2827
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
1
A review of pile test results and design from a London clay site
Un compte rendu sur les résultats d’essais sur pieux et leur dimensionnement sur un site d’argile
de Londres.
Powell J.J.M.
Geolabs Ltd, UK (formerly BRE Ltd UK)
Skinner H.
Donaldson Associates Ltd, UK (formerly BRE Ltd UK)
ABSTRACT: A large number of different types of piles have been installed on a well-characterised stiff clay site. The capacity of
piles tested soon after installation has been assessed by maintained load tests. A simple total stress design method in regular use in the
UK has been used to compare the results from the different types of pile. The results have been used to make comment on the choice
of parameters within the design method and the Eurocode factors for pile resistance quoted in the UK National Annex.
RÉSUMÉ : Un grand nombre de différents types de pieux ont été installés sur un site d’argile raide bien caractérisée. La capacité
portante des pieux testés peu après leur installation a été évaluée par essais de chargement statique. Une méthode de dimensionnement
simple en contrainte totale, en usage au Royaume-Uni, a été utilisée pour comparer les résultats des différents types de pieux. Les
résultats ont servis à commenter le choix des paramètres pour la méthode de dimensionnement ainsi que les facteurs de l’Eurocode
utilisés pour la résistance des pieux cités dans l’annexe national britannique.
KEYWORDS: Piles, pile tests, pile design.
1 INTRODUCTION.
Pile design in Europe has been changed by the adoption of the
Eurocodes, which articulate a number of different options for
the evaluation of pile capacity. A traditional UK procedure has
been compared for a number of pile types and relevant issues
for choice of parameters and resistance factors highlighted.
Three series’ of maintained load (ML) pile tests are
described and the data presented, on CFA, auger displacement,
driven and bored piles. These tests have given an opportunity
for comparisons to be made between different pile types on the
same site.
2 THE SITE
2.1
Location and geological setting
The site is located at Chattenden, northern Kent, approximately
30 miles south-east of central London. The site is underlain by
high plasticity London Clay to a depth of at least 44m.
The ground slopes gently at about 1:10. Successive emergent
shear surfaces uncovered in trial pits indicate that there has been
down-slope movement to depths of 1.5m in the past.
2.2
The site and soil properties
The site has been used by BRE as a shrinkable clay and in situ
testing trial site since 1987. Dummy foundations – pads, trench
fill and piles have been installed and monitored during seasonal
and vegetation-induced changes in water content and
consequent soil movements. The site has since been used to test
the behaviour of piles and pile installation and a number of in
situ test procedures.
2.2.1
Site investigation and soil parameters
Site investigations carried out over a number of years have
given information on index properties, stiffness, shear strength
(via in situ and laboratory tests) and in situ stresses. The
London Clay is of high plasticity, heavily overconsolidated and
anisotropic. Figure 1 shows some of the basic soil properties.
Shear strengths range from 40kPa to over 140kPa at 10m depth,
with K
0
reducing from 3 near the surface to 2 at depth.
Figure 1. Chattenden soil properties.
A number of CPT profiles were carried out over the piling trial
area, which showed that the strata were very uniform laterally
and vertically (Figure 1) with occasional claystone bands. In
one area there was an increased shear strength over the upper
5m, thought to be associated with the previous presence of trees.
Figure 1 shows undrained shear strengths interpreted from the
CPT (using N
kt
=20) and laboratory UU and CU tests on 100mm
samples; a general trend line is also shown through all data.
16
14
12
10
8
6
4
2
0
Depth (m)
18 19 20 21 22
Density
(kN/m 3 )
0 20 40 60 80 100
Water content
%
50 100 150 20
c u (kPa)
based on N k 20
Soil description
Soft to firm, fissured,
yellow brown clay with
occasional rootlets
Stiff, fissured, light
brown and light grey
mottled clay
Firm to stiff, fissured, light
brown clay with occasional
yellow brown silt partings
and scattered gypsum
Very stiff light brown clay with
occasional yellow brown silt
partings and scattered gypsum
Transition to stiff grey clay
3 THE PILES
3.1
Piling trials
Trials into the control of CFA drilling, pile types, ageing of
bored piles, pile improvements for reuse and pile testing
