Environmental Engineering Reference
In-Depth Information
Figure 4.32
Negative pore-water pressure measurements on as-compacted soil specimens using
axis translation technique (after Olson and Langfelder, 1965).
The null-type axis translation technique can be used to
measure negative pore-water pressures in the laboratory with
reasonable accuracy. High-air-entry disks with a maximum
air-entry value up to 1500 kPa are commercially available.
Theoretical studies on the axis translation technique sug-
gested that the technique is best suited to soils with a contin-
uous air phase (Bocking and Fredlund, 1980). The presence
of occluded air bubbles in the soil specimen can result in an
over estimation of the measured matric suction. In addition,
air diffusing through the high-air-entry disk can cause an
underestimation of the measured matric suction.
properties of the porous block. These properties are a function
of the water content of the porous block and matric suction
can be established through a calibration process. In the calibra-
tion process, the porous block is subjected to various applied
matric suction values, and its electrical or thermal properties
are measured. The measured electrical or thermal property of
the porous block under each equilibrium condition with the
soil can be used to determine the matric suction of the soil
through use of the calibration curve.
Indirect measurements of the water content of the sen-
sor based on electrical properties have been found to be
unsatisfactory due to the presence of dissolved salts in the
pore-water (Richards, 1974). On the other hand, indirect
measurements based of the thermal properties of a porous
block sensor show little effect from the dissolved salts in the
pore-water. As a result, the thermal-conductivity-type sen-
sor appears to be the most promising device for the indirect
measurements of matric suction (Richards, 1974).
4.2.8 Indirect Measurement of Soil Suction
Indirect measurements of matric suction can be made using
a standard porous block as a measuring sensor. A range
of porous materials have been examined for their water
retention characteristics in order to select the most appro-
priate material for making the sensor. These materials have
included nylon, fiberglass, gypsum plaster, clay ceramics,
sintered glass, and porous metals.
The porous block sensor must be brought into equilibrium
with the matric suction of the soil. At equilibrium, the matric
suctions in the porous block and the soil are equal. The matric
suction is inferred from the water content of the porous block
through a calibration process. The water content of the porous
block can be determined bymeasuring the electrical or thermal
4.2.8.1 Electrical Resistance Sensors
Electrical resistance sensors constitute the simplest type of
indirect sensor for the measurement of soil suction. Unfortu-
nately, these relatively inexpensive sensors do not have the
accuracy or the long-term stability in the calibration curve
for applications in geotechnical engineering. Gypsum blocks
with an internal electrical resistor are the oldest type of
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