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Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
There is scope for greater use of cantilever walls and walls and
systems with lower stiffness.
Figure 5. Midleton
10 ACKNOWLEDGEMENTS
8 DISCUSSION
The reasons for the very stiff behaviour of the retaining walls in
Irish glacial deposits are complex but are due to a combination
of the following factors:
Excavation induced depressed pore pressures (or even
suctions) in the material. These result in increased effective
stress and hence stability (Long et al., 2012a).
Some of the data used in this paper was kindly provided by Mr.
Tony O’Dowd of PJ Edwards & Co., Mr. Pat Fox of Murphy
International Ltd., Mr. Patrick Casey of Arup Consulting
Engineers and Mr. Eddie Meade of NVM Ltd. and their co-
operation is gratefully acknowledged.
The low permeability of the clay fraction of the material
(10
-10
m/s or less) meaning that the dissipation of the
depressed pore pressures takes a long time.
11 REFERENCES
BSI, 1994.
Code of practice for earth retaining structures
The material has extremely high strength and stiffness.
A possible approach for the design of temporary retaining
structures in the clay fraction of the material may be in the use
of undrained parameters in conjunction with the observational
approach. This approach should only be considered where the
predicted deflections using an effective stress approach would
be within defect limits to prevent the possibility of damage,
economic loss or unsafe situations. The risk associated with the
decision should be clearly assessed in terms of understanding of
the site geology, type and condition of structures to the rear of
the pile wall and the quality of monitoring procedures and
contingency measures put in place.
It seems there is scope for the greater use of cantilever
walls, less stiff walls (e.g. smaller diameter piles) and also
greater retained heights at least for temporary works purposes. It
must also be emphasised that an effective stress design
approach is always needed for the ultimate limit state analysis in
permanent works as the suctions will eventually dissipate.
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9 CONCLUSIONS
Case history data confirms that retaining wall behaviour in Irish
glacial deposits is extremely stiff.
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