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This paper reports the soil flow mechanisms observed in centrifuge tests around full-flow (T-bar and ball) penetrometers in layered clays. The layered clay samples consisted of soft-stiff, stiff-soft, soft-stiff-soft, and stiff-soft-stiff soil profiles. Particle image velocimetry (PIV), also known as digital image correlation (DIC), allowed accurate resolution of the flow mechanism around the faces of the T-bar and half-ball penetrated adjacent to a transparent window. For the T-bar, overall, a full symmetrical rotational flow around the T-bar dominated the behavior. A novel 'trapped cavity mechanism' was revealed in stiff clay layers, with the evolution of the trapped cavity being tracked. No soil plug was trapped at the base of the advancing T-bar regardless of penetration from stiff to soft layer or the reverse. For the ball, two key features of the soil flow mechanism were identified, including (i) a combination of vertical flow, cavity expansion type flow, and rotational flow for a fully embedded ball and (ii) a stiff soil plug trapped at the base of the ball advancing in a stiff-soft clay deposit. For both penetrometers, a squeezing mechanism mobilized as they approached a soft-stiff layer interface. Key words: centrifuge modelling, penetrometers, clays, failure, site investigation. Le present article rend compte des mecanismes d'ecoulement du sol. Ils ont ete constates lors d'essais en centrifugeuse autour de penetrometres a plein debit (barre en T et boule) dans des argiles stratifiees. Les echantillons d'argile stratifiee etaient constitues selon les profils de sol suivants : souple-resistant, resistant-souple, souple-resistant-souple et resistant-souple-resistant. Le procede de velocimetrie par images de particules (PIV), qui est aussi designee sous le nom de correlation d'images numeriques (CID), a fourni une definition precise du mecanisme d'ecoulement autour des faces de la barre en T et de la demi-boule infiltree a proximite d'une fenetre transparente. Dans l'ensemble, << le comportement de la barre en T >> a ete domine par un ecoulement rotationnel symetrique autour de la barre en T. Un nouveau mecanisme de cavite piegee a ete revele dans des argiles rigides stratifiees, l'evolution de la cavite piegee etant suivie. En effet, aucun bouchon de sol n'a ete piege a la base de la barre en T qui avancait, peu importe la penetration de la couche resistante a la couche souple ou l'inverse. En ce qui concerne la boule, il y a deux caracteristiques cles du mecanisme d'ecoulement du sol, qui ont ete relevees, notamment (i) une combinaison d'ecoulement vertical, d'ecoulement de type expansion de la cavite et d'ecoulement rotationnel pour une boule entierement enrobee et (ii) un bouchon de sol resistant piege a la base de la boule avancant dans un depot d'argile resistante et souple. Pour les deux penetrometres, un mecanisme d'ecrasement s'est mobilise a l'approche de l'interface entre la couche souple et la couche resistante. [Traduit par la Redaction] Mots-cles : modelisation au centrifuge, penetrometres, argiles, echec, etude du terrain., Introduction Accurate characterization of seabed sediments is crucial for safe and cost-effective design of offshore foundations. Recent exploration of hydrocarbon products in deep water has placed more reliance on in [...]

