Environmental Engineering Reference
In-Depth Information
at theWater Conservation Laboratory in Tempe, Arizona. The
Tempe cell has been subsequently manufactured by SoilMois-
ture Equipment Corporation of Santa Barbara, California.
A Tempe cell assemblage is shown in Fig. 5.70 and a
cross-sectional view is given in Fig. 5.71. A soil specimen
is placed onto the high-air-entry disk inside the retaining
cylinder of the Tempe pressure cell. An outlet tube located
at the base plate underneath the high-air-entry disk allows
the drainage of water from the soil specimen. Air pressure
is supplied through the inlet tube on the top plate.
A test is started by saturating the high-air-entry disk. The
soil specimen is also allowed to fully imbibe water at the
start of the test. Excess water is removed from the cell
after saturation of the soil specimen. The top plate is then
mounted and tightened into place. The lowest air pressure is
applied to the Tempe cell. The applied air pressure is equal
to the desired lowest matric suction value.
Water starts draining from the specimen through the high-
air-entry disk once the air pressure is applied. At equilibrium,
the matric suction in the soil is equal to the applied air pres-
sure. The time required to reach equilibrium depends upon
the thickness and permeability of the specimen and the per-
meability of the high-air-entry disk. The apparatus continues
to be used for sand soils that have an air-entry value (and resid-
ual water content) less than 100 kPa. The air-entry value of
the ceramic disk is slightly greater than 100 kPa. Changes in
water content are measured by weighing the specimen and the
cell after equilibrium is reached with the applied air pressure.
The procedure is repeated at higher applied air pressures
(i.e., high matric suctions). Once the highest matric suction
has been applied, the air pressure in the cell is released and
the soil specimen is removed. The water content correspond-
ing to the highest matric suction is measured by oven drying
the soil specimen. The final water content, together with
the previous changes in weight, is used to back calculate
the water contents corresponding to other applied suction
values. The matric suction values are plotted versus the cor-
responding water contents to give the SWCC.
temperature than the soil temperature. Water condensation
on the walls would introduce an error in the determination of
equilibrium water contents, particularly for long-term tests.
The vapor saturator is used to completely saturate the air
that flows into the volumetric pressure plate extractor. The
saturated air surrounding the soil specimen prevents the soil
from drying by evaporation.
The air trap is provided to collect air that may diffuse
through the high-air-entry disk. A “level mark” is provided
on the stem of the air trap as a reference point in measuring
the volume of water. A ballast tube is provided as a horizon-
tal storage for water flowing in or out of the soil specimen
under atmospheric conditions. A “level mark” on the ballast
tube also serves as a reference point. A burette is used to
store or supply water. The change in the volume of water
in the burette during the equalization process is equal to the
water volume change in the soil specimen.
Drying and wetting processes can be performed on the
same soil specimen when using the volumetric pressure
plate extractor. During the drying process, matric suction
is increased, and pore-water drains from the specimen into
the ballast tube. During the wetting process, matric suction
is decreased, and water in the ballast tube is slowly absorbed
back into the soil specimen. The drying branch of the SWCC
is first measured, followed by measurements for the wetting
branch. Further drying and wetting SWCCs can be measured
subsequent to the first cycle, if desired.
5.8.3 Test Procedure for Volumetric Pressure
Plate Extractor
The test procedure commences with the placement of the soil
specimen into the retaining ring. The ring and soil specimen
are then placed on top of the water-saturated high-air-entry
disk. The specimen is first saturated, and the hysteresis attach-
ments are connected to the extractor. The hysteresis attach-
ments are filled with water to the level mark. Air bubbles
should be flushed from below the ceramic disk at the start of
the test by running the roller over the connecting tube. This
action will push water from the air trap through the grooves
beneath the disk and cause air bubbles to move into the air
trap. The trapped air bubbles are then released by adjusting
the water level in the air trap to the level mark. This is accom-
plished by opening the stopcock at the top of the air trap and
applying a small vacuum to the outlet stem of the air trap. The
vertical position of the ballast tube is made level with the top
surface of the ceramic disk or with the center of the specimen
by placing the extractor on a support.
5.8.2 Volumetric Pressure Plate Extractor Apparatus
and Test Procedure
Figure 5.72 shows an assemblage of a volumetric pressure
plate extractor manufactured by SoilMoisture Equipment
Corporation. The maximum matric suction that can be
attained with the volumetric pressure plate extractor is 200
kPa. This apparatus can also be used to study hysteresis
effects associated with the drying and wetting of the soil.
A hysteresis attachment is required, as shown in Fig. 5.73.
The hysteresis attachments allows for the measurement of
the volume of water that flows in or out of the soil speci-
men. The hysteresis attachments consist of a heater block,
vapor saturator, air trap, ballast tube, and burette. The heater
block is attached to the top plate to prevent water from
condensing on the inside walls of the extractor. The heater
block maintains the walls of the extractor at a slightly higher
5.8.4 Drying Portion of SWCC
A starting point on the first drying curve is established by
applying a lowmatric suction to the soil specimen (i.e., raising
the air pressure in the extractor to a low pressure). Water starts
to drain from the soil specimen through the high-air-entry disk
to the ballast tube. Water should drain to the burette when the
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