Environmental Engineering Reference
In-Depth Information
1,000,000
160
Distilled water (0 kPa) - drying
Distilled water (0 kPa) - wetting
Salt solution (9700 kPa) - drying
Salt solution (9700 kPa) - wetting
In air (70000 kPa) - drying
In air (70000 kPa) - wetting
Ridley (1995) - drying
Ridley (1995) - wetting
Harrison & Blight (1998) - drying
Harrison & Blight (1998) - wetting
Leong et al. (2002) - drying
Leong et al. (2002) - wetting
140
100,000
From solution
120
10,000
100
1,000
80
60
100
40
From suction plate, pressure plate,
and pressure membrane extractor
10
20
1
0
20
40
60
80
100
120
0
5
10
15
20
25
30
35
Filter paper water content, %
Duration, days
(a)
(a)
1,000,000
120
Distilled water (0 kPa) - drying
Distilled water (0 kPa) - wetting
Salt solution (9700 kPa) - drying
Salt solution (9700 kPa) - wetting
In air (70000 kPa) - drying
In air (70000 kPa) - wetting
McQueen & Miller (1968) - drying
McQueen & Miller (1968) - wetting
Harrison & Blight (1998) - drying
Harrison & Blight (1998) - wetting
Leong et al. (2002) - drying
Leong et al. (2002) - wetting
From solution
100,000
100
10,000
80
1,000
60
100
40
From suction plate, pressure plate,
and pressure membrane extractor
10
20
1
0
20
40
60
80
100
120
0
5
10
15
20
25
30
35
Filter paper water content, %
Duration, days
(b)
(b)
Figure 4.79
Drying and wetting processes in coming to equilib-
rium (from Rahardjo and Leong, 2006): (a) data for Whatman No.
42 filter paper; (b) data for Schleicher and Schuell No. 589 filter
paper.
Figure 4.80
Time required for water content equilibration in fil-
ter paper (from Rahardjo and Leong, 2006): (a) equilibrium for
Whatman No. 42 filter paper; (b) equilibrium for Schleicher and
Schuell No. 589 filter paper.
Figures 4.81a and 4.81b present the equilibrium drying
and wetting water contents,
w
f
, for Whatman No. 42 and
Schleicher and Schuell No. 589 filter papers, respectively,
that were held over varying concentration salt solutions. The
equilibrium water content was taken as the water content
of the filter paper when there was no measurable change
in the weight of the filter paper with time. The hysteresis
observed was small (i.e., 1-5% in terms of the water content
of the filter paper), with the largest differences observed
for suctions less than 100 kPa. Hysteresis appeared to be
minimal when sufficient equilibration time was allowed.
Equalization Time.
Table 4.13 presents various equilibra-
tion times that have been used when making suction mea-
surements with filter paper. ASTM D5298-94 recommends
a minimum equilibration time of seven days. Two points are
important when examining the time required for the equi-
libration of the filter paper with a suction source. Time is
required for equilibration between the environment and suc-
tion source. The water vapor in the air space of a closed
container takes time to reach equilibrium. The equilibrium
time is particularly important when calibrating filter papers
using salt solutions. Time is also required for equilibration
between the filter paper and the suction source. The water
vapor in the air space must first reach equilibrium and then
the filter paper will come to equilibrium with the water vapor
in the air space.
Matric suction is applied instantly when filter paper is
calibrated using a pressure plate apparatus. The equilibra-
tion time is the time required for the filter paper to achieve
equilibrium with the applied matric suction. The filter paper
equilibration time depends on suction source, contact con-
dition, and suction level, as illustrated by the differences in
equilibration time reported in Table 4.13.
The equilibration of water vapor with the suction source
appears to be particularly important when an enclosed envi-
ronment with a soil specimen is used. Water must be lost or
gained from the soil specimen to the environment in order to
establish an equilibrium condition. A small enclosed envi-
ronment enables equilibrium conditions to be achieved more
rapidly (McQueen and Miller 1968a).
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