Environmental Engineering Reference
In-Depth Information
Figure 4.10 illustrates typical time responses of a small-
tip tensiometer inserted into decomposed volcanic soil. The
response is plotted in terms of matric suctions by referenc-
ing measurements to atmospheric air pressures. Sweeney
(1982) found that this type of tensiometer could only main-
tain matric suction equilibrium for about one or two days
before the suction readings began to drop. The time that
equilibrium can be maintained is a function of the matric
suction value being measured.
4.2.5.4 Quick Draw Tensiometers
Figure 4.11 shows a Quick Draw tensiometer equipped with
a coring tool and a carrying case. The Quick Draw ten-
siometer has proven to be a particularly useful portable
tensiometer to rapidly measure negative pore-water pres-
sures. The water in the tensiometer is subjected to tension
for only a short time while each measurement is being made.
Therefore, air diffusion through the ceramic cup over time is
minimized.
The Quick Draw tensiometer can repeatedly measure pore-
water tensions approaching 1 atm when it has been properly
serviced. The probe is maintained in water-saturated cotton
in a carrying case when the Quick Draw tensiometer is not in
use. The rapid response of the Quick Draw tensiometer in a
sand soil is illustrated in Fig. 4.12.
Figure 4.10 Typical time responses of small-tip tensiometer
placed in decomposed volcanic soil (after Sweeney, 1982).
remove air bubbles from the top of the tensiometer tube.
Subsequent removal of air bubbles can be performed by
flushing water through the coaxial tube.
Flushing is conducted by circulating water from the top
of the tube and opening the water vent screw to the atmo-
sphere. Water containing air bubbles will be forced into the
inner nylon tube and released to the atmosphere through
the opened water vent. This procedure may be required on
a daily basis when matric suctions are greater than about
50 kPa. It is sometimes difficult to obtain an accurate suction
measurement because water moves from the tensiometer into
the soil each time the tensiometer is serviced. The movement
of water into the soil reduces the soil suction.
The small ceramic tip with the flexible tube allows the
installation of the tensiometer into a relatively small soil
specimen during laboratory experiments. As an example, the
small-tip tensiometer has been installed in a consolidation
specimen to measure changes in matric suction during the
collapse of compacted silt (Tadepalli, 1990).
4.2.5.5 Tensiometer Performance for Field
Measurements
Tensiometers have been used to measure negative pore-water
pressures for numerous geotechnical engineering applica-
tions. One example is a cut slope consisting of 5-6m of
colluvium overlying deep weathered granite in Hong Kong.
Figure 4.13 illustrates the construction of the shaft, along with
Carrying case
Probe
Coring toll
Cleaning rod
Null knob
Dial gauge
Porous ceramic
sensing tip
Figure 4.11 Quick Draw tensiometer with coring tool and carrying case manufactured by Soil-
moisture Equipment Corporation.
 
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