2387
Displacement response to axial cyclic loading of driven piles in sand
Réponse en déplacement au chargement cyclique axial de pieux battus dans le sable
Rimoy S., Jardine R., Standing J.
Imperial College London
ABSTRACT: Interactive axial cyclic loading stability charts have been developed to guide the assessment of axial cyclic capacity
degradation of piles driven in sands. Less guidance is available regarding displacement accumulation and cyclic stiffness response at
full scale. This paper focuses on axial cycling experiments of six full–scale steel open–ended pipe–piles at a marine sand site in
Dunkerque, France. Multiple suites of cyclic loading were applied, interspersed with reference static tension capacity tests. The piles’
stable, meta-stable and unstable capacity responses are identified with reference to a site-specific normalised cyclic interaction
stability diagram. The stiffness response and rates of accumulated displacement associated with each style of cycling are reported. It is
shown that under stable loading, the piles’ cyclic stiffnesses remain constant or decline marginally. Similar trends are observed with
meta-stable tests up to onset of an eventual cyclic failure, after which stiffness degrades rapidly. Unstable tests displayed shorter
periods of modest change before marked losses of cyclic stiffness. The patterns of accumulated displacement growth show more
complex relationships with the cyclic loading parameters that can be expressed in multi-surface 3-D plots.
RÉSUMÉ : Des diagrammes interactifs de stabilité cyclique ont été développés afin d’évaluer la dégradation cyclique des pieux battus
dans les sables. Peu de données sont disponibles à échelle réelle en ce qui concerne les déplacements. Cet article s’intéresse aux essais
cycliques axiaux de six pieux tubulaires en acier à base ouverte dans un site de sable marin à Dunkerque. Plusieurs séries de
chargement cyclique ont été appliquées, entrecoupées d’essais statiques référentiels en traction. Les réponses stable, méta-stable et
instable de capacité des pieux sont identifiées en relation avec un diagramme normalisé de stabilité cyclique. La réponse en termes de
rigidité et de taux de déplacement accumulé associée à chaque type de chargement cyclique est ensuite présentée. On montre que sous
un chargement stable, la rigidité cyclique reste constante ou diminue légèrement. On observe des tendances similaires dans les essais
méta-stables jusqu'à l'apparition d'une éventuelle rupture cyclique, après laquelle la rigidité se dégrade rapidement. Les essais
instables ont montré de courtes périodes de léger changement avant de fortes pertes de rigidité cyclique. Les schémas de croissance
des déplacements cumulés montrent des relations avec les paramètres de charge cyclique plus complexes qui peuvent être exprimées
dans des représentations 3-D.
KEYWORDS: axial cyclic loading/ pile stiffness/ accumulated displacements/ offshore engineering/ renewable energy
MOTS-CLÉS: chargement cyclique axial/rigidité du pieu/déplacements cumulés/ingénierie offshore/énergies renouvelables
1 INTRODUCTION
The axial cyclic response of driven pile foundations can be
critical in the design of offshore oil and gas platforms, and
multi-piled wind turbines, towers and pylons. Lateral and
moment loads imposed by wind or wave action can be large
compared to self-weights, leading to multiple modes of axial
and lateral cyclic loading on the foundation piles. Lateral
loading model tests have been reported that tracked the gradual
rotation and stiffness of monopiles (Leblanc et al. 2010);
however less guidance is available on full-scale displacement
accumulation and stiffness responses under axial cycling.
Jardine et al. (2012) reviewed the potential effects of cyclic
loading on offshore pile foundations and considered how these
may be addressed in practical design for a range of
geomaterials. They note that loads vary with platform weight,
water depth, metocean environment and structural form. Of the
15 field research studies they identified, only one concerned
silica sands, that at Dunkerque, France reported by Jardine &
Standing (2000, 2012). Merritt et al. (2012) describe how the
most severe tens or hundreds of cycles imposed in storms are
the most critical to pile performance. The Jardine & Standing
(2000) field study investigated behaviour up to 1000 cycles.
Karlsrud et al. (1986), Poulos (1988) and Jardine & Standing
(2000) have used cyclic stability diagrams to guide the
assessment of pile axial cyclic behaviour. These consider the
interaction effects of cyclic and mean loads (normalised by
static capacity before cycling) and the number of cycles applied.
Such interaction diagrams may be zoned to identify a cyclically
stable (S) region where there is no reduction of load capacity
after N cycles, a meta-stable (MS) region where some reduction
of load capacity occurs after N cycles and an unstable (US)
region where cyclic failure develops within a small specified
number of cycles. Jardine & Standing (2012) used a similar
scheme in interpretation of their field tests at Dunkerque (Figure
1) where multiple cyclic loading tests performed that were
interspersed with reference static tension capacity (Q
T
) tests.
This paper focuses on further interpretation of the same axial
cycling experiments. The axial static and cyclic stiffness
responses are discussed and the accumulated cyclic
displacement trends associated with each mode of cycling are
examined, referring to the site specific normalised cyclic
interaction stability diagram.
-0.2 0.0 0.2 0.4 0.6 0.8 1.0
0.0
0.2
0.4
0.6
0.8
1.0
Set 2
Set 3
3
206
24
13
1
12
41
1
9
27
345
>221
>200
No cyclic failure
First failure
Cyclic failure after previous cyclic or static failure
Q
cyclic
/Q
T
Q
mean
/Q
T
>1000
S = Stable cycle zone
MS = Metastable cycle zone
US = Unstable cycle zone
US
MS
S
Set 1
Figure 1. Axial cyclic interaction diagram for the full–scale pile tests in
Dunkerque silica marine sands (Jardine & Standing 2012).
