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Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
All the tests were conducted at initial effective confining
pressure of 25 kPa and applied cyclic deviatoric stress of 50.91
kPa. It is observed from the figure that the degree of saturation
during shearing has a significant effect on permanent axial
strain response of pond ash specimen. With increase in degree
of saturation the permanent axial strain values increase. A 110%
increase in permanent axial strain is observed when the degree
of saturation increases from 52.70% to 95%.
Fig. 6 shows the relationship between accumulation of
permanent axial strain and number of applied loading cycles at a
constant effective initial confining pressure of 15 kPa for a
range of applied deviatoric stress levels. It can be clearly seen
that with increasing deviator stress levels the magnitude of
accumulated permanent strains increases with loading cycles.
Depending on the level of applied stress, at small stress levels
specimens experienced some value of permanent strain but at
high stress levels test specimens have achieved failure after a
finite number of applied load cycles.
Furthermore, as the stress level exceeds a specific value
(critical stress), the permanent axial strain accumulates rapidly
with the number of applied load cycles which exhibits the
unstable conditions in terms of excessive permanent
deformation in the test specimen. The test results reported here
suggest that at stress levels greater than or equal to 91.64 kPa,
permanent strain accumulates rapidly. Hence applied cyclic
deviatoric stress should not exceed this value, so as to avoid the
excessive plastic strain in the subgrade.
Figure 7. Relationship between permanent axial strain rates (log scale)
versus permanent axial strain
4
CONCLUSIONS
This study aimed in understaing and characterising the
developement of traffic load induced cumulative permanent
axial strain in the compacted ash specimens in repated loading
triaxial (RLT) tests. The investigation aimed specifically at
evaluation of the magnitude of the permanent axial strain, with
combination of various applied deviatoric stress and confining
stress level, and the factors affecting it, as it has not been done
before. The following conclusions are drawn from the
investigation.
The occourance of permanent strain under traffic loading is
controlled by several factors. It incresaes with increase in
number of load cycles, applied cyclic deviatoric stress, and
degree of saturation, and decreases with increase in initial
effective confining pressures.
Figure 6. Relationship between permanent axial strain versus number
of applied load cycles in undrained conditions
If the ash specimen is subjected to deveiatoric stress smaller
than the critical stress, peramanent strain increases at the
beginning of test, and reaches a peak value after a finite number
of applied load cycles, and then remains constant till the end of
test or practically unchanged, attributing to stable state. If cyclic
deviatoric stress is higher than the critical stress then the strain
will change permanently with number of applied loading cycles
attributing to unstable state. Hence the amplitude of permanent
strain represents a boundary between two fundamentally
different kinds of one-way cyclic behavior in the compacted
pond ash specimen under induced repeated traffic loading.
Fig. 7 shows the associated permanent strain rate during the
one-way cyclic triaxial tests in undrained conditions. A total of
seven tests with combination of various deviatoric stress and
confining stress values have been presented. Two different
cases viz. stable and unstable states are considered and labeled
on the figure for the illustration of permanent deformation
behaviour under repeated loading in this study.
5 ACKNOWLEDGEMENTS
The experimental work presented in this paper was conducted at
the Geotechnical Engineering Laboratory at the Indian Institute
of Technology Kanpur, India. The first author gratefully
acknowledges Indian Institute of Technology Kanpur for
providing the one-way cyclic triaxial testing device for
conducting the experiments successfully.
As it is seen in the figure, in stable state, during the applied
load cycles the permanent strain rate decreases gradually and
reaches a constant value depending on the cyclic stress level
applied to the specimen. Where ash material is in stable
equilibrium and can be said to be in shake down range and
would be permitted in the subgrade. In this case total
accumulated strain is sufficiently small. In contrast, in unstable
state the permanent strain rate decreases very slowly depending
on the applied stress level than that observed in the stable state.
It would result the failure in the subgrade and should be
prevented.
6 REFERENCES
Ladd R.S. 1978. Preparing test specimens using undercompaction.
Geotech. Testing J.
1 (1), 16-23.
Prakash K. and Sridharan A. 2006. A geotechnical classification system
for coal ashes.
Proc. Inst. Civ. Eng. Geotech. Eng. (UK)
159 (GE2),
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