Measurement and Estimation of State Variables - Unsaturated Soil Mechanics in Engineering Practice - page 125

Matric suction equalization Shearing

300

280

u a

Top
Middle
Bottom

260

240

220

130 h

200

180

0 25 50 75 100 125 150 175 200 225

Elapsed time, t (h)

Figure 4.24 Use of Nanyang Technological University mini high-range tensiometers at three
locations along triaxial specimen during shear at net confining pressure of 25 kPa (from Meilani
et al., 2002).

4.2.6.6 Moisture Transfer When Using High-Suction
Tensiometer
Moisture can transfer between the high-suction tensiometer
and the soil in both the liquid and vapor phases. Moisture from
the suction probe can evaporate into the soil in the vicinity of
the ceramic disk. Liquid water transfer occurs through direct
contact between the soil and the ceramic disk of the high-
suction tensiometer. Tension appears to be developed in the
high-suction tensiometer as a result of combined vapor and
liquid flowbetween the soil and the ceramic disk. Ifmoisture is
continuously lost from the system through evaporation during
the measurement of suction, there is a continuous increase in
observed pore-water tension. The amount of moisture lost
through evaporation may be minuscule; however, this loss
can have a significant effect on the suction measurement. The
response of the high-suction tensiometer can be divided into
the following three phases (Fig. 4.25):

a relatively short period of time (e.g., 5-10 m). In
phase I, liquid water transfer is more dominant than
the evaporation process (curve 1). Curve 2 (Fig. 4.25)
occurs when tension in the high-suction tensiometer has
already developed due to evaporation that took place
before the suction probe was put in contact with the
soil. However, the process of suction equalization due
to liquid water transfer will quickly become dominant
when there is good contact between the soil and the
high-suction tensiometer. The tension developed due to
evaporation decreases and equilibrium is reached with
the suction in the soil.
Phase II: Following phase I there is a process during
which suction equalization due to liquid water transfer
is dominant but slow. At the end of this phase, equi-
librium between the high-suction tensiometer and the
suction in the soil is reached. The liquid water transfer
from the soil to the high-suction tensiometer can off-
set the loss of moisture from the suction probe due to
evaporation.

Phase I: This is the initial phase of tension develop-
ment. Significant tension is rapidly developed during

Phase I Phase II Phase III

(Rapid
evaporation)

(Slow equalization
of matric suction)

(Slow drift due to
moisture loss)

Curve 2

Approximate
equilibrium

Curve 1

Phase I Phase II Phase III

(Slow drift due to
moisture loss)

(Rapid equalization
of matric suction)

(Slow equalization
of matric suction)

Time