Measurement and Estimation of State Variables - Unsaturated Soil Mechanics in Engineering Practice - page 138

Figure 4.47 Laboratory measurement of matric suction on high-plasticity clay from Darke Hall,
Regina, Canada, using thermal conductivity suction sensors ( w = 35 . 1%).

to equilibrium with the water in the soil. The responses of
both sensors were monitored at various elapsed times after
their installation. The results indicate that the time required
for the initially dry sensor to come to equilibrium with the
soil specimen was less than the equilibrium time required
for the initially saturated sensor to come to equilibrium.
On the basis of numerous laboratory experiments, it would
appear that the thermal conductivity suction sensors that were
initially dry yielded a matric suction value which was close
to the correct value. In general, the initially dry sensor should
yield a value which is slightly high. On the other hand, the
initially wet sensor should yield a value which is lower than
the actual soil suction. Table 4.6 gives the interpretation of
the results presented in Figs. 4.45, 4.46, and 4.47.
It is suggested that if only one sensor is installed in an
undisturbed sample, the sensor should be initially dry. It may
take 4-7 days before suction equilibrium is achieved. If the
sensors are left in situ for a long period of time, there should
be no need to take readings any more often than about once

every 3 h. Thermal conductivity suction sensors are not fast-
acting sensors and as such are not meant to register rapid
changes in pore-water pressure.
Laboratory measurements of matric suction have been used
to establish the negative pore-water pressures in undisturbed
samples of Winnipeg clay taken from various depths within a
railway embankment (Sattler et al., 1990). The samples were
brought to the laboratory for matric suction measurements
using thermal conductivity suction sensors. The measured
matric suctions were corrected for the removal of the over-
burden stress and plotted as a negative pore-water pressure
profile (Fig. 4.48). The results indicated that the negative
pore-water pressures approached zero at the average water
table. In general, the pore-water pressures tended to be more
negative than the hydrostatic line through the water table.

4.2.9.4 Thermal Conductivity Suction Measurements
in the Field
Field measurements of matric suction under a controlled
environment have been conducted in the subgrade soils
of a Department of Highways indoor test track at Regina,
Saskatchewan (Loi et al., 1992). The temperature and
the relative humidity within the test track facility were
controlled. Twenty-two thermal conductivity suction sensors
were installed in the subgrade of the test track. The subgrade
consisted of highly plastic clay and glacial till. The sensors
were initially air dried and installed into predrilled holes
at various depths in the subgrade. The sensor outputs were
recorded twice daily.
Typical matric suction measurements made on compacted
Regina clay and glacial till soils at the test track are shown
in Fig. 4.49. Consistent readings of matric suction ranging

Table 4.6 Interpretation of Laboratory Matric Suction
Measurements

Initially Initially

Dry Wet Best

Water Sensor, Sensor, Estimate,

Soil Type Content, % kPa kPa kPa

Sceptre clay 39.3 120 100 114

34.1 136 108 126

Regina clay 35.1 160 150 157