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Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
-0.5
-0.3
-0.1
0.1
0.3
0.5
0.0
0.2
0.4
0.6
0.8
1.0
q / p'
0
( )
p' / p'
0
( )
UCT/V
UCT/H
Figure 5. Effective stress paths of axially and radially loaded samples.
0.0
0.2
0.4
0.6
0.8
1.0
0
2
4
6
8
10
12
1
p' / p'
0
( )
4
UCT/V UCT/H UCT/HV
(a)
-6.0
-4.5
-3.0
-1.5
0.0
1.5
3.0
0
2
4
6
8
10
12
14
q
(%)
N ( )
(b)
Figure 6. Comparison between the behaviour of similar samples
subjected to unidirectional and multidirectional loading: (a) mean
effective stress degradation and (b) deviatoric strain variation with the
number of cycles.
3.3
Effect of multidirectional loading on the liquefaction
resistance
Figure 6 compares the response of similar samples of Coimbra
sand subjected to unidirectional (UCT/V and UCT/H) and
multidirectional loading (UCT/HV). It can be observed that the
degradation of the mean effective stress is considerably faster in
the case of the sample subjected to multidirectional loading. In
particular, the number of cycles required to achieve 5 % double
amplitude of shear strain under multidirectional loading is about
65 % and 38 % less than those required under axial and radial
loading conditions, respectively.
4 CONCLUSIONS
The results presented herein show that the behaviour of sand is
highly dependent on the loading conditions. As suggested by
previous studies (e.g. Ishihara & Yasuda 1975), the pore water
pressure (pwp) build-up in tests where non-uniform cyclic
loading is applied differs from that observed in tests using
uniform loading conditions. In the former, a dramatic increase
of pwp occurs when the largest load peaks are applied, while, in
the latter, an almost constant rate of pwp generation is observed.
Consequently, the location of these large peaks clearly affects
the number of cycles required for the onset of liquefaction.
Furthermore, even when the variation of deviatoric stress
imposed to similarly prepared samples during testing is
identical, different loading directions lead to different cyclic
responses. Indeed, the results show that liquefaction resistance
is greater under axial loading than under radial loading
conditions.
Lastly, the application of a multidirectional loading pattern
leads to a faster degradation of the mean effective stress than
that observed in unidirectional cyclic loading tests.
Due to the random nature of earthquakes, significant
uncertainty exists regarding the characteristics of the loading
conditions to which soil deposits are subjected during seismic
events. Therefore, the evaluation of the impact of the loading
mode on liquefaction resistance presents a significant challenge
in Geotechnical Earthquake Engineering. This is particularly
important when designing laboratory testing programmes,
where simplifications are obviously necessary. This paper offers
insight into this topic by demonstrating that both non-uniform
cyclic loading and the application of relatively simple total
stress paths have a considerable effect on the number of cycles
required for the onset of liquefaction.
5 ACKNOWLEDGEMENTS
This research is financed by FEDER funds (Operational
Competitiveness Programme – COMPETE), and by national
funds, through FCT – Foundation for Science and Technology,
under research contract «FCOMP-01-0124-FEDER-009790».
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