220
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
437.0%, plastic limit 52.86%, specific surface area 700 m2/g
and ion exchange capacity 88.63+/-6.51 meq/100g. The
influence of swelling and water content of bentonite samples on
the measured values of total shear strength and shear strength
parameters was observed using the direct shear device. In a
series of tests, under conditions of varying effective stress,
hydration times were changed depending on the results of
oedometric tests in order to simulate different levels of
bentonite swelling. Swelling behavior of bentonite was defined
through long-term oedometer tests with varying effective stress.
In order to prevent the change in chemical and mineralogical
composition of bentonite, demineralised water was used as the
test fluid. Shear displacement rate was 1 mm/min. This
displacement rate enables relatively short shear stage in relation
to previously finished hydration stage, that is, the reduced
impact of additional hydration and creep during shear stage on
the test results.
2
OEDOMETER SWELL TEST
Bentonite swelling tests were performed using standard
oedometer cells of 74 mm in diameter. The placement
procedure was relatively simple i.e. pouring of granular
bentonite into the oedometric cell was performed without the
application of external loading. The initial water content (as-
received) of granules was approximately 12%. Identical
amounts of bentonite were used for all specimens, making sure
that the bentonite is not compacted, but only slightly flattened.
After installation, the specimens were loaded to normal stress
levels of 50, 100 and 200 kPa. The next step was to add
demineralised water into the cell, leaving the specimens to swell
under applied normal stress levels for the next 276 days. At the
end of experiments, specimens were taken out of the device, and
final moisture content was determined.
Test results showed the highest level of swelling and
relative vertical deformation for those specimens that were
under the lowest levels of normal stress. Therefore, the intensity
of swelling decreased as normal stress level increased. It is
evident on the basis of the swelling curves that relative vertical
deformation (swelling) of specimens after the period of 276
days was 65.80% (6.787 mm) under normal stress of 50 kPa;
38.54% (3.945 mm) under normal stress of 100 kPa and 13.93%
(1.339 mm) under normal stress of 200 kPa.
The analysis of time required for the primary swelling stage
indicates that these times were identical for all normal stress
levels (Figure 1). In this particular case, the time for completion
of the primary swelling was approximately 31 days, looking at
all three series.
On the basis of analysis of vertical deformation
development for these specimens upon completion of the
primary swelling stage, it is evident that in the period which
remained the stage of secondary compression started. Following
conclusions were drawn by observing vertical deformations
over the remaining period of 245 days during which there was
secondary compression of the specimens:
• with specimen subjected to normal stress intensity of 50
kPa, there was compression by 0.092 mm, resulting in vertical
deformation of 0.54%;
• with specimen subjected to normal stress intensity of 100
kPa, there was compression by 0.196 mm, resulting in vertical
deformation of 1.36%;
• with specimen subjected to normal stress intensity of 200
kPa, there was compression by 0.203 mm, resulting in vertical
deformation of 1.82%.
Figure 1. Bentonite swell tests.
Therefore, same as with the primary swelling, the rate of
vertical deformation in the stage of secondary compression and
how it develops with time also depends on normal stress levels,
but in this case the intensity of secondary compression increases
with the increase of normal stress level.
The specimens were subjected to secondary compression
over a long period of time, so it is assumed that the impact of
changing temperature in the laboratory on the vertical
deformation curves is possible during the period of secondary
compression. A combination of very low vertical deformations
and variable temperatures in the laboratory during measurement
may lead to a change in the rate of vertical deformation
increment during secondary compression. Some changes in
temperature in the laboratory were expected, and it is assumed
that they affect measuring sensors used in this test.
3
DIRECT SHEAR TEST
3.1
Laboratory testing program
Clay geosynthetic barriers are composite materials. Considering
the specific form of clay geosynthetic barriers, their shear
strength is mainly tested using modified direct shear devices.
This test was aimed at quantifying the performance of bentonite
clay component within the clay geosynthetic barrier. It was
conducted on a sample of granular bentonite, not including the
geosynthetic component. The shear strength tests on some
unreinforced and particularly on reinforced clay geosynthetic
barriers indicate that special attention is required relating to the
size of specimen. However, the specimen size is not crucial
when testing the shear strength of bentonites. Therefore, a
standard direct shear device with box dimensions of 60×60 mm
was used in this study.
Previous studies of the shear strength of bentonite indicate
that the key influence on its behavior comes from the property
of swelling i.e. moisture content in the specimen. In order to
establish the influence of bentonite swelling on its shear
strength, specimens were tested in three series under normal
stress of 50, 100 and 200 kPa, with varying hydration times (7,
14 and 21 days).
The specimen placement procedure consisted of pouring
bentonite into the shear box. The as-received water content level
in granules was approximately 12%. The same amount of
granulated bentonite was always used, making sure that the
bentonite is not compacted, but only slightly flattened. The
described procedure ensured approximately identical initial
values for thickness, dry mass and dry density of all specimens
(their thickness was approximately 8 mm). This kind of
procedure was believed to provide a representative simulation
for the conditions under which bentonite is used as part of clay
geosynthetic barriers. After the placement, normal stress
loading was applied on the specimens immediately followed by
the initiation of hydration procedure. After finished hydration
