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Proceedings of the 18
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International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18
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International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
equivalent to that of the spudcan. The soil deformations were
directed predominantly vertically down in the 2
nd
layer and
laterally out in the lower (3
rd
) soft layer. The soil around the
spudcan edges just started to flow back into the cavity formed
above the spudcan. It can be seen that, under this relatively high
confining stress in an embedded layer, the load spread angle is
about 8
in carbonate sand and 19
in silica sand. The load
spread angle is sometimes taken as the dilation angle (Lee et al.
2009; Teh et al. 2009). As such, it can be concluded that the
interbedded carbonate sand layer showed less dilatancy.
Furthermore, the trapped plug height (and hence the bearing
base) is slightly lower for carbonate sand.
In both deposits, with the progress of penetration, the
dilatancy was suppressed quickly and hence a plug with the
shape of an inverted truncated cone, bounded by clear shear
planes, was formed in the stronger (2
nd
) layer and moved down
with the spudcan. Continual backflow provided a seal above the
advancing spudcan and limited the cavity depth.
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8
100
Figure 5. Effect of interbedded sand mineralogy on dilation and load
spread angle (Tests HS1 and HS2, Table 2).
5 CONCLUDING REMARKS
This paper reported results from a series of simple shear tests
for characterising carbonate sand dredged directly from
Australian North-West Shelf. The stress-strain behaviour was
compared with those of silica sand, focusing particularly on
dilatancy. To examine the influence of dilatancy on foundation
performance, a series of centrifuge model tests were carried out
on spudcan foundations penetrating four-layer soils, with a
carbonate or silica sand layer interbedded in soft clay layers.
The following key conclusions can be drawn from the results
presented in the paper.
1. The dilatancy of carbonate sand was affected strongly
by the confining stress. Even for relative density as low
as 5%, in contrast to silica sand, dilative behaviour was
shown to occur, reflecting the greater interlocking
compared to silica sand.
2. With the increase of confining stress, dilatancy of
carbonate sand was suppressed quickly, and eventually
diminished completely at a relatively low stress level,
due to particle degradation. In contrast, silica sand
showed dilatant behaviour at stresses > 1000 kPa.
3. This distinctive characteristic influenced the behaviour
of continuously penetrating spudcan foundations,
causing a less severe punch-through failure in an
interbedded carbonate sand compared to that in silica
sand layer, with significantly lower bearing capacity.
6 ACKNOWLEDGEMENTS
The research presented here was undertaken with support from
the Australian Research Council through the Linkage Project
LP110100174. The work forms part of the activities of the
Centre for Offshore Foundation Systems (COFS), currently
supported as a node of the Australian Research Council Centre
of Excellence for Geotechnical Science and Engineering and in
partnership with The Lloyd’s Register Educational Trust. This
support is gratefully acknowledged, as is the assistance of the
drum centrifuge technician, Mr. Bart Thompson and soil
technician, Mrs. Satoko Ishigami.
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