Environmental Engineering Reference
In-Depth Information
ballast tube is full of water. This is accomplished by applying
a small vacuum to the top of the burette and carefully opening
the stopcock on the burette. The outflow of water from the
specimen stops when equilibrium is reached.
Diffused air is removed from the grooves underneath the
ceramic disk using the pumping procedure described above.
Air may diffuse through the pore-water and the water in the
high-air-entry disk during the test. The air is called “diffused
air” and it comes out of the solution in the grooves beneath
the ceramic disk. The removal of diffused air should be
performed each time equilibrium is reached with a particular
applied suction. The accuracy of the measurement of water
flow from the soil specimen is increased through use of this
technique. Also, the accumulation of diffused air below the
ceramic disk may prevent the uptake of water by the soil
during the wetting process.
The above testing procedure can be repeated at each increas-
ingmatric suction (i.e., increasing air pressure in the extractor)
until the drying curve is complete. The water volume readings
from the burette will provide the information necessary for the
calculation of the equilibrium water contents for each applied
pressure.
quite common that impedance from the high-air-entry disk
will not allow the coefficient of permeability of the soil to
be measured.
5.8.6 Pressure Plate Extractors for Drying Tests
Pressure plate extractors such as the type manufactured by
SoilMoisture Equipment Corporation can be used to mea-
sure the SWCC (see Figs. 5.74-5.76). The extractors are
commonly used to apply various matric suctions to one or
more soil specimens, and the test is called a pressure plate
test [ASTM D2325]. The pressure plate extractor consists
of a high-air-entry ceramic disk contained in an air pressure
chamber. The high-air-entry disk is saturated and is always
in contact with water in a compartment below the disk. The
compartment is maintained at zero water pressure.
Tempe cells and volumetric pressure plate extractors are
two types of extractors which can be used on single soil speci-
mens in the low range of matric suction. Pressure plate extrac-
tors for the high-matric-suction range (i.e., up to 1500 kPa)
are presented in Figs. 5.74, 5.75, and 5.76. The pressure
membrane extractor utilizes cellulose membranes instead of
a high-air-entry ceramic disk. In addition, a compression
diaphragm is provided on the lid of the pressure membrane
extractor. A pressure regulator maintains a slightly higher
pressure behind the diaphragm than inside the extractor in
order to keep the soil specimens in contact with the cellu-
lose membrane. The increased contact pressure is particularly
important for the high range of matric suctions where the soil
specimens may shrink and an air gap may tend to form at the
contact with the high-air-entry disk.
Soil specimens are placed on top of the high-air-entry
disk, and the airtight chamber is pressurized to the desired
matric suction. The disk does not allow the passage of air
as long as the applied matric suction does not exceed the
air-entry value of the disk. The air-entry value of the disk is
related to the diameter of the pores in the ceramic disk. The
air-entry value of the disk and the strength of the pressure
chamber control the maximum air pressure which can be
applied to a soil specimen.
The application of matric suction to a soil causes the pore-
water to drain to the water compartment through the disk.
At equilibrium, the soil will have a reduced water content
corresponding to the increased matric suction. Since more
than one soil specimen is generally tested, it is necessary to
dismantle the chamber and measure the weight of each speci-
men after equilibrium at the applied pressure. This procedure
is commonly used with 5- and 15-bar ceramic plate extrac-
tors when several specimens are tested at the same time. The
plot of equilibrium water content versus the logarithm of the
corresponding soil suction constitutes the SWCC.
5.8.5 Wetting Portion of SWCC
The volumetric pressure plate test can be continued through
a wetting process upon completion of the drying portion
of the test. The soil matric suction is reduced by decreas-
ing the air pressure in the extractor. A decrease in the air
pressure causes water to flow from the ballast tube back
into the soil specimen. The water volume required for the
backflow may be in excess of the water volume in the bal-
last tube. In this case, water should be added to the ballast
tube by opening the burette stopcock. Equilibrium is reached
when the water flow from the ballast tube into the specimen
has stopped. Following equilibrium, the water levels in the
air trap and the ballast tube are again adjusted to the level
mark, and the burette reading is taken for computation of
water volume change. The procedure is repeated at decreas-
ing matric suctions until the desired range of the wetting
curve is obtained. Subsequent cycles of drying and wetting
can also be performed if desired.
The final water content of the specimen corresponding to
the last matric suction is measured at the end of the test. The
final water content and the water volume changes between
two successive applied pressures are used to calculate the
equilibrium water contents. A typical plot of matric suction
versus water content for the drying and wetting processes
is shown in Fig. 5.77. The plot illustrates the effect of hys-
teresis between the drying and wetting curves.
The volumetric pressure plate extractor has also been used
to measure the coefficient of permeability in unsaturated
soils using the outflow method described by Klute (1965a).
The coefficient of permeability of the high-air-entry disk
must be measured to ensure that there is no flow impedance
resulting from the low permeability of the ceramic disk. It is
5.8.7 Typical Results from Pressure Plate
Extractor Tests
The SWCCs obtained from pressure plate tests provide infor-
mation regarding the water phase constitutive surface for
Search WWH ::
Custom Search