1244
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Figure 4 Ch 150m Deformation Ratio & Fill Height v Time
5 PEAK DEFORMATION RATIO & FILLING RATES
The variation of peak deformation ratio associated with the
maximum filling rate has been investigated for the Limerick
Tunnel project as well as other published case histories of multi-
stage embankments in Ireland. The maximum filling rate is
defined herein as the local filling rate typically measured over a
period of a few weeks immediately prior to the peak
deformation ratio being observed. This is not the same as the
slower average filling rate for a discrete stage in construction.
The data is presented in Figure 5 and includes 18 data points
from Limerick Tunnel representative of 6 km length of
embankment on PVD improved ground and a range of depths of
soft ground from 3 to 11m. Also included are 9 data points from
4 sites in Cork, Bunratty Co. Clare, Derry and Athlone with a
diverse range of embankment geometry from 2:1 to 3:1 side
slopes (Derry & Athlone with toe stability berms); depth of soft
ground from 6 to 13m; range of C
v
typically from 0.5 to 3 m
2
/yr
and PVD spacing 0.6 to 1.4 m (Dunkettle Cork did not contain
PVD and exhibited a range of C
v
from 7 to 16 m
2
/yr).
Figure 5 Peak Deformation Ratio v Max Filling Rate
While there is considerable scatter in the data, there is a
discernable trend of increasing deformation ratio and thus
decreasing stability as the local filling rate increases. Below
maximum filling rates of 0.5 and 1m / week, deformation ratios
generally are below 0.25 and 0.5 respectively, suggesting stable
conditions. An exception to this occurs at A2 Maydown site in
Derry where significant deformation ratio of 0.65 occurred
during construction at a modest maximum filling rate of only
0.4 m / week. This was found to be caused by a pre existing
failure plane and remedied by addition of toe stability berms. At
Limerick Tunnel filling rates in excess of 2 m / week were
consistently associated with large observed deformations or
failures. Local failures were strongly influenced by the presence
of creeks, ditches or excavations and occurred at filling rates
above 1m / week. Peak ratios were nearly always observed
during first stage filling at embankment heights of 4m or less.
Only 4 peak ratios (15% of the total for all sites) occured at
embankment heights over 5m during second stage or near final
embankment height.
6 CONCLUSION
Deformation ratios offer a reliable method for controlling
stability of multi-stage embankments when used in conjunction
with pore pressure instrumentation. An assessment of critical
deformation ratio values during loading should be based on
modelling of the specific embankment geometry and soft
ground properties, trial embankments and case history
precedence in similar conditions. For the specific conditions at
Limerick Tunnel a limit deformation ratio of 0.6 was shown to
give satisfactory performance and acceptable stability.
Excessive lateral deformation related to local failure
occurred at several locations in the vicinity of creeks, ditches
and hisotrical excavations located within 10m of the
embankment toe. More general failure of embankments
occurred at 2 locations during construction, in both cases related
in part to excessive filling rates. Local filling rates in excess of
1 m / week have a significantly higher risk of failure and rates
below 0.5m / week are advisable to sustain a well controlled,
stable stage filling for typical Irish soils.
7 ACKNOWLEDGEMENTS
The author would like to acknowledge the National Roads
Authority and DirectRoute (Limerick) Limited for their kind
permission to publish the data contained within this Paper. The
views expressed in this paper are solely those of the author.
8 REFERENCES
Buggy, F. J. & Peters, M., 2007. Site Investigation and Characterisation
of Soft Alluvium for Limerick Southern Ring Road - Phase II,
EI
Proc. Conf. Soft Ground
, Port Laoise, paper 1.6.
Buggy, F. J. and Curran, E., 2011. Limerick Tunnel Approach Roads –
Design Construction & Performance.
Geotechnical Society of
Ireland Meeting, Engineers Ireland,
Dublin 8th December, 2011.
CIRIA Report C185, 1999. Observational Method in Ground
Engineering.
CIRIA
London.
Connolly, C., Davitt, S. & Farrell, E. 1990. The Design and
Construction of Bunratty Bypass
, Engineers Ireland Meeting
Limerick.15th October 1990.
Creed, M. J. 1996. Dunkettle Embankments, Glanmire Co. Cork – Pore
Water Pressures Reviewed.
Engineers Ireland Conference Road
Embankments on Soft ground in Ireland.
Dublin.
Dauncey, P.C., O’Riordan N.J. & Higgins J. 1987. Controlled failure
and back analysis of a trial embankment at Athlone.
Proc. IX
Europ. Conf. Soil Mechnanics.
Dublin pp 21 – 24.
Jardine, R.J. 2002. Stability and Instability: soft clay embankment
foundations and offshore continental slopes.” Keynote Paper.
International Symposium on Coastal Geotechnical Engineering In
Practice.
Volume 2, Balkema, Rotterdam. pp 99-118.
Kamrat-Pietraszewska, D., Buggy, F., Leoni, M. & Karstunnen, M. 2008.
Numerical Modelling of an Embankment on PVD-Improved soft
Alluvium – Limerick Southern Ring Road Phase II.
BCRI 08
Symposium
NUI Galway pp 443 – 452.
Ladd, C.C. 1991. Stability Evaluation During Staged Construction.
ASCE Journal Geotech. Engng Divn.
Vol 117. No. 4. pp 540 -615.
Long, M. & O’Riordan N., 2001. Field behavior of very soft clays at
Athlone embankments.
G
é
otechnique
Vol. 51 (4) pp 293 – 309.
Raven, K. & Anketell-Jones, J. 2012.A2 Maydown to City of Derry
Airport – Construction over Soft Ground.
Proc. Conf. Geotechnics
on Irish Roads 2000 - 2010
, Portlaoise. 11 October 2012.
Wakita, E. & Matsuo, M. 1994. Observational design method for earth
structures constructed on soft ground.
G
é
otechnique
Vol. 44 (4) pp
747 – 755.
