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An important step in load and resistance factor design (LRFD) is the selection of the characteristic values of uncertain soil parameters, which can be quite subjective despite the simplicity of LRFD. This paper assesses five statistical methods for the selection of characteristic values for design with LRFD, focusing on the design robustness. A framework based on the consideration of safety, cost, and design robustness is proposed for assessing these selection methods. This framework is illustrated with an example, the design of a drilled shaft in sand using LRFD, in which the best overall method for selecting the characteristic values is suggested. The implication of the outcome of this study is quite significant in geotechnical engineering practice, as it provides guidance on the selection of the characteristic values for design with LRFD. Key words: characteristic value, geotechnical design, load and resistance factor design (LRFD), reliability, robustness. Une etape importante dans la conception des coefficients de charge et de resistance (<< LRFD >>) est la selection des valeurs caracteristiques de parametres de sol incertains, ce qui peut etre assez subjectif malgre la simplicite de LRFD. Cet article evalue cinq methodes statistiques pour la selection de valeurs caracteristiques pour la conception avec LRFD, en se concentrant sur la robustesse de la conception. Un cadre base sur la prise en compte de la securite, des couts et de la robustesse de la conception est propose pour evaluer ces methodes de selection. Ce cadre est illustre par un exemple, la conception d'un puits fore dans du sable utilisant LRFD, dans laquelle la meilleure methode globale de selection des valeurs caracteristiques est suggeree. L'implication des resultats de cette etude est assez significative dans la pratique de l'ingenierie geotechnique, car elle fournit des indications sur la selection des valeurs caracteristiques pour la conception avec LRFD. [Traduit par la Redaction] Mots-cles : valeur caracteristique, conception geotechnique, conception des coefficients de charge et de resistance (<< LRFD >>), fiabilite, robustesse., 1. Introduction Uncertainties in soil parameters, solution models, and applied loads make it difficult to ascertain the predicting performance of a geotechnical design, which complicates the design decision. To deal [...]

This paper presents a gradient-based robustness measure for robust geotechnical design (RGD) that considers safety, design robustness, and cost efficiency simultaneously. In the context of robust design, a design is deemed robust if the system response of concern is insensitive, to a certain degree, to the variation of noise factors (i.e., uncertain geotechnical parameters, loading parameters, construction variation, and model biases or errors). The key to a robust design is a quantifiable robustness measure with which the robust design optimization can be effectively and efficiently implemented. Based on the developed gradient-based robustness measure, a robust design optimization framework is proposed. In this framework, the design (safety) constraint is analyzed using advanced first-order second-moment (AFOSM) method, considering the variation in the noise factors. The design robustness, in terms of sensitivity index (SI), is evaluated using the normalized gradient of the system response to the noise factors, which can be efficiently computed from the by-product of AFOSM analysis. Within the proposed framework, robust design optimization is performed with two objectives, design robustness and cost efficiency, while the design (safety) constraint is satisfied by meeting a target reliability index. Generally, cost efficiency and design robustness are conflicting objectives and the robust design optimization yields a Pareto front, which reveals a tradeoff between the two objectives. Through an illustrative example of a shallow foundation design, the effectiveness and significance of this new robust design approach is demonstrated. Key words: gradient, optimization, Pareto front, reliability analysis, robust design, sensitivity index, shallow foundation. Cet article presente une mesure de la robustesse basee sur le gradient pour la conception robuste geotechnique qui considere la securite, la robustesse du concept et l'efficacite economique simultanement. Dans le contexte de la conception robuste, un concept est considere comme robuste si la reponse du systeme concerne est insensible, jusqu'a un certain degre, a la variation de facteurs de bruit (c'est-a-dire les parametres geotechniques incertains, les parametres de chargement, les variations de construction, et les biais et erreurs du modele). La cle d'un concept robuste est une mesure de robustesse quantifiable avec laquelle l'optimisation du concept robuste peut etre implantee efficacement. Un cadre d'optimisation de concept robuste est propose, base sur le developpement de la mesure de robustesse basee sur le gradient. Dans ce cadre, la contrainte de conception (securite) est analysee a l'aide de la methode du second moment de premier ordre avance (SMPOA) en considerant les variations des facteurs de bruit. La robustesse du concept, en termes d'indice de sensibilite (IS), est evaluee a l'aide du gradient normalise de la reponse du systeme aux facteurs de bruit. Cette reponse peut etre calculee efficacement a partir du sous-produit de l'analyse SMPOA. Avec le cadre propose, l'optimisation du concept robuste est realisee avec deux objectifs : robustesse du concept et efficacite economique, tandis que la contrainte de conception (securite) est satisfaite en atteignant un indice de fiabilite cible. De facon generale, l'efficacite economique et la robustesse du concept sont des objectifs conflictuels; l'optimisation de la robustesse presente le principe de Pareto, ce qui revele un intermediate entre les deux objectifs. L'article presente l'efficacite et l'importance de cette nouvelle approche de conception robuste par un exemple illustre de la conception d'une fondation peu profonde. [Traduit par la Redaction] Mots-cles : gradient, optimisation, principe de Pareto, analyse de fiabilite, concept robuste, indice de sensibilite, fondation peu profonde., Introduction Due to uncertainties of operating conditions, manufacturing tolerances, and model errors, the performance of a product (referred to herein as 'product performance') can be scattered and uncertain. A desired [...]

