1058
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
normal inter-particle contact force provides a stabilizing effect
on an unsaturated soil by inhibiting slippage at particle contacts
and enhancing the shear resistance of the unsaturated soil
(Wheeler et al., 2003). Secondly, an increase in s induces the
shrinkage of soil specimen (Ng and Pang, 2000). Due to the
stronger inter-normal force between particles and higher
density,
M
R
measured during cyclic loading-unloading is larger
at higher suctions. Further inspection of this figure reveals that
the relationship between
M
R
and s is nonlinear along a wetting
path, along which soil suction is smaller than initial suction.
Given the same increase in s, the percentage of increase in
M
R
is
much larger in the lower suction range. At a cyclic stress of 30
kPa,
M
R
doubles when s increases from 0 to 30 kPa, while only
increases by 10% when s increases from 30 to 60 kPa. Along a
drying path, the increase rate of M
R
with increasing s is almost
constant. The different results observed in different suction
ranges are likely related with AEV of a soil specimen. The
different results observed in different suction ranges are
probably because the bulk water effects dominate soil behaviour
when matric suction is lower than AEV of soil specimen (here
about 60 kPa) and meniscus water effects dominate soil
behaviour when matric suction exceeds AEV (Ng and Yung,
2008).
Comparing average steady state values of
M
R
measured at
the last 5 cycles at zero suction but at two different temperatures
shown in Figure 5, average
M
R
measured at 20
℃
(W0T20) is
almost identical to that measured at 40
℃
(obtained W0T40). At
the four levels of cyclic stress, the maximum difference in
M
R
at
20
℃ and at
40
℃ is about 7%.
In the temperature ranges
studied, the thermal effect may be considered to be negligible at
zero suction. This negligi le thermal effect at zero suction on
M
R
seems to be in agreement with previous experimental
evidence. Romero et al. (2003) and Uchaipichat and Khalili
(2009) observed from their oedometer tests that soil stiffness
during unloading seems to be independent of thermal
conditions. To fully understand the thermo-hydro-mechanical
effects on
M
R
, further experimental and theoretical studies at
different suction values under different temperature conditions
are needed.
6 SUMMARY AND CONCLUSIONS
Three series of cyclic triaxial tests have been carried out to
investigate
M
R
of an unsaturated silt at different temperatures in
a newly developed suction and temperature controlled cyclic
triaxial apparatus.
By studying the relationship between normalised M
N
r
/M
1
r
and
the number of load applications (N), it is evident that measured
M
R
is sensitive to N values at zero suction but it is almost
insensitive to N at different suctions. At zero suction,
M
R
measured at cyclic stress of 70 kPa increases with N by about
20% during 100 cycles of loading-unloading. When suction is
equal and larger than 30 kPa,
M
R
measured at the same cyclic
stress is almost independent of N. For unsaturated CDT
specimens tested, a steady resilient response was achieved
within 100 cycles of loading-unloading.
For a given stress level the increase of M
N
r
/M
1
r
with
increasing N is more significant at higher temperature at zero
suction. This observation may be explained by the fact that
yielding stress of soil specimen is smaller at higher temperature.
Measured
M
R
is found to be dependent on cyclic stress level
and suction value. It decreases with cyclic stress because soil
stress-strain behaviour under cyclic loads is highly non-linear.
On the other hand,
M
R
increases significantly with suction.
When suction increases from 0 to 250 kPa,
M
R
increases by up
to one order of magnitude. This is attributed to suction induced
additional inter-particle normal force which stiffens soil
specimen.
It is clear that more theoretical and experimental work are
needed to understand unsaturated cyclic soil behaviour and
engineering properties under different suction and temperature
conditions.
7 ACKNOWLEDGEMENTS
The research grant 2012CB719805 of 2012CB719800 provided
by the Ministry of Science and Technology of the People's
Republic of China through the National Basic Research
Program (973 project) is gratefully acknowledged. In addition,
the authors would like to thank the Research Grants Council of
the Hong Kong Special Administrative Region (HKSAR) for
financial support from research grant HKUST6/CRF/12R.
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