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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
water content. The horizontal line describes the border between
the frozen and unfrozen areas at the final state of the test. With
the onset of freezing the suction increases and the water is
drawn into the area of the frost line. The slower the frost
penetrates, the more water can reach the frost line. This leads to
an increasing water content from the top down and small ice
lenses become visible. In the closed system, the consolidating of
the unfrozen area leads to a reduction of the initial water
content. This water redistribution is also the reason for the slight
heave increase after reaching the steady state.
Analyzing suction in a closed system is limited by laws of
physics. If the water pressure falls below the atmospheric
pressure or increases the suction, the boiling temperature
decreases and the water changes its physical state from liquid to
vapor even at lower temperatures. This can occur in free pore
water as well as in the water of the pressure transducers. Since
gas is able to expand in vacuum, the pore water pressure will
change if a gas bubble builds up in the system. In order to delay
this process, the water used for the tests is conditioned in a
vacuum to release dissolved gases in advance. Nevertheless,
suctions greater than 0.9 bar are difficult to reach. After
reaching the maximum suction and the potential development of
gas bubbles the measured suction drops. The water reallocation
that occurs in closed systems is supported by the gas forming,
because additional water can flow to the frost front when the
gas expands.
The magnitude of the measured suction in closed-system
freezing varies significantly between the different materials. In
tests with fine sand, no suction could be measured. Accordingly
no water intake was observed in tests with an open system. The
results for quartz powder only show a low suction. Therefore,
the consistency of the results will be verified in further tests. In
general the highest suction values were measured for kaolin
with absolute values around 0.9 bar and bentonite with 0.8 to
0.9 bar. Also silt reached high suction values of 0.4 to 0.9 bar.
Lower values were measured in the tests with limestone powder
in the range of 0.2 to 0.6 bar.
Figure 8. Dependency of the maximum suction on the applied surcharge
Figure 8 shows a comparison of the suction values for kaolin
and silt with the same surcharge. A clear correlation between
the maximum suction and the surcharge can be deduced from
the results of silk. Hence, the higher surcharge is not only
hindering the intake of water into the specimen in the open
system, but also influencing the suction as principle of the water
flow. Constantly high ultimate suction values greater than 0.8
bar were measured in the tests with kaolin under different
surcharges. Compared to the results of silk, kaolin shows a
stronger suction for surcharges greater than 50 kN/m². This also
explains the less intense water reallocation of silk in a closed
system and as well as the lower amount of water intake in the
open system compared to the same tests done with kaolin. For
kaolin under the same test conditions, the ultimate suction
values show no correlation with the surcharge. The extremely
high suction values suggest that the greatest possible suction
could not be reached in these experiments due to the changing
aggregate state of the water that is finally limiting the
measurement. Consequently, the theoretical possible suction for
kaolin is greater than 1 bar, as long as the water has not passed
from liquid into a gaseous state.
5 CONCLUSIONS
An experimental apparatus was presented to run tests
determining the suction in closed systems and analyzing ice
lenses development in open systems. It was demonstrated, that
in materials allowing a development of ice lenses as well as an
intake of water into the specimen in open-system freezing, a
corresponding suction could be measured in the closed system.
The test data is used to verify and extend the theoretical model.
Besides the dependency of suction on the applied surcharge also
the correlation between suction and ion concentration in the
pore water and in particular, the material specific properties
such as the specific surface area and the surface charge shall be
taken into account. Accordingly, further soil properties need to
be determined as well as the ion concentration in the free pore
water. In the main the magnitude of the cation exchange
capacity corresponds to the magnitude of the measured suction.
Transferring the results from closed system to open system and
to ice lenses development the determining factor for the water
migration is suction. However, the hydraulic permeability is
crucial for the amount of water that is moved. Bentonite shows
a pronounced suction but in comparison to kaolin or silt under
the same surcharges only little water can be moved in a certain
time due to the lower coefficient of hydraulic conductivity.
6 ACKNOWLEDGEMENTS
The authors would like to thank the Deutsche
Forschungsgemeinschaft for supporting this research project.
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