Environmental Engineering Reference
In-Depth Information
Ten small holes (i.e., 0.08mm in diameter) were drilled
in the base of the apparatus to provide drainage for water
from the high-air-entry ceramic disk. The holes also needed
to be sufficiently small to remain filled with water. Some
diffusion of air through the ceramic disk did not affect the
test results.
The specimens were then consolidated under selected
isotropic net confining stresses. Each specimen required
about 1 h for the consolidation stage to be complete.
After isotropic consolidation was complete, the first matric
suction was applied to the soil specimen through control of
the pore-air and pore-water pressures.
The GDS pressure-volume controller was connected to the
pore-water pressure at the base plate. The pore-water pres-
sure was controlled through a 5-bar high-air-entry ceramic
disk sealed onto the base pedestal of the cell. The soil spec-
imen was placed in contact with the high-air-entry disk. The
pore-water pressure in the soil specimen was held constant
while water volume change was measured. The net con-
fining stress was maintained at a constant value. During the
drying stage, matric suction was increased by decreasing the
pore-water pressure at the base plate while maintaining the
pore-air pressure. For the wetting stage, the matric suction
was decreased by increasing the pore-water pressure at the
base plate.
The amount of water that drained from the soil specimen
and the total volume change of the specimen during drying
and wetting were recorded. Equalization of matric suction
was deemed to be complete when there was negligible water
flow either into or out of the soil specimen. The SWCCs
were conducted under several net confining stresses (i.e.,
σ
5.9.5 Modified Triaxial Tests
A modified triaxial cell is a rather elaborate apparatus that
can be used to measure the SWCC. The triaxial equipment
needs to have a high-air-entry disk sealed onto the base
pedestal and a low-air-entry material used to control the air
pressure at the top of the soil specimen.
There are a number of advantages associated with the use
of the modified triaxial equipment. First, the triaxial appa-
ratuses provide greater flexibility in terms of the stress path
that can be followed. It is possible to apply confinement to
the soil specimen while data are collected for the SWCC.
Second, it is possible to measure both overall volume change
as well as water volume change and thereby obtain more
accurate measurements of the volume-mass properties. It
is also possible to use volume change measuring devices
(e.g., GDS pressure-volume controllers) and thereby inde-
pendently measure changes in water volume and air volume
(Bankole et al., 1996). When air volume change is measured,
it is not necessary to measure overall volume changes in the
soil specimen.
A series of triaxial tests were performed by Thu et al.,
(2007b) where the net confining pressure was altered. The
study involved tests on several statically compacted kaolin
specimens that were prepared in the same manner. The soil
specimens were compacted to the maximum dry density and
optimumwater content in 10 layers, each layer being 10 mm in
thickness using the standard Proctor method. The height and
diameter of the specimens were approximately 100 and 50
mm, respectively. Mini high-suction probes (Meilani et al.,
2002) were installed along the height of the specimens to
observe pore-water pressures during the SWCC tests.
Liquid limit, grain size analysis, specific gravity, and
hydraulic conductivity tests were conducted to determine the
index properties of the compacted kaolin. The kaolin was
of high plasticity (i.e., MH according to the USCS). Com-
paction tests on the kaolin showed a maximum dry density
of 1
.
35 Mg / m
3
and an optimum water content of 22%.
A modified triaxial apparatus (Fredlund and Rahardjo,
1993a) was used and a series of net confining stresses were
applied to the soil specimens. The axis translation technique
(Hilf, 1956) was adopted for the application of matric suc-
tion to the soil specimens. The soil specimens were initially
saturated. The air pressure line was replaced by a water
pressure line connected to a digital pressure and volume con-
troller (GDS pressure volume device) to inject water from
the top of the specimen. The specimen was saturated while
applying a cell pressure
σ
3
and a back pressure
u
w
. Full sat-
uration was achieved with
B
parameters greater than 0.97
(Head, 1986).
10, 50, 100, 150, 200, 250, 300 kPa).
Figure 5.92 shows the SWCC in terms of degree of sat-
uration with respect to matric suction under a net confining
stress of 100 kPa. Figure 5.92 also indicates the procedure
for determining the air-entry value. Figure 5.92 shows how
the air-entry value of the soil increases with increasing net
confining stress. The increase in air-entry value appears to be
related to the reduced void ratio associated with an increased
net confining stress.
−
u
a
=
5.9.6 Column Tests
The SWCCs are often required for coarse, cohesionless soils
and it is possible to obtain satisfactory results through use of a
simple column test (Fig. 5.93). The distance above the water
level at the base of the column can be converted into an equiv-
alent matric suction value by assuming hydrostatic conditions
for the water within the column. Columns approximately 1m
in height have proven satisfactory when the air-entry value
of the soil is less than about 7 kPa and residual conditions
are around 10 kPa. It is possible to run the column test in
a wetting mode or a drying mode and thereby measure the
wetting and drying SWCCs, respectively.
Wetting SWCCs are most commonly measured using col-
umn tests. The wetting SWCC is measured by allowing
water to be drawn up into the soil in the column from water
supplied at the base of the column. Water content measure-
ments can be made at various heights in the column once
equilibrium has been achieved. Figure 5.93c shows the wet-
ting SWCC for three coarse sands tested as a column test. A
Tempe cell test was used to obtain the drying curve (Yang
et al., 2004b).
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