3186
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
we
n the basis that they would be settling 25 mm to
40
tion
re consistent with the expected load changes and associated
ground response across the raft footprint.
When including settlement reducing piles (SRP) in the
calculation, rather than modelling them as a spring, it was
assumed that they acted as a constant load. This was deemed
acceptable o
mm, at which point the shaft resistance would be fully
mobilised.
Calculations were undertaken for three stages of loading /
soil response:
1. Undrained excava using the undrained stiffness
profile in Fig. 5; this analysis suggested heave up to 30
mm might occur.
2. Semi-drained net loading using a stiffness profile based
on E
= 0.75E
u
which was considered to be a reasonable
estimate for the situation at the end of construction
when it was estimated that 40
10% consolidation might
have been achieved.
3. Drained net loading including uplift due to groundwater
using a stiffness profile based on E
= 0.50E
u
which was
assessed using elastic theory and the degree of
anisotropy in stiffness suggested by Pennington et al.
earing
cted
settle 40 – 60 mm with local maxima of 60 – 90 mm
s than
5 mm; in the absence of SRP where a similar range of
e contact pressures on this boundary to
approximately the same values present prior to the
ts of bearing piles supporting the hotel
hich with the measures introduced were estimated to be
however it is thought that this is the first such application in
tial impacts outside the site boundary.
ch
no
by constraining those within
the site boundary, and thus protect neighbouring buildings from
potentially damaging movement.
(1997), i.e. G
0(hh)
/G
0(hv)
2.
3.2
Settlement control and role of SRP
In the design, an overall factor-of-safety with respect to b
capacity failure in excess of three was demonstrated for the raft.
However, mitigation measures were required in order to:
Reduce excessive contact pressures and limit raft
settlements to less than 40 mm, as a plain raft was expe
to
under heavily loaded columns and building cores.
Limit raft settlements along sensitive boundaries to les
2
settlement values as indicated above were expected.
Minimise net load changes adjacent to the diaphragm wall
on the hotel boundary which was achieved by introducing
SRP to limit th
redevelopment.
Minimise movemen
w
less than 1.5 mm.
This was achieved by the introduction of SRP at the required
locations. Their use in the first instance is well documented
terms of mitigating poten
4 CONCLUSIONS
Use of precedent knowledge of the ground’s response to load
ange has allowed a calculation model to be derived that is
better conditioned to predict ground movements in Gault Clay.
In this case, the use of a piled raft, though more complex to
design, provided a clear cost and time advantage to a fully piled
solution. Furthermore, the use of a well-conditioned pseudo-
nlinear elastic soil model allowed further savings by reducing
the number of SRPs needed to achieve the performance criteria.
SRP have been employed in a novel way in order to limit
vertical ground movements off-site
5 ACKNOWLEDGEMENTS
The authors would like to thank Bovis Lend Lease and the
Grand Arcades Partnership for allowing us to report on our
contribution to this project, and Dr. David Nash of Bristol
University with whom the first author spent a very interesting
and productive afternoon discussing the Gault Clay.
The work presented here was undertaken while the authors
worked at Card Geotechnics Ltd., UK.
6 REFERENCES
Burland J.B. 1995. Piles as settlement reducers, in – Proc. XIX
Convegno Nazionale di Geotecnica, Pavia, 21–34.
Cooke R.W. 1986. Piled raft foundations on stiff clays – a contribution
to design philosophy, Géotechnique 36(2), 169–203.
Lings M.L. Nash D.F.T. Ng C.W.W. and Boyce M.D. 1991. Observed
behaviour of a deep excavation in Gault Clay: a preliminary
appraisal, Proc. 10th European Conf. SMFE, 467–470.
Lings M.L. Pennington D.S. and Nash D.F.T. 2000. Anisotropic
stiffness parameters and their measurement in a stiff natural clay,
Géotechnique 50(2), 109–125.
Love J.P. 2003. Use of settlement reducing piles to support a raft
structure, Proc. ICE Geotechnical Engineering 156(GE4), 177–181.
Marsh A.H. and Greenwood N.R. 1995. Foundations in Gault Clay, in –
Geol. Soc. Spec. Publ. no. 10, Eng’g Geol. of Constr., 143–160.
Nash D.F.T. Lings M.L. and Ng C.W.W. 1996. Observed heave and
swelling beneath a deep excavation in Gault Clay, Proc. Intl.Symp.
on Geot. aspects of underground constr. in soft ground, London,
191–196.
Ng C.W.W. and Nash D.F.T. 1995. The compressibility of a carbonate
clay, in - Compression and Consolidation of Clayey Soils,
Yoshikuni & Kusakabe (eds), 281–286.
Pennington D.S. Nash D.F.T and Lings M.L. 1997. Anisotropy of G0
shear stiffness in Gault Clay, Géotechnique 47(3), 391–398.
Poulos H.G. 2001. Piled raft foundations: design and applications,
Géotechnique 51(2), 95–113.
Price G. and Wardle, I.F. 1986. Queen Elizabeth II conference centre:
monitoring of load sharing between piles and raft, Proc. ICE, Part 1
80(6), 1505–1518.
Reul O. and Randolph, M.F. 2003. Piled rafts in overconsolidated clay:
comparison of in situ measurements and numerical analyses,
Géotechnique 53(3), 301–315.
Samuels S.G. 1975. Some properties of the Gault Clay from the Ely-
Ouse Essex water tunnel, Géotechnique 25(2), 239–264.
