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Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Figure 5. CFA alpha related to installation.
4.1.5
Results
–
displacement piles- driven
The square section driven piles (TP1 and TP2) gave an
α
of
around 1.0 in Table 3. The normalized plots shown in Figure 5
are linear but with a slightly softer response than the bored piles
but reaching capacity at a similar displacement of 3-4mm.
4.1.6
Results
–
range of alpha for typical piles
Various sources give values for
α
for different pile types (e.g.
Burland et al 2012) and selected relevant values are shown
below in Table 4.
Some sources vary the α value so that it
decreases after a threshold with increasing shear strength, which
effectively creates a maximum value for the achievable shear
stress. Others vary α with vari
ations in the c
u
/σ’
vo
. The values
quoted in Table 3 reflect the soil conditions and pile lengths and
diameters on the test site.
Table 4. Typical values of alpha for London Clay (similar L/D)
Pile type
Range of alpha
(α)
Bored
0.45-0.5
CFA
0.6
Driven
0.8
The tests on the various pile types reported here show:
Bored piles: tests on piles installed in well controlled
conditions were at the upper range of
the typical α values;
CFA piles: showed variation dependent on pile
installation, and
α values varying
from those close to bored
piles on this site to values much higher than ones typically
quoted for CFA; on average values for
‘typical’ CFA piles
on this site were some 30% higher than values normally
quoted;
Auger displacement piles: showe
d α values similar
or
slightly lower than the bulk of the CFA results, when an
appropriate diameter was selected. For screw displacement
piles this was the outer diameter;
Driven piles:
showed very high α values,
significantly
above those typically quoted.
4.1.7
Results
–
range of results compared with Eurocode 7
UK National Annex
The UK National Annex quotes resistance factors to be applied
to the shaft capacity, for various pile types. These values are
summarised in Table 5.
The variation in R4 values between pile types could be taken
to imply a difference in anticipated variability in capacity.
Based on the results found in these studies, a far greater
variability is to be expected from CFA piles than bored piles.
However these piles were
constructed under ‘supervision’ and
so should be well controlled and reflect the inherent variability
of the construction methods and what can be achieved.
No comment can be made in this study as to the effects of
time to concreting for bored piles, test methodologies or driven
piling, as the database is too small.
Table 5. R4 values for shaft resistance only.
Pile type
R4 without load
tests
R4 with load
tests
Bored
1.6
1.4
CFA
1.6
1.4
Driven
1.5
1.3
5 CONCLUSION
Total stress estimates for ultimate capacity in clay soils are
common in the UK. This paper shows pile tests on different pile
types and the
α
value associated with them on one uniform site.
O’Brien
and Bown (2008) show, based on a large database
of pile tests, that the
α
-c
u
approach is unreliable. In this study,
all the quoted sources of variation (shear strength, test
methodology, failure definition) other than installation have
been reduced as far as possible. Where the results shown here
are compared with other data, these other sources of varibility
must be considered.
All the
α
found in this study are higher than the literature for
relevant pile and soil types. While this might be partly a
function of shear strength, the selected values for shear strength
are in accordance with tests on 100mm diameter samples, and
the higher capacity can better be explained by greater control.
The testing reported here shows greater variability was found
for CFA compared with bored piles, not necessarily implied by
the R4 factors. Displacement pile capacity was similar to a CFA
pile of relevant diameter (here the outer diameter). In addition
under these conditions the driven piles were seen to be very
effective.
6 ACKNOWLEDGEMENTS
The piles were installed and testing assisted by Cementation
Skanska and BBGE (Stent) under European, DTI and FBE
funded projects with BRE.
7 REFERENCES
Brown M.J. and Powell J.J.M. 2012. Comparison of rapid load pile
testing of driven and CFA piles installed in high OCR clay.
Soils
and Foundations
.
Burland J., Chapman T., Skinner H. and Brown M. 2012.
ICE Manual
of Geotechnical Engineering
. pp1570. ICE Publishing.
Butcher A.P., Powell J.J.M., Kightley K. and Troughton V. 2008.
Comparison of behaviour of CFA piles in London clay as
determined by static, dynamic and rapid testing methods.
Proc 5th
Int Symp on Deep Foundations on Bored and Augered Piles
. 8-10
Sept 2008. pp 205-212.
Fernie R., Bourne-Webb P. Shotton, P. and Tester P. 2006.
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Butcher, J J M Powell and H D Skinner (Eds). pp 187-198. IHS
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ICE 2007.
ICE Specification for Piling and Embedded Retaining Walls
,
2nd edition. Thomas Telford Publishing.
O’Brien A.P. and Bown A.P. 2008. Piled Foundations –
Emerging
Design Methods.
Ground Engineering Conference
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Powell J.J.M. and Brown M.J. 2006. Statnamic pile testing for
foundation reuse.
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236. IHS BRE Press, EP73
Powell J.J.M. and Skinner H. 2006. Capacity changes of bored piles
with time.
Reuse of Foundations for Urban Sites: Proceedings of
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, Watford, UK. A P Butcher, J J M Powell
and H D Skinner (Eds). pp 237-248. IHS BRE Press, EP73.
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–
834.
0.5
0.6
0.7
0.8
0.9
1
0
0.1
0.2
0.3
0.4
0.5
back-calculated alpha
{depth / rev} / pitch
Range for bored piles (Table 3)
CFA (Tables 2,3)
