942
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
-10
0
10
20
30
40
50
100
150
200
Percent Improvement in
Deviatoric stress (kPa)
Effective Confining Pressure (kPa)
Area Ratio = 7.9%
Area Ratio = 17.8%
Area Ratio = 31.2%
0
50
100
150
0
50 100 150 200 250 300
Shear Stress (kPa)
Effective Normal Stress (kPa)
A
c
/
A
s
= 31.2%
(
c'
=
18 kPa,
'=21
o
)
A
c
/
A
s
=7.9%
(
c'
=
0 kPa,
'=20.7
o
)
Control
c'
=
0 kPa,
'=21
o
)
A
c
/
A
s
= 17.8
(
c'
=
0 kPa
,
'
=
23
o
)
Figure 6. Dependency of the percent improvement in deviatoric stress at
failure on the effective confining pressure
Figure 8. Failure envelops for control and reinforced clay specimens.
5. ACKNOWLEDGEMENTS
Figure 7. Dependency of the percent improvement in deviatoric stress at
failure on the area replacement ratio.
-10
0
10
20
30
40
50
5 10 15 20 25 30 35
Percent Improvement in
Deviatoric stress (kPa)
Area Replacement Ratio (%)
100 kPa
150 kPa
200 kPa
The authors would like to acknowledge the support of the
University Research Board (URB) at the American University
of Beirut for funding this research program.
6. REFERENCES
Alamgir M., Miura N., Poorooshasb H.B. and Madhav M.R. 1996.
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For specimens reinforced with an area replacement ratio of
17.8%, the friction angle
' was found to increase to 23
o
(compared to 21
o
for the control clay) with the effective
cohesion intercept
c’
remaining at zero. On the other hand,
samples with an area replacement ratio of 31.2% showed no
improvements in the friction angle compared to the control
specimens (
'=21
o
), but were associated with a non-zero
c’
value of 18 kPa. The non-zero
c’
could be related to the relative
reduction in the percent improvement in the deviatoric stress for
samples tested at the higher confining pressure of 150 kPa and
200 kPa as indicated in Figs. 6 and 7.
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4. CONCLUSIONS
Based on the results of 12 consolidated drained triaxial tests the
following conclusions can be drawn on the effect of partially
penetrating sand columns on the drained response of soft clay:
Fattah M.Y., Shlash K.T. and Al-Waily M.J.M. 2011. Stress
concentration ratio of model stone columns in soft clays.
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34(1), 1-11.
1. The mode of failure of the test specimens was governed
by bulging that was relatively uniform for specimens
reinforced at a small area replacement ratio of 7.9% and
concentrated in the lower half of the specimens for the
higher area replacmenent ratios of 17.8% and 31.2%. For
an area ratio of 31.2%, specimens tested at confining
pressures of 150 kPa and 200 kPa exhibited clear shear
planes in the lower half of the specimens indicating
elevated stress concentrations in the unreinforced clay.
Gniel J. and Bouazza A. 2009. Improvement of soft soils using geogrid
encased stone columns.
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27(3), 167–175.
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7(3), 42-49.
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Engineering
114(8), 930–943.
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claybed stabilized with stone and reinforced stone columns.
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, Seoul, Korea, 322-329.
2. The specimens tested with the lower area replacement
ratio of 7.9% did not show any improvement in the load
carrying capacity. For the higher area replacement ratios
of 17.8% and 31.2%, average improvements of 20% and
32% were observed in the deviatoric stresses at failure,
respectively. For the higher confining pressures of 150
kPa and 200 kPa, the rate of improvement in the
deviatoric stress at failure was found to decrease as the
area replacement ratio was increased from 17.9% to
31.2%. This could be due to the premature formation of
shear planes in the lower half of the specimens
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vibrated stone columns in soft clay.
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Najjar S.S., Sadek S. and Maakaroun T. 2010. Effect of sand columns
on the undrained load response of soft clays.
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Geotechnical and Geoenvironmental Engineering
136(9), 1263-
1277.
3. An analysis of the Mohr-Coulomb envelopes indicated
that for an area replacement ratio of 17.8%,
' increased
from 21
o
(control clay) to 23
o
, while for an area
replacement ratio of 31.2%,
c’
increased from 0 (control
clay) to 18 kPa with
' remaining constant at 21
o
.
Sivakumar V., McKelvey D., Graham J. and Hughus D. 2004. “Triaxial
tests on model sand columns in clay.
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Sivakumar V., Jeludine D.K.N.M., Bell A., Glyn D.T. and Mackinnon
P. 2011. The pressure distribution along stone columns in soft clay
under consolidation and foundation loading.
Geotechnique
61(7),
613-620.
