2687
Improved Safety Assessment of Pile Foundations Using Field Control Methods
Évaluation améliorée de la sécurité des fondations sur pieux à l'aide de méthodes de contrôle
in situ
Bilfinger W.
Vecttor Projetos, São Paulo, Brazil
Santos M.S.
Liberty Seguros, São Paulo, Brazil
Hachich W.
Polytechnic School – University of São Paulo, Brazil
ABSTRACT: The theme of foundation safety has historically deserved special attention in both theory and practice due to the need to
find optimized solutions which balance cost and safety. Safety against bearing capacity failures (ultimate limit states) continues to be
a key topic, particularly in pile foundations, as opposed to other foundation types in which serviceability limit states tend to dominate
safety considerations. The paper presents a new approach, in which Bayesian inference is used to combine bearing capacity
predictions and field controls, so as to improve reliability assessment and, possibly, lead to more economic design. For bearing
capacity predictions semi-empirical procedures based on SPT blow-count are frequently used, and those are the ones addressed in the
paper. Rebound and set obtained during pile driving are generally used for uniformity control only, but the paper explores the
possibility of combining this duly interpreted information with the design predictions, so as to achieve more economical foundations,
while maintaining the prescribed level of safety against failure. The extension of the approach to the case where pile load tests (both
static and dynamic) are also available is straightforward and discussed in referenced papers.
RÉSUMÉ : Le thème de la sécurité des fondations a historiquement fait l’objet d’une attention particulière, dans la théorie et la
pratique, en raison de la nécessité de trouver des solutions optimales entre coût et sécurité. La sécurité vis-à-vis des états limites
ultimes est notamment importante dans le cadre des fondations sur pieux, par opposition aux autres types de fondations, dans lesquels
les états limites de service dominent les considérations de sécurité. Cet article présente une nouvelle approche, dans laquelle
l’inférence bayésienne est utilisée pour combiner les prédictions de capacité portante et les contrôles in situ, afin d'améliorer
l'évaluation de la fiabilité et, éventuellement, conduire à un projet plus économique. Des procédures semi-empiriques fondées sur le
SPT sont fréquemment utilisées pour la prévision de la capacité portante, et ce sont celles traitées ici. Le refus élastique et
l’enfoncement obtenus au cours du battage des pieux sont généralement utilisés pour le seul contrôle de l'uniformité, mais cet article
explore la possibilité de combiner ces informations, dûment interprétées, avec les prédictions de projet afin de parvenir à des
fondations plus économiques, tout en maintenant le niveau prescrit de sécurité vis-à-vis de la rupture. L'extension de l'approche, au
cas où des essais de chargement de pieux (statique et dynamique) sont également disponibles, est simple; elle est discutée dans les
documents référencés.
KEYWORDS: pile, set, rebound, foundation, safety, bayesian, inference, ULS
1 INTRODUCTION
Foundation safety is a primary concern of civil engineers. Given
the serviceability requirements of modern buildings, safety is
frequently governed by serviceability limit states. Even if safety
against such limit states must always be confirmed, pile
foundations are most often designed on the basis of bearing
capacity predictions, i.e., ultimate limit states. Semi-empirical
procedures based upon field tests such as SPT or CPT are a
common choice for such predictions.
The paper presents a new approach, in which Bayesian
inference is used to combine bearing capacity predictions and
field controls, so as to improve reliability assessment, and
possibly lead to more economic design.
2 THE PROPOSED APPROACH
The idea behind the proposed approach is that foundation
design based on semi-empirical bearing capacity prediction
models can benefit from the incorporation of duly interpreted
field controls during construction.
Even if field controls are almost always used exclusively to
guarantee that uniform behavior is attained, it is believed that
such controls carry quantifiable information that can be
translated into more efficient foundation solutions.
The proposed incorporation mechanism is Bayesian
updating. This paper relies heavily in the work of Baecher and
Rackwitz (1982), which has been explored in detail by Santos
(2007) and by Hachich and Santos (2006).
3 SEMI EMPIRICAL PREDICTION PROCEDURES
Bearing capacity prediction is one of the key analyses required
by pile foundation design. Several semi-empirical procedures
are available, based on different geotechnical investigation
methods, such as SPT, CPT, pressuremeter, dilatometer, and
others.
The use of the SPT to estimate bearing capacity of piles is
still current practice in Brazil and other countries (Poulos et al.
2001). “Case 1” bearing capacity predictions, as categorized by
Poulos (1989), are the commonly adopted procedure.
Several bearing capacity prediction methods using the SPT
have been developed since Meyerhof (1956), including the
relatively recent SPT 97, by the Florida DOT.
In Brazil the most widely used such method is D&Q, the
Décourt and Quaresma method (Décourt and Quaresma 1978,
Décourt 1982, Décourt 1991). For this reason, D&Q is retained
in the paper as the ultimate load prediction method.
Table 1 presents the moments of the random variable
R=log
10
(K), where K = P
OBS
/P
PRED
. The first line is based on
the values originally used to develop the method, where P
OBS
was derived from static load tests on precast concrete piles and
P
PRED
=P
D&Q
. The second line is based on the statistical analysis
of a database of 189 dynamic load tests in precast concrete piles
