2799
Safety theory in geotechnical design of piled raft
Théorie sur la sécurité pour la réalisation de radier sur pieux.
Lorenzo R., Zubeldia E.H., Cunha R.P.
University of Brasilia, GPFees Geotechnical Research Group (
)
ABSTRACT: A significant amount of literature describing and discussing the behavior of piled raft foundations have been produced
in the last thirty years; nevertheless, a method to introduce the safety in the design of this kind of foundation has not been established
yet. Hence, they have been omitted in most Standards and Codes around the world and have been used in just few cases. Until now,
the performance of piled rafts has been analyzed using a global factor-of-safety approach. The application of the Limit State Method
to geotechnical designs of this kind of foundation is presented with the purpose of establishing a methodology for the calculation of
the bearing capacity of piled rafts. The partial coefficients necessary to describe the safety in this method are defined for the ultimate
limit state. The application of the Safety Theory and probabilistic methods in the geotechnical design is presented, as well as the
mathematic formulation for its implementation in piled raft foundation. A methodology for the adjustment of partial coefficients (to
be used for the method of limit states) based on probabilistic methods is proposed. The expressions for the adjustment to be used in
piled raft foundations using the selected design method are finally presented.
RÉSUMÉ : Une importante partie de la littérature décrivant et examinant le comportement de radier sur pieux a été réalisée au cours
des trente dernières années. Néanmoins, une méthode pour introduire la sécurité dans la conception de ce type de fondation n'a pas
encore été établie. En conséquence, on remarque son oubli dans la plupart des normes et des codes dans quasiment tous les pays et on
relève son utilisation dans très peu de cas. Jusqu'à présent, la sécurité du radier sur pieux a été analysée en utilisant un facteur global
permettant l'approche sécuritaire. L'application de la méthode des états limites pour la conception géotechnique de ce type de
fondation est présentée dans le but d'établir une méthodologie de calcul de la capacité portante du radier sur pieux. Les coefficients
partiels nécessaires pour incorporer la sécurité dans cette méthode sont définis pour les états limites ultimes. L'application de la
théorie basée sur la sécurité et l'utilisation des méthodes probabilistes dans la conception géotechnique sont explicitées, ainsi que les
formules mathématiques pour sa mise en œuvre en ce qui concerne les radiers sur pieux. Une méthodologie pour l'ajustement des
coefficients partiels (à utiliser pour la méthode des états limites) basées sur des méthodes probabilistes est proposée. Enfin, les
expressions pour l'ajustement à utiliser dans le cas des radiers sur pieux, en utilisant une méthode de conception spécifique, sont
obtenues.
KEYWORDS: pile raft, safety theory, limit state, probabilistic method, partial coefficient.
1 INTRODUCTION
After the publication of the Technical Committee Report TC18
of the International Society of Soil Mechanics and Geotechnical
Engineering (ISSMGE) in 2001, the use of piles as settlement
reducers has increased, with the corresponding design savings.
Nevertheless, this kind of foundation is seldom used, because of
the complexity generated by the analysis of the load transfer
mechanism between the piles and the raft, and also because of
the difficulty in determining the
load-settlement
curves. All this
mechanisms are influenced by the interaction between the
elements (raft-piles-soil) of the system. (Sales, 2000).
It´s worth adding that, despite the TC18, the lack of Design
Codes for piled raft foundation (PR) in many countries has had
a great influence in the slow pace incorporation of this kind of
foundation in engineering projects (Ahner, Soukhov, & König,
1997).
Based on the concepts of Limit States, most of the engineering
schools (and Standards) use
partial coefficients
as safety
approach. Notwithstanding, this is not applied in foundation
design (Quevedo 2002, Eurocode 7 2005) and let alone in deep
foundations in which the
global safety factor
approach is still
used in most codes and standards. Some of the disadvantages of
this approach are: a) it does not explicitly take into account the
material´s variability y; b) it uses nominal loads, while the
structural design uses design loads, i.e. characteristic loads
affected by weighting coefficients (Caneiro 2003).
The definition of a global safety factor for a PR is very difficult,
because it has to involve both the raft bearing capacity and the
corresponding pile group capacity. A rigorous safety analysis
has to take into account the interaction of these distinct elements
(raft and piles) with the incorporation of the soil.
2 CALCULATION METHODS FOR THE ULTIMATE
BEARING CAPACITY
The design of any foundation has two main stages: first, the
bearing capacity analysis (ultimate state) and second, the design
settlement estimation (service state). This article focuses in
safety studies related to the ultimate (state) stage which,
according to Mandolini (2003), is the stage that generally
governs the design of PRs with a raft breath between 6 to 14m.
Methods for evaluating the bearing capacity of PR are based in
empirical correlations or in numerical analyses calibrated by
field tests. These are very simple methods, but they can only be
used in similar conditions to those where the in situ tests or
instrumentation were carried out.
One of the more recent methodologies for the PR design is the
one developed by Sanctis & Mandolini (2006). The authors
