Page 67 - Numerical Analysis and Modelling in Geomechanics

48 A.A.JAVADI
accommodate several aspects of the behaviour and loading of unsaturated soils.
The basic requirement is that it must allow independent control and/or
measurement of all individual components of stress, i.e., total stress, σ, pore-
water pressure, u , and pore-air pressure, u .
a
w
11
A triaxial cell was modified and used for the shear strength tests. The flow
of air was controlled from the top and the flow of water was controlled and
measured from the bottom of the specimen. Three geotechnical digital systems
(GDS) controllers were used to control and measure the cell pressure, the
deviator stress, and the pore-water pressure and pore-water volume change. The
controllers were connected to a computer and a complete set of data was
recorded by the computer.

PORE-WATER PRESSURE CONTROL
For controlling the pore-water pressure, a 5 bar, 0.635 cm thick, high entry ceramic
disk was sealed onto the pedestal of the triaxial cell. The disk has small pores
and allows the passage of water but resists the flow of air and in this way serves
to separate the pore-air and pore-water pressures. So long as the difference
between the pore-air and pore-water does not exceed the “air entry value” of the
disk, there is a continuous column of water from the specimen to the pore-water
pressure transducer below the porous disk and the pore-water pressure can then
be independently controlled and measured.

PORE-AIR PRESSURE CONTROL
A special hydrophobic membrane was used to control the pore-air pressure. The
hydrophobic membrane was fitted between the two parts of a specially designed
top cap. The hydrophobic membrane allows the passage of air but resists
water flow. As long as the difference between the pore-water and pore-air
pressures is less than the capacity of the membrane, there will be no water flow
from the top cap to the air pressure line.

Testing procedure
The testing procedure consisted of three main tests. The difference between the
tests was the saturation procedure and application of the stresses.

TEST SERIES 1:
INCREASING AIR PRESSURE
The first test series was a multistage triaxial drained test. After the specimen was
prepared and saturated, the stresses associated with the first stage of the test were
applied through an isotropic consolidation procedure and the specimen was
allowed to consolidate under the applied stresses. Table 2.5 shows the stresses