Environmental Engineering Reference
In-Depth Information
10
-5
Drying permeability function calculated using statistical
method based on drying SWCC and
k
s
= 5.0
×
10
−
6
m/s
10
-6
10
-7
10
-8
10
-9
10
-10
10
-11
Coefficient of permeability obtained
from instantaneous profile method
10
-12
0.1
1
10
100
1000
10,000
100,000
Matric suction (
u
a
-
u
w
), kPa
Figure 7.41
Permeability function for the MRF 60-40 mixture computed using instantaneous
profile and statistical method (after Krisdani et al. 2009).
layer. The method is not practical when the matric suctions
exceed approximately 50 kPa because of the extremely slow
drainage process.
The instantaneous profile method also has a number of
disadvantages:
1. The method is time consuming and may take several
months to conduct.
2. A proper flow rate for wetting the soil is difficult to
choose. The flow rate limitation particularly applies
when performing the test in the laboratory. If the flowrate
is too high, the gradually changing suction versus water
content change profiles cannot be discerned. Daniel
(1983) suggested flow rates in the range of 0.2-5 cm
3
/
day. Flow rates of this magnitude are difficult to control.
3. The accuracy of the test is related to the space between
water content versus suction monitoring points. Theo-
retically, the closer the water content versus suction
monitors, the more accurate is the calculated unsatu-
rated permeability function. However, the use of too
many sensors may cause soil disturbance and affect
the water infiltration process.
Four Theta probes were installed for water content mea-
surement as well as four tensiometers for suction measure-
ment. The sensors were installed along the column at heights
of 100, 300, 600, and 800mm above the pedestal. Miniature
tip tensiometers with a high-air-entry ceramic cup were con-
nected to a pressure measuring device through a small plastic
tube. The ceramic cups were saturated and installed in the
soil. The working suction range of the tensiometers was
limited to between 0 and 90 kPa. Theta probe devices (i.e.,
ML2x model) from Delta-T Devices (Miller and Gaskin,
1997) were used to monitor water content. The voltage sig-
nal from the sensor was transformed to the volumetric water
content through use of a calibration curve.
The soil is initially placed into the column in a dry state.
During the wetting process, the dry soil in the column turned
to a darker color as water rose in the column. The change
in color can be used to provide information on the rate at
which the wetting front advanced. This information, along
with either the water content or soil suction measurements,
was used to calculate the permeability function for the soil. A
series of unsaturated capillary rise tests were conducted using
the soil column apparatus. The grain-size distribution curves
for five soils identified as GW-GMwith sand, SMwith gravel,
SC with gravel, sandy ML, and CL with sand are shown in
Fig. 7.43. The SWCCs for each of the soilswere independently
measured and the results are shown in Fig. 7.44.
7.6.4 Measurement of Wetting Permeability Function
Using Wetting-Front Column Technique (Li et al., 2009)
The “wetting-front” advancing column test has been per-
formed as either an infiltration test or a capillary rise test.
Only the capillary rise procedure and analysis are described
in this section. Figure 7.42 shows the soil column device
developed by Li et al. (2009) with water content and soil
suction measuring devices mounted along the walls of the
column. A water flow control and measurement system was
also part of the apparatus. The acrylic column was 120mm
in diameter and 1000mm high.
7.6.4.1 Method of Data Interpretation from
Wetting-Front Column Tests
Figure 7.45a shows the capillary rise of water from the bot-
tom of the column. The top of the soil column is at section
A
.
The water content versus suction measurement sensors were
installed at section
B
. The observed wetting front rises with
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