Geology Reference
In-Depth Information
[B] Permeability Test
For trial no. 1 of the permeability test, the head,
h
, is
10.9
-
5.4
, or
5.5 cm; the quantity of water discharged,
Q
, is
250 cm
3
; and the total
time of discharge,
t
, is
65 s
. Furthermore, the length between manome-
ter outlets,
L
, was determined to be
11.43 cm
, and area,
A
, was computed
to be 81.07 cm
2
. With these data known, the coefficient of permeability,
k
, can be computed using Eq. (18-3):
QL
Ath
k
(18-3)
(
250
)(
11.43
)
(81.07)(
65
)(5.5)
k
0.0986 cm
s
To correct for permeability at 20°C, the ratio of viscosity at 23°C to that
at 20°C is determined from Table 18-2 to be 0.9311. The permeability at
20°C is therefore
k
20°C
(0.0986)(0.9311)
0.0918 cm/s
In like manner, the value of
k
20°C
for trials no. 2 and 3 can be calculated
to be 0.0925 and 0.0911 cm/s, respectively. Taking an average gives
0.0918
0.0925
0.0911
Average
k
20¡ C
0.0918 cm
s
>
3
These results, together with the initial data, are summarized in the
form on pages 265 and 266. At the end of the chapter, two blank copies
of this form are included for the reader's use.
As mentioned previously, because permeability varies with void ratio, it
is recommended that permeability tests be performed on three different
specimens of the same soil sample, with each specimen having a differ-
ent void ratio. A relationship between void ratio and permeability can
be established for a given soil by plotting, on semilogarithmic graph
paper, a graph of permeability (logarithmic scale) versus void ratio
(arithmetic scale). This graph will usually approximate a straight line.
Then, whenever the permeability of that soil
in situ
is needed, a sample
can be taken at the project site, its void ratio determined, and the asso-
ciated permeability obtained from the graphical relationship.
Such a graph of permeability versus void ratio is shown in
Figure 18-3. The graph was obtained by plotting the results of three
different permeability tests corresponding to three different void ratios
on three different specimens of the same soil sample. The plotted point
labeled “A” represents the results of the permeability test related in the
section “Numerical Example.” The other plotted points represent the
results (not covered here) of two other permeability tests at different
void ratios on the same sample.
GRAPH
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