Measurement and Estimation of State Variables - Unsaturated Soil Mechanics in Engineering Practice - page 155

used to produce a controlled soil suction environment, as
shown in Fig. 4.75.
The equilibrium water content of the filter paper should
correspond to the matric suction in the soil if the filter paper
has been placed in contact with the soil-water. On the other
hand, the equilibrium water content of the filter paper should
correspond to the total suction of the soil if the paper has
had no contact with the soil-water. Originally it was assumed
that the same calibration curve could be used for determining
matric suction and total suctions. More recent information
on the filter paper calibration curves is presented later.
There appears to have been greater success in measuring
total suction than matric suction. The filter paper method
can be used to measure soil suction over a wide range of
values. The measurements are generally performed in the
laboratory by equilibrating a filter paper with an undisturbed
or disturbed soil sample obtained in the field. Measurements
should be made in a controlled temperature environment.

C 6 Cl 5 OH, or 0.005% HgCl have been used to pretreat the
filter papers. There have also been other studies that have
suggested that the results from pretreated and untreated
filter papers have given the same results (Hamblin, 1984;
Chandler and Gutierrez, 1986).
The most common practice is to have the filter paper ini-
tially dry and then allow water to be absorbed from the
soil specimen during equilibration. Filter paper calibration
curves have historically been established using initially dry
filter papers. It has been suggested that hysteresis in the filter
papers may affect the calibration curve (Lykov, 1961). More
recent studies on hysteresis and other factors are presented
in the next section.
The filter paper should be oven dried for several hours
prior to usage. The dry filter paper should then be cooled and
stored in a desiccant container. A soil specimen on which
suction is to be measured can be placed in a container. The
soil specimen should almost fill the container in order to
reduce the time to equilibration. The no-contact procedure
can be achieved by placing two dry filter papers on a per-
forated disk seated on top of the soil specimen, as shown in
Fig. 4.74. The contact procedure should have three stacked
filter papers placed in direct contact with the soil specimen,
as illustrated in Fig. 4.74. The contact filter papers can be
placed below the soil specimen or the soil specimen can be
cut in half and the filter papers placed such that there is con-
tact both above and below the filter papers. The center filter
paper is generally used for the measurement of soil suction
while the outer filter papers are primarily used to protect the
center paper from soil contamination.
The sample container can be sealed with plastic electrical
tape once the filter paper and the soil specimen are placed in
the large container. The sealed container is then stored in an
insulated box for the equilibration period. The ambient tem-
perature should remain within

4.3.3.2 Measurement and Calibration Techniques
for the Filter Paper Method
The measurement and calibration technique for the filter
paper method was written into a tentative ASTM (1997b)
standard (D5298-94, Committee D18 on Soil and Rock).
The filter paper sensor must be of the ash-free, quantita-
tive type II as specified by ASTM standard E833. Whatman
No. 42 and Schleicher and Schuell No. 589 white ribbon
are two commonly used brands of filter paper. A typical
filter paper has a disk diameter of 55 mm. Filter papers
from the same brand are considered to be “identical” in the
sense that all filter paper disks are assumed to have the same
calibration curve.
The equipment associated with the filter paper method
consists of large and small metal containers, an insulated
box, a balance, and a drying oven. The large container must
be able to contain a soil specimen of approximately 200 g.
The container should be treated to prevent rusting. The large
container must have an air-tight lid. The container is used
to equilibrate the soil specimen and the filter paper for a
period of several days. The small container with a volume
of approximately 60 cm 3 is used to contain the filter paper
during oven drying for its water content measurement. The
small container should be as light as possible because of the
small mass of the filter paper.
An insulated box can be used to store the large containers
with the soil specimens and filter papers during the equili-
bration period. The box should be maintained at a constant
temperature within

1 ◦ C during the equilibration
period (Al-Khafaf and Hanks, 1974).
The filter papers can be removed from the large container
at the end of the equilibration period. A pair of tweezers can
be used when handling the filter papers. Each filter paper is
placed into a small metal container for the measurement
of water content. The water contents of each of the filter
papers used for the no-contact procedure can be indepen-
dently measured. The water content of the center filter paper
is of primary importance for the contact measurement.
Extreme care must be exercised when measuring the small
mass associated with the filter papers. The filter paper should
be transferred from the large container to the small container
within a short period of time (e.g., 3-5 s ). A short period for
transferring the filter paper is necessary to minimize water
loss or gain between the filter paper and the surrounding
atmosphere. The small container containing the filter paper
must be closed and weighed immediately in order to deter-
mine the mass of the filter paper and the adsorbed water.
The container along with the filter paper is then placed
in an oven at a temperature of 110

±

1 ◦ C. An accurate balance with a min-
imum capacity of 20 g and a readability of 0.0001 g should
be used when weighing the filter paper for determination of
water content.
The filter papers should be pretreated prior to usage to
prevent fungal and bacterial growth during the equilibration
period (Fawcett and Collis-George, 1967; McQueen
and Miller, 1968a). Solutions of 3% pentachlorophenol,

±

5 ◦ C. The lid of the

±