Liquefaction often causes damage to infrastructure such as buildings, bridges, and lifelines. Liquefaction-induced ground movements such as settlement and lateral spread are of major concern to engineers who have to evaluate seismic risk. This paper deals with evaluation of liquefaction-induced settlement using a cone penetration test (CPT). Existing CPT-based models often overestimate liquefaction-induced settlement. In this paper, a database of case histories of settlement in recent earthquakes is compiled and used to calibrate the model bias of a CPT-based model, from which a simplified procedure is developed that allows for estimation of the probability of exceeding a specified settlement at a given site. Reasonable results are obtained using the developed simplified CPT-based model, as demonstrated in the examples presented. Key words: liquefaction, settlement, case histories, cone penetration test (CPT), probability, maximum likelihood, model bias. La liquefaction cause souvent des dommages aux infrastructures, telles que les batiments, les ponts et les services publics. Les mouvements de terrain induits par la liquefaction, comme les tassements et l'etalement lateral, representent une difficulte pour les ingenieurs ayant a evaluer le risque sismique. Cet article traite de revaluation des tassements induits par la liquefaction a l'aide d'essais de penetration du cone (EPC). Les modeles existants bases sur les EPC surestiment souvent le tassement induit par la liquefaction. Dans cet article, une base de donnees d'histoires de cas de tassements produits lors de seismes recents est compilee et utilisee pour calibrer le biais d'un modele base sur les EPC, et ainsi une procedure simplifiee est developpee. Cette procedure permet d'estimer la probabilite de depasser un tassement specifie sur un site donne. Des resultats raisonnables sont obtenus grace au modele simplifie base sur l'EPC developpe dans cette etude, comme le demontrent les exemples presentes. [Traduit par la Redaction] Mots-cles: liquefaction, tassement, histoires de cas, essais de penetration du cone (EPC), probabilite, probabilite maximale, biais d'un modele., Introduction Earthquake-induced liquefaction of soils often causes damage to infrastructure such as buildings, bridges, and lifelines. Thus, it is essential to be able to estimate the potential of liquefaction occurrence [...]

Ensuring the safety of undersea shield tunnels constructed in soft marine deposits that are subject to strong seismic motion represents a major engineering challenge. An example of one such undersea shield tunnel is the 2.70 km-long subsea highway tunnel crossing under the Gulf of Suai in the Rongjiang River Estuary in Shantou, China. Using the generalized response displacement method, the authors developed a nonlinear seismic response analysis of the segmental lining for the 2.7 km-long Suai tunnel. In the proposed longitudinal seismic response analysis, the engineering geology characteristics and nonlinear dynamic behavior of the Suai seabed soil, the nonuniform mesh layout of the free-field site, the artificial boundary conditions and nonuniform seismic input, simulation model, and the parameters of soil-tunnel interaction systems, are considered in detail. Special emphasis is given to the irregular unloading-reloading rules for the stress-strain hysteresis loop, the seismic input at artificial boundary nodes, and the spatial incoherency of ground motions in the seabed site. The opening width at the ring intersegment under simultaneous actions of longitudinal, transversal, and vertical seismic motions is critical for seismic safety. The proposed methodology was deemed conservative, as demonstrated by a comparison with dynamic transient analysis using the three-dimensional finite element method. The results of this pilot study should be of use in the construction of future long subsea shield tunnels in the high seismic intensity region. [ABSTRACT FROM AUTHOR]

A total of 153 cyclic triaxial tests were performed on 19 sand–gravel mixtures over the strain range of 10–5–10–2, which shows that the particle characteristics have significant influence on the small-strain shear-modulus G0, shear modulus reduction G/G0, and damping ratio λ. An improved Hardin's equation for the G0 was proposed to consider the effects of particle characteristics and consolidation state. Furthermore, the KZM model correlation was improved to characterize the G/G0 curve. A four-parameter equation of λ with parent rock type, particle gradation, and consolidation condition was also introduced to characterize the λ curve. [ABSTRACT FROM AUTHOR]

The shear wave velocity (Vs)-based method of assessing the liquefaction susceptibility of soils has given geotechnical engineers a promising alternative for determining soil resistance to seismic liquefaction. Through the use of an expanded global case histories of soil liquefaction, the authors obtained a new calibrated correlation equation for liquefaction triggering analysis. This new liquefaction triggering correlation was found to be reasonably conservative, and yet insensitive to the variations in the components of the adopted simplified analysis framework. A probabilistic version of the liquefaction triggering correlation, obtained with the weighted maximum-likelihood technique, was validated using a weighted empirical probability approach. Three zones of liquefaction potential, transitioning from non-liquefaction to liquefaction, were then defined based on liquefaction probability contours. [ABSTRACT FROM AUTHOR]