Characterisation of model uncertainties for laterally loaded rigid drilled shafts

Abstract

This paper presents a critical evaluation of model factors for laterally loaded rigid drilled shafts (bored piles). Both the lateral or moment limit and hyperbolic capacity are considered to make explicit the dependence of model factors on the criterion for interpreting 'capacity' from load test data. Although the hyperbolic capacity may be closest to the theoretical ultimate state or upper bound, results indicate that it generally does not produce a mean model factor of 1. When the measured capacity is interpreted consistently from load test data, the coefficient of variation (COV) appears to remain relatively constant between 30% and 40%. The range of the mean bias for the lateral or moment limit is 0 . 67 to 1 . 49, whereas that of the hyperbolic capacity is 0 . 98 to 2 . 28. Based on available data, a log-normal probability model appears adequate for reliability analysis. Laboratory-scale load tests conducted in uniform soil deposits prepared under controlled laboratory conditions are ideal for establishing benchmarks on the probable magnitude of uncertainty arising from model idealisations alone. However, the limited range of geometric and geotechnical parameters in a laboratory load test database may not produce a representative mean model factor. A field load test database typically contains more diverse geometric and geotechnical parameters, but it entails an unknown degree of extraneous uncertainties. A comparative study indicates that model statistics are surprisingly robust and appear not to be seriously affected by the above concerns (possibly because of normalisation). Model factors from drained analysis seem to be more variable than those from undrained analysis. A more detailed examination indicates that the higher COV of about 40% for these drained model factors arises because they are not completely random. There are reasons to believe that applying a more complete force system for drained analysis could minimise some of the undesired correlations and reduce the COV to a level comparable to undrained analysis.Cet exposé présente une évaluation critique de facteurs de modèles pour des puits percés rigides à charge latérale (piles forées). Nous prenons en compte la limite latérale et la limite de moment ainsi que la capacité hyperbolique afin de rendre explicite la dépendance des facteurs de modèles vis-à-vis du critère servant à interpréter la 'capacité' d'après les données d'essais de charge. Bien que la capacité hyperbolique puisse être la plus proche de l'état ultime théorique ou de la limite supérieure, les résultats indiquent qu'en général cela ne produit pas un facteur de modèle moyen de 1. Lorsque la capacité mesurée est interprétée de manière consistante d'après les résultats des essais de charge, le coefficient de variation (COV) paraît rester relativement constant, entre 30% et 40%. La gamme de l'inclinaison moyenne pour la limite latérale ou la limite de moment se situe entre 0,67 et 1,49, alors que celle de la capacité hyperbolique se situe entre 0,98 et 2,28. D'après les données dispo...

Closure to “Characterization of Model Uncertainty for Cantilever Deflections in Undrained Clay” by D. M. Zhang, K. K. Phoon, H. W. Huang, and Q. F. Hu

“…13 is best appreciated from the coefficient of variation (COV) of the modified MSD model factor ε 0 MSD which is about 0.3. Phoon and coworkers (e.g., Phoon and Kulhawy 2005;Dithinde et al 2011) have demonstrated that a COV between 0.3 and 0.4 is typical for model factors of simplified calculation methods. Hence, the COV of the modified MSD model factor is not atypical from this perspective.…”

Characterization of model uncertainty in predicting axial resistance of piles driven into clay

“…Marcos et al (2013) evaluated the bias in resistance interpretation criteria for driven precast concrete piles in compression for drained and undrained condition. Studies associated with the effect of bias in failure criteria on the model statistics and reliability analysis were performed by Phoon and Kulhawy (2005) and Zhang et al (2005). In this article, the measured resistance is defined as the load corresponding to a pile head settlement of 10% of the pile diameter B.…”

Slope Stability and Reliability Analysis of Earth Embankment Constructed for the Doubling of Railway Track

“…Geotechnical structures, especially embankments or slopes of great heights, constructed using soil as natural material, possess serious stability issues and require detailed investigation for the load carrying capacity, settlement, or deformation pattern as well as stability against failure (Duncan and Wright, 2005). Further challenges are posed due to uncertainties associated with input geotechnical parameters arising due to (i) inherent soil variability, (ii) model transformation uncertainty and (iii) testing errors (Phoon and Kulhawy, 1999a&b;Phoon and Kulhawy, 2005;Uzielli et al 2007). Conventionally, geotechnical design uses Allowable Stress Design (ASD), in which a reasonable factor of safety based on engineering judgment, experience and confidence level of the designer, is used.…”