Measurement and Estimation of State Variables - Unsaturated Soil Mechanics in Engineering Practice - page 114

Surface tension T s changes slightly with temperature (see
Chapter 2). The air-entry value of a disk is largely controlled
by the radius of curvature R s of the largest pore in the
disk. The pore sizes in the ceramic are controlled by the
manufacturing procedure and the sintering technique used to
produce the ceramic disks. The air-entry value of a ceramic
disk increases as the largest pore size in a disk is decreased.
The ability of the high-air-entry disk to withstand a dif-
ference between air and water pressures makes the disk a
suitable separator for the direct measurement of negative
pore-water pressures in an unsaturated soil. The disk is used
as an interface between the unsaturated soil and the pore-
water pressure measuring system. The water in the ceramic
disk provides continuity between the pore-water in the soil
and the water in the measuring system. At the same time,
free air is not allowed to pass through the high-air-entry disk
into the measuring system.
The separation of air and water pressures by a high-air-
entry disk can be achieved only as long as the matric suction
in the soil does not exceed the air-entry value of the disk.
Once the air-entry value of the disk is exceeded, free air will
freely pass through the disk and enter the measuring system,
as shown in Fig. 4.6. The release of air in the measuring
system forces water through the ceramic disk back to the
soil specimen. The end result is an erroneous measurement
of the pore-water pressure in the soil.
The properties of several high-air-entry disks manufac-
tured by Soilmoisture Equipment Corporation are tabulated
in Table 4.3. The disks are identified by specified air-entry
values expressed in bars (i.e., 1 bar

between the sides of the ceramic disk and the triaxial cell
pedestal. A gradient can be applied across the high-air-entry
disk by applying an air pressure to the water above the
ceramic disk. The volume of water flowing through the disk
is measured using a water volume change indicator.
The air-entry value and the coefficient of permeability of
high-air-entry disks should be measured prior to usage in
testing unsaturated soils. Figure 4.6 presents the air pas-
sage characteristics of high-air-entry disks from Soilmois-
ture Equipment Corporation. The measured air-entry values
appear to be higher than the nominal values specified by the
manufacturer. The results of measurements of the air-entry
values and the coefficients of permeability for various high
air-entry disks are summarized in Table 4.4.

4.2.5 Direct Measurements (Low Suctions
up to 100 kPa)
There are several suitable devices that can be used for the
direct measurement of negative pore-water pressures. For
example, there are several low-range tensiometers that are
commercially available. Tensiometers utilize a high-air-entry
ceramic cup as the interface between the measuring system
and the negative pore-water pressure in the soil. The axis
translation technique can be used in the laboratory to extend
the range over which matric suctions can be measured. The
axis translation technique is discussed later in this chapter.

4.2.5.1 Tensiometers
Tensiometers measure the negative pore-water pressure in
a soil. The tensiometer consists of a high-air-entry, porous
ceramic cup connected to a pressure measuring device
through a small-bore tube. The tube is usually made from

100 kPa). The water
coefficient of permeability of a ceramic disk can be mea-
sured by mounting the disk in a triaxial apparatus and plac-
ing water above the disk. It is important that there is a seal

=

4

Air-entry values
of
1 bar high air-entry disk

3

Thickness = 6.4 mm

Air blew
through
the disk

Air blew
through
the disk

Thickness = 10.0 mm

2

1

100

110 120 130 140 150 160

Matric suction, u a - u w (kPa)

Figure 4.6 Air passage characteristics of 1-bar, high-air-entry disks (from Rahardjo, 1990).