Measurement and Estimation of State Variables - Unsaturated Soil Mechanics in Engineering Practice - page 120

Figure 4.14 Measured matric suction profiles along shaft through colluviums and residual soil
in Hong Kong (after Sweeney, 1982).

bubbles begin to form in water. Vapor bubbles are triggered
at gaseous or other hydrophobic surfaces called potential
cavitation nuclei.
It is normally assumed that water will began to boil (or
cavitate) when the absolute pressure above the water surface
approaches the vapor pressure. This is true; however, if the
radii of the cavitation nuclei in the water are sufficiently small,
the water will be able to sustain a high tensile stress without
undergoing cavitation. For example, if the radius is assumed
to be the size of a water molecule (i.e., about 3

10,000 kPa that was sustained for over a week. Guan (1996)
studied the behavior of a small volume of water subjected to
high tensions using a high-air-entry ceramic disk and a high-
range-pressure transducer. Based on past studies it appears
that there is a cavitation tension that is somewhat related to
each prepressurization procedure and each particular suction
probe. A tension can be sustained for several hours or days
if the applied tension does not exceed the cavitation tension.
The methods used by physicists for measuring high ten-
sions in water are not directly adaptable to the measurement
of soil suction. Gilbert (1960) attempted to measure high
tensions in sucrose solutions in a triaxial cell. Ridley (1993)
prepressurized a small volume of water in a tensiometer-
type device (Fig. 4.15) and measured matric suctions up
to 1500 kPa. Ridley and Burland (1993) found reasonable
agreement between matric suction values in London clay
measured using the Ridley-tensiometer-type probe and the
filter paper method. It was suggested that the microstructure
of the soil, particularly the size of the voids, may affect the
measurement of matric suction when using the high-suction
probe. The device was later modified to measure matric suc-
tion in osmotically controlled oedometer tests (Dineen and
Burland, 1995).
A direct high-suction tensiometer was developed at the
University of Saskatchewan for the direct measurement of
high soil suctions. Studies by Guan (1996) indicated that the
direct high-suction tensiometer could sustain tensile stresses
of 1250 kPa. The suction probe has been tested for measur-
ing suctions in various types of soils.

10 − 7 mm),
at the inception of the formation of a gas bubble, the tensile
strength of water should be about 480,000 kPa.
The tensile strength of water measured by physicists is
widely scatteredwithmost values greater than 500 kPa. Liquid
water under high tension is in a metastable state (Temperley
and Chambers, 1946; Temperley, 1947; Chapman et al., 1975;
Green et al., 1990). Rupture of the water can occur due to the
presence of potential cavitation nuclei in the liquid and on
the container walls. Tiny crevices on the surfaces of container
walls and dust particles are a common source of cavitation
nuclei. High positive pressures can suppress cavitation nuclei
in the water and on container walls. The end result is a signifi-
cant increase in the tensile strength of the water (Knapp, 1958;
Winterton, 1977). Research studies have focused mainly on
the maximum tensile strength of water as opposed to how long
a tensile stress can be sustained by the water.
Chapman et al., (1975) and Winterton (1977) measured
high tensions in water which were sustained for a few min-
utes. Henderson and Speedy (1980) reported a tension of

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