
/P)
ln(P
0
/P)
0
1.60
1
0
60
1.51
1.06
0.06
120
1.42
1.13
0.12
240
1.26
1.27
0.24
480
0.99
1.62
0.48
At any instant the potential
P
is the height of water in the standpipe (above the
overflow) and
P
0
=
=
t
0
. Figure 7.12 shows the ln(
P
0
/
P
) plotted against
time. The data points fall close to a straight line. Hence, from Eq. (7.9) the coefficient
of permeability is given by
1.60 m at
t
(0.001)
2
aL
A
ln(
P
0
/
P
)
t
70
×
×
0.1
0.1
100
≈
10
−
6
m/s
k
=
=
×
1
×
(0.1)
2
(
π
/4)
×
