Shear Strength of Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 546

(a)

(b)

Figure 11.32 Triaxial base plate for unsaturated soil testing: (a) plan view of base plate with
its outlet ports; (b) cross section of base plate with high-air-entry disk.

accumulated as a result of air diffusion through the ceramic
disk. A conventional triaxial base plate may also require
a modification in order to accommodate the application of
air pressure to the top of the soil specimen. Figure 11.32a
shows valve C being used for this purpose.
Va l v e A in the base plate (Fig. 11.32) can be used to
control the pore-water pressure and to measure the water
volume change during a drained shear test. Valve A can
be connected to a water volume change indicator to mea-
sure water flow in or out of the soil specimen (Bishop
and Donald, 1961; Bishop and Henkel, 1962; Head, 1986).
Va l v e B on the base plate can be used to measure the
pore-water pressures during an undrained shear test. It can
also be used to measure the applied pore-water pressure
during a drained test. The pore-water pressure can be mea-
sured using either a null-type indicator (Bishop and Henkel,
1962) or a pressure transducer. Pressure transducers are now
most commonly used for measuring pore-water pressure
(Head, 1986).
A diffused air volume indicator can be connected to valve
B . It is used to measure the volume of diffused air accumu-
lating below the high-air-entry disk. Valve C is connected
to the pore-air pressure control or measuring system. The
cell pressure is controlled through valve D .

11.4.3 Saturation Procedure for High-Air-Entry Disks
It is preferable to not boil the ceramic disk and the base plate
in an attempt to produce complete saturation of the high-
air-entry disk. Boiling may produce fine cracks between
the epoxy and the high-air-entry disk as a result of dif-
ferential thermal properties of the materials involved. The
high-air-entry disk can best be saturated by first passing
water through the disk and then using pressurization to force
air into solution. Once the disk has been saturated, it appears
to remain saturated as long as it is kept in contact with water.
Ceramic disks are relatively easy to saturate because of
their hydrophilic nature. The following procedure is sug-
gested to ensure saturation of a high-air-entry disk mounted
in the triaxial or direct shear apparatus (Fredlund, 1973a).
The ceramic disk should first be properly mounted onto the
testing apparatus. Distilled, deaired water should be added to
the chamber to a height of about 25mm above the ceramic
disk. The triaxial cell or the direct shear box chamber should
then be assembled. The water in the cell can be subjected to
an air pressure of approximately 600 kPa. Water is allowed
to flow through the porous disk for approximately 1 h.
Air bubbles that collect below the ceramic disk can be
periodically flushed. The valves connecting the water com-
partment to the measuring system should then be closed.