Environmental Engineering Reference
In-Depth Information
diffused air is measured by recording the volume change
burettes before and after flushing the base-plate compart-
ment. An alternating air pulse is applied to the top of one
of the burettes through use of a squeeze bulb. The puls-
ing motion flushes the air from the compartment below the
high-air-entry disk.
The Fredlund pressure plate cell has an ability to measure
the overall volume change of the soil specimen during the
test. The movement of the load plate indicates the move-
ment of the top surface of the soil specimen. The movement
of the load plate can be measured using a dial gauge or
a linear vertical displacement transducer (LVDT). The vol-
ume change of the soil specimen can be computed based on
vertical movement of the load plate provided the specimen
does not contract diametrically. The pressure plate cell is
equipped with either a small heater or a heating jacket that
can maintain the chamber temperature slightly above ambi-
ent temperature in the laboratory. The heater helps prevent
condensation of water vapor inside the chamber.
A “hanging-column” accessory is also available for the
application of low suctions (Fig. 5.90).
Soil specimens can be saturated either in the pressure plate
cell or prior to testing by partial submergence in water. The
saturated soil specimen is placed on a saturated ceramic
stone. A vertical load (i.e., token or higher loads) can be
applied to the soil specimen.
Water moves out from the soil specimen and drains
through the ceramic disk until equilibrium is established
with the applied suction. The water level in the volume
indicator tubes shows the amount of water released and
movement ceases when equilibrium is attained. Following
equilibrium, the water levels in the volume indicator tubes
are recorded, and generally diffused air is flushed from
the compartment below the soil specimen. The volume
indicator tubes should be read before and after flushing. The
tops of the water volume change tubes should be covered
with metal foil in order to minimize the evaporation of
water. The water content of the soil specimen is computed
based on back calculation of the volume indicator tube
readings. The overall volume change of the soil specimen is
also measured using the initial and final vertical movement
measurements.
The above procedure can be repeated by applying pressure
decrements to obtain the “wetting” curve. At the end of
the test, the soil specimen is removed from the cell and its
water content is determined by oven drying the specimen.
The volume of the soil specimen can be measured using
calipers.
Four different ceramic stones rated as 1, 3, 5, and 15 bars,
are capable of withstanding air pressures of 100, 300, 500,
and 1500 kPa, respectively. The appropriate ceramic stone
should be selected based on the type of soil being tested
and the suction range of greatest interest. It may be neces-
sary to change the high-air-entry disks during the course of
one test.
0.895
0.890
0.885
0.880
0.875
0.870
0.865
0.860
0.1
1
10
100
Matric suction, kPa
Figure 5.86
Drying and wetting void ratio curves measured on
a sand soil using the Wille pressure plate cell with volume change
measurements (after Lins and Schanz, 2004).
100
Drying;
e
0
= 0.89;
σ
= 0 kPa
Wetting;
e
0
= 0.89;
σ
= 0 kPa
Drying;
e
0
= 0.66;
90
= 0 kPa
Wetting;
e
0
= 0.66;
σ
= 0 kPa
σ
80
70
Transition zone
60
Saturated zone
(Increasing suction)
Residual zone
50
40
30
Dense:
ψ
aev
= 2.0 kPa
Loose:
ψ
aev
= 1.5 kPa
20
10
0
10
6
0.1
1
10
100
1,000
10,000
100,000
Matric suction, kPa
Figure 5.87
Drying and wetting degree-of-saturation data mea-
sured on sand using the Wille pressure plate cell (after Lins and
Schanz, 2004).
Vertical loads can be applied to the soil specimen either
through use of dead weights placed on the loading plate
or through use of a pneumatic pressure loading system
(Fig. 5.89). A pressure “compensator” is used to balance the
uplift force generated by internal air pressure acting on the
loading shaft. The loading shaft slides up and down through
the compensator on a set of O-rings with minimal friction.
The apparatus has the ability to measure changes in the
volume of water in the soil specimen without dismantling the
apparatus during testing. This is accomplished by measuring
water released (or absorbed) from the specimen using two
volume indicator tubes on the pressure panel. The base-plate
compartment is connected to two volume indicator tubes on
the pressure panel via plastic tubing. The volume indicator
tubes are graduated to read the amount of water released or
absorbed during the test.
The diffusion of air into the compartment below the high-
air-entry disk (during a test) is registered as a water volume
change. The diffused air bubbles can be removed from the
system through use of a flushing technique. The amount of
Search WWH ::
Custom Search