Environmental Engineering Reference
In-Depth Information
The water above and below the ceramic disk equalizes to the
pressure applied in the cell. It is suggested that the air pres-
sure be applied for approximately 1 h, during which time
the air in the porous disk dissolves in water. The valves
connected to the water compartment can then be opened
for approximately 10min to allow the water in the disk to
flow into the compartment. The air bubbles are then flushed
from below the high-air-entry disk. The above procedure
can be repeated several times after which the high-air-entry
disk should be saturated. The disk should remain covered
with water until a soil specimen is ready to be mounted
onto the disk. The suggested procedure for saturating the
ceramic disk is similar to that described by Bishop and
Henkel (1962).
The coefficient of permeability of the ceramic disk can
be measured during the saturation process by recording the
quantity of water passing through the high-air-entry disk.
The coefficient of permeability is of interest in evaluating
the compliance of the pore pressure measuring system and
ensuring that the ceramic disk has no cracks. The measured
coefficient of permeability should dramatically increase once
the ceramic disk has even a small crack.
Figure 11.33 System for controlling pore-air pressure.
measure in a closed system because of the ability of air
to diffuse through rubber membranes, water, polythene
tubing, and other materials. This is particularly true when
considering the long test periods required when testing
unsaturated soils.
11.4.4 Pore-Air Pressure Control or Measurement
Pore-air pressure is controlled at a specified value when
performing a drained shear test (e.g., consolidated drained
or constant-water-content test). Pore-air pressure is mea-
sured when performing an undrained shear test (e.g., con-
solidated undrained test where the air and water phases are
not allowed to drain).
Pore-air pressure can be controlled or measured through
use of a porous element which provides continuity between
the air voids in the soil and the air pressure system. The
porous element must have a low attraction for water or an
extremely low air-entry value in order to prevent water from
entering the pore-air pressure system. The porous element
can be a fiberglass cloth disk (Bishop, 1961b) or a coarse
porous disk (Ho and Fredlund, 1982a).
The arrangement for applying a controlled air pressure
to the soil specimen in a triaxial apparatus is shown in
Fig. 11.33. A 3.2-mm-thick coarse corundum disk is placed
between the soil specimen and the loading cap. The disk
is connected to the pore-air pressure control through a hole
drilled in the loading cap. A small-bore polythene tube then
connects the loading cap to the base plate of the triaxial
cell. Pore-air pressure can be controlled at a desired pres-
sure using a pressure regulator from an air supply. In the
case of a modified direct shear apparatus, the air pressure
line is connected to the top of the chamber enclosing the
shear box.
The measurement of pore-air pressures can be achieved
using a small pressure transducer mounted on the loading
cap. The volume of the measuring system should be
kept to a minimum when measuring pore-air pressures in
an undrained mode. Pore-air pressure is also difficult to
11.4.5 Water Volume Change Measurement
The past few decades have witnessed a number of new appa-
ratuses for the measurement of the volume of water flowing
in or out of soil specimens. The volume of water flowing in
or out of unsaturated soils can be quite small. It is important
that the volume of water movement in or out of the soil spec-
imen be accurately measured. The quantity of water flowing
in or out of the soil specimen can be quite small while
at the same time possible leakages and air diffusion can
produce significant errors. Consequently, measuring water
volume changes has proven to be a challenge when testing
unsaturated soils.
Conventional twin-burette volume change indicators
should be slightly modified prior to usage for testing
unsaturated soil specimens (Fig. 11.34). A small-bore
burette (e.g., 10mL volume) should be used as the central
tube in order to achieve a volume measurement accuracy
of 0.01 mL. Leakage needs to be essentially eliminated for
tests that take long periods of time. There is a preferred
procedure for making water volume change measurements
when using twin burettes (Fredlund, 1973a). The burette
opposite to the direction that the values point gives the
most reliable volume change reading. The other burette also
measures volume changes associated with water leakage
and air diffusion.
The diffusion of pore-air through membranes around the
soil specimen can be greatly reduced by using a composite
membrane consisting of one or two sheets of slotted alu-
minum foil (Dunn, 1965). Silicone grease can be placed
between the two aluminum sheets. Rubber membranes can
be placed next to the soil specimen and on the outside of
the aluminum sheets. Further details on the control of water
 
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