Soil-Water Characteristic Curves for Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 235

1997). Most of the laboratory measurements have been
made within agriculture-related disciplines. Over the years
it became quite acceptable to perform SWCC measurements
in agricultural disciplines without applying total stresses to
the soil specimen. The data was primarily used to assess the
availability of water for plant growth near ground surface.
Most laboratory testing equipment has been developed
without consideration of applying total stresses and without
the ability to measure volume change as matric suction is
applied. The design of some pressure plate apparatuses is
somewhat restrictive when obtaining data for geotechnical
engineering applications (Ng and Pang, 1999, 2000a).
Geotechnical engineers have attempted to utilize soil testing
equipment used in other disciplines, but at the same time
several new devices have been developed that are better
suited for geotechnical engineering applications.
It has become common practice to measure matric suc-
tions below 1500 kPa and total suctions above about 1500
kPa. Data sets for a particular soil are generally plotted with
matric suction and total suction on the same graph (i.e.,
along the abscissa on a logarithm scale). It has also become
common practice to simply measure the desorption branch
of SWCCs. The desorption branch is the easiest and most
rapid branch of the SWCC to measure. Soil specimens are
commonly placed into a pressure plate apparatus, initially
saturated, and allowed to come to equilibrium prior to the
start of the test. Therefore, the initial matric suction in the
soil specimen is released to zero at the start of the test.
The SWCC has proven to be the most important
unsaturated soil property function to measure in unsaturated
soil mechanics. The SWCC provides the key relationship
needed to implement unsaturated soil mechanics in geotech-
nical engineering practice. SWCCs have been measured in
agriculture-related disciplines since the 1930s. A number
of devices have been developed for applying a wide range
of soil suction values. Typical pressure plate apparatuses
are Tempe cells (100 kPa) (Reginato and van Bavel,
1962) (Figs. 5.70 and 5.71), volumetric pressure plates
(200 kPa) (Figs. 5.72 and 5.73), and large pressure plate
apparatuses (500 and 1500 kPa) (Figs. 5.74, 5.75, and 5.76);
(Fredlund and Rahardjo, 1993a). These apparatuses use the
measurement of either a water mass or a water volume to
allow the back calculation of equilibrium water contents
corresponding to applied matric suctions.
ASTM designation D6836-02 (2008) provides a detailed
description for the determination of the SWCCs using
several testing procedures: (i) hanging column, (ii)
pressure plate extractor (with volumetric measurements and
gravimetric water content measurements of water content),
(iii) chilled-mirror hygrometer, and (iv) centrifuge. For
suction values greater than 1500 kPa, small soil specimens
are allowed to come to equilibrium in a fixed relative
humidity environment (i.e., vacuum desiccators). Various
molar salt solutions are used to create the fixed relative
humidity environments.

Soil specimen
High air-entry disk

Figure 5.68 Single specimen pressure plate cell developed at
University of Saskatchewan, Saskatoon, Canada.

The use of pressure regulators is satisfactory for matric
suctions greater than about 5 kPa. At matric suctions less
than about 5 kPa, an inverted column of water can be used to
apply small negative pressures to the water below the base
of the high-air-entry disk. The air pressure in the cham-
ber is maintained at atmospheric conditions when using an
inverted column to apply small suction values.
Matric suctions can be applied as high as about 1500 kPa
using pressure plate equipment. Generally 1500 kPa
has formed the dividing line between matric suction
measurements and total suction measurements.
A controlled relative humidity environment is used to
establish a fixed total suction. A small soil specimen is
brought into equilibrium with the surrounding vapor pres-
sure. Figure 5.69 shows the cross section of a desiccator
with a salt solution placed in the bottom to create a constant-
relative-humidity environment. The relative humidity envi-
ronment is converted to total suction through use of the Lord
Kelvin equation (Fredlund and Rahardjo, 1993a).
Tens of thousands of SWCC data sets have been
measured in various countries of the world (Fredlund,

Figure 5.69 Vacuum desiccator for equilibrating small soil spec-
imens in constant-relative-humidity environment created by con-
trolled salt solution.