2885
Compressive resistance of piles, an update
Résistance à la compression des pieux, une mise à jour
Tol van A.F.
1,2
, Stoevelaar R.
1
, Bezuijen A.
1,3
1
Deltares;
2
Delft University of Technology, Netherlands;
3
Ghent University, Ghent, Belgium.
Jansen H.L.
Fugro Geoservices,Netherlands
Hannink G.
Engineering Consultancy Division, City of Rotterdam, Netherlands
ABSTRACT: The research consortium Delft Cluster/CUR recently published the results of a study of axial pile capacities. It emerged
that the capacity of displacement piles calculated using the Dutch method considerably overestimates the actual capacity measured in
static pile load tests. The identification and quantification of any concealed safety factor is a possible way of preventing the reduction
of the pile capacity factors that could follow from the test results at least in part. This paper presents the results of a study of
concealed safety factors. The conclusion of the study is that it is useful to examine time effects in greater detail, primarily focusing on
the quantification of the effect and the determination of the impact of load variations. It is recommended to continue with research
into the impact of compaction on pile-base capacity in combination with the sequencing of installation of displacement piles.
RÉSUMÉ: Le consortium de recherche Delft Cluster/CUR a récemment publié les résultats d'une étude sur la capacité axiale des
pieux. Il est apparu que la capacité des pieux de déplacement calculée à l'aide de la méthode néerlandaise surestime considérablement
la capacité réelle mesurée lors des essais de chargement statique. L'identification et la quantification des facteurs de sécurité cachés
est un moyen de prévenir la réduction des facteurs de capacité dans la méthode NEN, du moins en partie. Cet article examine les
résultats d'une étude des facteurs de sécurité cachés. La conclusion de l'étude est qu'il est utile d'examiner les effets du temps plus en
détail, en se concentrant principalement sur la quantification de l'effet et la détermination de l'impact des variations de charge. En
outre, nous recommandons la poursuite des recherches sur l'impact de la compaction sur la capacité de la base du pieu en combinaison
avec le séquençage de l'installation de pieux de déplacement.
KEYWORDS: pile capacity, displacement pile, set-up, group effect.
1 INTRODUCTION
Research looking at the axial capacity of foundation piles (van
Tol et al., 2010) has shown that calculating the capacity using
the method set out in the Dutch standard (NEN 9997-1, 2012)
results in a considerable overestimation of the capacity as
compared to measurements in load tests. The study referred to
properly equipped load tests conducted in France, Belgium and
the Netherlands in which it was possible to distinguish between
pile-base capacity and shaft capacity. It emerged that adequate
tests for the reliable validation of the design rules (French,
Belgian and Dutch) were available only for driven, soil-
displacement, prefabricated piles (concrete and close-ended
tubular piles). Calculations of the shaft capacity in accordance
with the NEN standards proved to be a good match for the
values generated by load tests, although the coefficient of
variation is large at approximately 30%. The measured pile-base
capacities, however, proved on average to be only 70% of the
predicted values. The overestimation of capacity increased with
the depth driven in the sand, see Figure 1. Piles located at a
depth of more than 8 D in the sand layer were found to have a
pile base capacity of 60% of the predicted value.
Since the capacity calculation is too optimistic, and since no
failures have been observed in practice, it is thought that there
must be concealed safety factors in the system. The
identification and quantification of those factors may prevent a
future reduction of the installation factors in the Dutch standard
in whole or in part.
This paper will focus primarily on the results of the literature
study looking at concealed safety factors. It will discuss the
following areas:
i
the improvement in capacity over time
ii
residual stresses in the pile
iii
limit values
iv
group effects and
v
wind load in relation to negative skin friction.
0.0
0.2
0.4
0.6
0.8
1.2
1.4
1.6
perfect fit
measured/calculated
0
5
10
15
20
25
Length/Diameter in sand
Figure 1. Comparison of measured and calculated pile-base capacities as
a function of penetration in the sand (Stoevelaar et al. 2011).
2 IMPROVEMENT OF CAPACITY IN TIME
Extensive research has been conducted into the increase of pile
capacity over time. In the past, it was mainly thought that this
phenomenon was a factor related to piles in clay, but it has also
emerged that the capacity of piles in sand increases with time.
Most research involved steel tubular piles and the load tests
were usually conducted under tension loading. Axelsson (2000)
conducted a study of time effects in prefabricated concrete piles
loaded in compression. This study will be discussed further
here.
In predominantly silty sand, an instrumented prefabricated
concrete pile with a cross-section of 235 x 235 mm was driven
to a depth of 13 m below ground level. The pile was equipped
with a pressure sensor at the pile base, and pressure sensors at
