Soil-Water Characteristic Curves for Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 242

Curves B and C have lower water contents than curve A , with
the difference indicating irreversible volume change.
Curve D in Fig. 5.81 was obtained from initially slurry
specimens where the soil structure was slightly disturbed.
Curve A for the natural soil joined curve D at a matric suc-
tion of 6300 kPa, indicating the maximum suction to which
the clay may have been subjected during its geological his-
tory. The deviations of the natural soil curves A , B , and C
from the initially slurry soil curve D represent the influence
of past drying and wetting cycles.
Another curve plotted in Fig. 5.81 is curve G , which relates
the water content to the matric suction of a continuously dis-
turbed soil specimen. Curve G is different from the other mea-
sured SWCCs mainly because the soil structure was contin-
uously being changed throughout the test. An SWCC should
be measured on a single specimen or several specimens with
“identical” initial soil structures. Curve G appears to be unique
for London clay. State points along the drying curve D or
curve A will move to corresponding points on curve G when
disturbed at constant water content. Similarly, state points
along any wetting curve will move to curve G when dis-
turbed at constant water content. Disturbance can take the
form of remolding or thoroughly mixing the specimens. Sim-
ilar relationships to curve G have also been found for other
soils. There appears to be a unique relationship between water
content and matric suction for a disturbed or remolded soil,
regardless of the stress path (Croney and Coleman, 1954).

7. The apparatus must be easy to assemble and safe to oper-
ate under high pressures.
8. Attentionmust be paid to themeasurement of air diffused
through the high-air-entry disks.

Some of the features desirable for engineering applica-
tions have been met in equipment developed by geotechnical
engineers in recent years. There has been an attempt to con-
tinuously monitor water volume change and overall volume
change in order that all volume-mass soil properties can be
measured (e.g., w, S, and e ). There are several recent appa-
ratuses that have been developed that better meet the needs
for measuring the SWCC in geotechnical engineering. Some
of the desired specifications for an apparatus to measure the
volume-mass properties along matric suction and applied
total stress paths are as follows. The suction range of oper-
ation should be up to at least 500 kPa and possibly up to
1500 kPa. The air pressure source to operate the device will
also need to be at least 500 or 1500 kPa.
It is desirable to independently apply total stresses to the
soil. The application of isotropic stresses may be preferable;
however, it is considerably more economical to develop
equipment for K 0 loading. It is also desirable that both
the water volume change and overall volume change of the
specimen be measured in order that all volume-mass soil
properties can be determined (e.g., w, S, and e ).
It is important that provision is made to independently mea-
sure the volume of air which might diffuse through the water
in the high-air-entry disk. The measured water flowing from
or to the soil specimen must be corrected for diffused air flow.
It appears preferable to test individual soil specimens for
geotechnical engineering purposes. The initial state of the
soil should be recorded (i.e., initially remolded at a high
water content, compacted or undisturbed). It is preferable
if the apparatus can accommodate both drying and wetting
procedures.
Several pressure plate cells have been developed over
the past few years that meet many of the above-mentioned
requirements. One cell was developed at the Technical Uni-
versity of Catalonia (UPC) in Barcelona, Spain (Gens et al.,
1995; Romero et al., 1995). Wille Geotechnik (Germany)
has also developed a pressure plate cell with many of the
above-mentioned features (Lins and Schanz, 2004). As well,
GCTS United States has developed a similar pressure plate
cell (Pham et al., 2004).

5.9 SINGLE-SPECIMEN PRESSURE PLATE
DEVICES FOR GEOTECHNICAL ENGINEERING
Most pressure plate apparatuses were originally developed for
testing soils for agricultural applications. Pressure plate equip-
ment and laboratory test procedures have been transferred
into geotechnical engineering as unsaturated soil mechanics
emerged. However, the testing of unsaturated soils in geotech-
nical engineering has a number of additional requirements that
should be addressed. Some of the desirable features benefi-
cial to geotechnical engineering applications can be listed as
follows:

1. It is desirable to test individual soil specimens.
2. It is desirable to test undisturbed soil samples.
3. It is desirable to be able to apply in situ total stresses in
the laboratory when testing the soil samples.
4. It is desirable to be able to measure changes in overall
volume as well as changes in the amount of water in the
soil specimen.
5. It is desirable to be able to measure both the drying and
wetting SWCCs.
6. Increased accuracy is required with respect to perform-
ing and interpreting the SWCC tests.

5.9.1 Barcelona Cell
The Barcelona K 0 pressure plate was developed in 1999 for
testing sand-bentonite mixtures at UPC (Hoffmann et al.,
2005). The suction-controlled oedometer can also accom-
modate the application of vertical load to the soil specimen.
Figure 5.83 shows the design drawings of the cell. The base