Measurement and Estimation of State Variables - Unsaturated Soil Mechanics in Engineering Practice - page 154

A cooling system is used to reduce the temperature on
the surface of mirror to the dew-point temperature. A pho-
toelectric cell detects the first sign of condensation on the
mirror, a sign that signals the dew point. The dew-point
temperature is measured using a thermocouple. An infrared
thermometer is used to measure the temperature of the cham-
ber, which is assumed to be the same as the temperature of
the soil specimen.
The vapor pressure above the soil specimen in the cham-
ber and the saturated vapor pressure at the same temperature
are computed using the dew-point and specimen tempera-
tures, respectively. Kelvin's equation is then used to calcu-
late the total suction of the soil specimen.
The WP4-T Water PotentiaMeter has proven to be a valu-
able laboratory tool for measuring total suction in the mid- to
high-total-suction range. The WP4-T apparatus can be cali-
brated using a standard solution of 0.5M KCl, which yields
a total suction value of 2190

Figure 4.74 Use of contact and noncontact filter paper tech-
nique for measuring matric and total suction, respectively (after
Al-Khafaf and Hanks, 1974).

100 kPa at 25 ◦ C. The device
is reliable and easy to use for total-suction measurements as
high as 300,000 kPa. It is particularly useful for studying
unsaturated soil behavior in the residual soil suction range.

±

calibrate the water content of the filter paper against soil
suction. The filter paper method is based on the assumption
that a filter paper will come to soil suction equilibrium with
the soil suction in the water phase.
Equilibrium can be reached by either liquid or vapor mois-
ture exchange between the soil and the filter paper. When a
dry filter paper is placed in direct contact with a soil speci-
men, it is assumed that moisture will flow from the soil to
the paper until equilibrium is achieved (Fig. 4.74). When
filter paper is suspended above a soil specimen (i.e., no
contact with the soil), vapor transfer can occur between the
filter paper and the soil until vapor pressure equilibrium is
achieved. The water content of the filter paper is measured
once vapor pressure equilibrium is established.
The water content of the filter paper corresponds to a
suction value provided by the filter paper calibration curve.
McQueen and Miller (1968a) have made extensive mea-
surements of the calibration curve for filter paper, and some
results are shown in Fig. 4.75. Several methods have been

4.3.3 Filter Paper Method
The filter paper method can be applied over a wide range of
soil suctions, namely, from a few kilopascals to several hun-
dred thousand kilopascals (Fawcett and Collis-George, 1967;
McQueen and Miller, 1968a). However, the measurements
must be performed with great care. Only the noncontact fil-
ter paper procedure (i.e., the filter paper is not in contact
with the soil) can be assured of measuring total suction.
The contact filter paper procedure (i.e., the filter paper is
in contact with the soil) may measure either the total or the
matric suction depending on the degree of contact between
the soil and the filter paper.
The filter paper method for measuring soil suction was
developed in the soil science discipline and has mainly been
used in agriculture and agronomy (Gardner, 1937; Fawcett
and Collis-George, 1967; McQueen and Miller, 1968a; Al-
Khafaf and Hanks, 1974). Attempts have been made to use
the filter paper method in geotechnical engineering (Ho, 1979;
Tang, 1978; McKeen, 1981; Ching and Fredlund, 1984a;
Gallen, 1985; McKeen, 1985; Chandler and Gutierrez, 1986).
The filter paper method has been used, for example, in the
study of airport pavement subgrades and the swelling poten-
tial of profiles in expansive soils (McKeen, 1985). The filter
paper method has gained limited acceptance in geotechnical
engineering, but the technique is deserving of further research
and application.

4.3.3.1 Principle of Measurement (Filter Paper Method)
The filter paper method has been used to measure either total
suction or matric suction of a soil. The filter paper is used as
a sensor. The filter paper method is classified as an “indirect
method” of measuring soil suction since it is necessary to

Figure 4.75 Suggested calibration curve for filter paper (after
McQueen and Miller, 1968a).