No abstract

No abstract

The analysis of bored piles, or drilled shafts, at the service limit state is important when foundation settlements are critical to the operation of a structure. The t-z method is a widely used soil-structure interaction model for the analysis of drilled shaft settlement. In current practice, nominal values of soil stiffness and strength parameters are used to determine settlement based upon the t-z method. However, the nominal values can vary from one designer to another, making the results somewhat inconsistent. By considering reliability-based design principles, probabilistic relationships can be incorporated into the settlement analysis of the drilled shaft, and thus design uncertainty can be quantified. Following this approach, load and resistance factor design (LRFD) procedures may be utilised and resistance factors established for use in design. Using a t-z model and the Monte Carlo simulation process, probability distributions are determined for drilled shaft capacity at the service limit state. Resistance factors are calculated based upon these relationships. The drilled shaft geometry and the shaft/soil interface parameters are varied so that their effects on the resistance factors may be understood.KEYWORDS: limit state design/analysis; numerical modelling; piles; settlement; soil/structure interaction L'analyse de pieux forés, ou d'arbres percés, à l'état limite de service est importante lorsque le tassement des fondations joue un rôle critique dans l'utilisation d'une structure. La méthode t-z est un modèle très répandu d'interaction sol -structure pour l'analyse du tassement des arbres percés. Dans les applications actuelles, on utilise des valeurs nominales de rigidité du sol et des paramètres de résistance afin de déterminer le tassement sur la base de la méthode t-z. Toutefois, les valeurs nominales peuvent varier d'un concepteur à un autre, en produisant ainsi des résultats quelque peu irréguliers. En examinant des principes conceptuels basés sur la fiabilité, des rapports probabilistes peuvent être incorporés dans l'analyse du tassement de l'arbre foré, et on est alors en mesure de quantifier l'incertitude conceptuelle. En suivant ce principe, il est possible d'utiliser des procédures d'étude du facteur de charge et de résistance, et d'établir des facteurs de résistance, qui seront utilisés dans la conception. On procède à la détermination de distributions de la probabilité en appliquant le modèle t-z et la technique de simulation Monte-Carlo, pour la capacité des arbres percés à l'état de service limite. Des facteurs de résistance sont calculés sur la base de ces rapports, et on varie la géométrie des arbres percés et les paramètres d'interface arbre/sol afin de comprendre leurs effets sur les facteurs de résistance. INTRODUCTIONThe geotechnical design of drilled shafts, also known as bored piles, has traditionally followed the allowable stress design (ASD) methodology at the axial ultimate limit state, assuming full resistance of the soil along the length of the shaft and at the tip. In the ...

The utility of the load and resistance factor design (LRFD) approach is being increasingly recognized for the design of drilled shafts. The current LRFD methodologies of drilled shaft design would be more efficient if reliability based design approaches were used for service limit state design. In this paper, the "t–z" methodology is utilized to develop probabilistic approaches for axial service limit state analysis of drilled shafts. Two different models of the soil–shaft interaction are implemented for load displacement calculations: (1) an ideal elastoplastic model, and (2) a hyperbolic model. For both of these soil–shaft interactions, Monte Carlo simulation is used to obtain a large set of load–displacement curves assuming lognormal distributions for the shaft–soil interface properties. The load–displacement curves are analyzed to develop two alternate methods for determining the probability of drilled shaft failure at the service limit state. As a result, cumulative distribution histograms are developed for drilled shaft load capacities at allowable head displacements. These results may be utilized to obtain resistance factors that can be applied to LRFD based service limit state design.Key words: drilled shaft, serviceability, failure probability, load displacement relation, "t–z" method.

Load–displacement analysis of a single deep foundation element can be accomplished by utilizing a soil–structure interaction model, such as the “t–z” model. By combining the soil–structure interaction model with a probabilistic analysis technique, such as Monte Carlo simulation, methods to rationally incorporate variability in the model parameters can be developed. As a result, the service limit state load capacity of a single deep foundation element can be computed for an allowable total head displacement. However, in design, differential settlement between individual foundation elements is often the event of interest. This paper develops a reliability-based design methodology for deep foundations based on a differential settlement design criterion. The design methodology is developed for various levels of uncertainty in the model parameters. The results are presented in the form of cumulative distribution functions that, combined with the calculated service limit state load capacity, form the basis for serviceability design of deep foundations based on a differential settlement criterion.