Environmental Engineering Reference
In-Depth Information
310
290
270
250
230
210
190
170
Water
NaOH (pH = 10.9)
150
130
10
6
1
10
100
1000
10,000
100,000
Suction, kPa
Figure 4.65
Initial and recalibration curves for University of Saskatchewan thermal conduc-
tivity suction sensors submerged in NaOH (pH 10.9) for 3 weeks (after Shuai et al., 2002).
800
700
600
500
400
300
HCl (pH = 4.25)
NaOH (pH = 10.9)
Best fit (HCl)
Best fit (NaOH)
200
100
0
0
100
200
300
400
500
600
700
800
Initial calibration reading, mV
Figure 4.66
Regression analysis of “before” and “after” calibrations on University of
Saskatchewan thermal conductivity suction sensors subjected to extreme chemical environments
(after Shuai et al., 2002).
4.3 MEASUREMENT OF TOTAL SUCTION
The free energy of the soil water (i.e., total suction) can
be determined by measuring the vapor pressure adjacent to
the soil-water or the relative humidity in the soil. The direct
measurement of relative humidity in a soil is performed using
a device called a psychrometer. The relative humidity in a
soil can be indirectly measured using filter paper as a measur-
ing sensor. The water content of the filter paper is calibrated
against total suction. One or more filter papers are then equi-
librated with the vapor pressure in the soil (i.e., total suction).
Environmental changes (i.e., weather at the ground surface)
are the primary cause of a change in the pore-water pres-
sures in a soil mass. Matric suctions in a soil mass change
as a result of moisture infiltration and evaporation at the
ground surface. Osmotic suction in the soil does not appear
to be highly sensitive to modest changes in the water con-
tent of the soil. As a result, a change in the total suction is
quite representative of a change in the matric suction. Total
suction measurements can sometimes be used in geotechni-
cal engineering to indicate changes in matric suction. This
is mainly true in the high-suction range when temperature
fluctuations are small.
4.3.1 Psychrometers
Thermocouple psychrometers can be used to measure the
total suction in a soil by measuring the relative humidity in
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