There is a historic lack of research examining inner soil plugging and outer lateral soil deformation during installation of pipe piles in clay. Furthermore, the influence of the pile tip geometry on soil deformation has not been investigated. Cast-in-place concrete pipe (PCC) piles are a recent generation of pipe piles for soft ground improvement applications, and are cast in place using an innovative double-walled steel casing with a tapered tip. This paper investigates the effect of tip geometry on soil flow mechanisms inside and outside the PCC pile casing during installation in clay. Large-deformation finite element (LDFE) analysis was conducted simulating the continuous penetration process of the casing installation. The LDFE results were validated against (i) field monitoring data in terms of soil lateral displacement and heave outside the casing and (ii) centrifuge test data in terms of penetration resistance, with excellent agreement obtained. An extensive parametric study was then performed encompassing a practical range of parameters. The geometry of the casing tip was shown to have significant influence on the soil movements inside and outside the casing, but minimal effect on the casing penetration resistance. The optimal tapered tip with a tip angle of 45° and minimal extended tip length beneath the tip angle allowed more soil to be pushed inside the casing and less lateral displacement outside the casing, which dictates the amount of concrete required to fill the cavity inside the pile and the pile construction sequence, respectively. Design expressions are proposed for estimating soil movements inside and outside the casing with optimal tip geometry being installed. Key words: pipe piles, casing installation, casing tip, clay, soil deformation, large-deformation finite element (LDFE) analysis. Historiquement, il y a un manque de recherche examinant le phenomene de formation de bouchon de sol interne et la deformation laterale externe pendant l'installation de pieux tubulaires dans l'argile. De plus, on n'a pas examine comment la geometrie de la pointe du pieu influe sur la deformation du sol. Les pieux tubulaires en beton coule sur place (PCC) sont une generation recente de pieux tubulaires en matiere d'applications pour l'amelioration des sols mous, et sont coules sur place au moyen d'un tubage en acier a double paroi avec une pointe conique. Cette etude a pour but d'examiner l'effet qu'a la geometrie de la pointe sur les mecanismes d'ecoulement des sols a l'interieur et a l'exterieur du tubage du pieu PCC lors de l'installation dans l'argile. On a fait l'analyse par elements finis appliquee aux deformations importantes (LDFE) simulant le processus de penetration continu de l'installation du tubage. Les resultats de LDFE ont ete valides contre (i) les donnees de suivi sur le terrain en matiere de deplacement lateral du sol et de soulevement a l'exterieur du tubage, et (ii) les donnees d'essai au centrifuge en matiere de resistance a la penetration, et la concordance s'est averee excellente. On a ensuite realise une vaste etude parametrique englobant une large gamme de parametres. On a demontre que la geometrie de la pointe du tubage influe de facon significative sur les mouvements du sol a l'interieur et a l'exterieur du tubage, mais l'effet sur la resistance a la penetration du tubage est minimal. La pointe conique optimale ayant un angle de pointe de 45° et une longueur de pointe en extension minimale fait en sorte qu'une plus grande quantite de sol a ete poussee dans le tubage et moins de deplacement a eu lieu a l'exterieur du tubage, ce qui prescrit le remplissage en beton requis a l'interieur de la cavite du pieu et la sequence de la construction du pieu, respectivement. On a propose des expressions de conception pour l'estimation des mouvements du sol a l'interieur et a l'exterieur du tubage d'installation ayant une geometrie optimale de la pointe. [Traduit par la Redaction] Mots-cles : pieux tubulaires, installation du tubage, pointe du tubage, argile, deformation du sol, analyse par elements finis appliquee aux deformations importantes (LDFE)., Introduction PCC pile and construction process Recently, large-diameter, cast-in-place concrete pipe (PCC) piles are increasingly being used in soft ground improvement applications (improving the bearing capacity and reducing the settlement [...]

Difficulties are encountered, in soft clay and silt sediments, during extraction of the spudcan foundations of 'mobile' jack-up rigs. This paper reports results from centrifuge model tests undertaken to provide a simple, effective, and practical means for easing spudcan extraction in soft normally consolidated and lightly overconsolidated clay. The tests were carried out on model spudcans of two different geometries typically used in the field. The spudcans were extracted from penetrations of 2~3 diameters and after an operation period of ~19 months. The measured pore pressure at the bottom face of the spudcan during extraction process provided insight into the development of suction at the interface between the spudcan base and the underlying soil. The tests were carried out with and without a sand layer placed locally in between the spudcan and the mudline at the onset of penetration. In the field, a gravel layer can be placed by means of a flexible fall pipe system. The presence of a sand layer (with a thickness less than the spudcan tip length) beneath the penetrated spudcan assisted in easing leg extraction difficulties allowing the base suction to be reduced by 73%~83% and hence the breakout force by 31%~32%. For a given penetration depth, the installation resistance was also increased by 24%~26%. This yielded a 15%~17% reduction of spudcan penetration depth under a given preload and hence the breakout force as much as 50%. Guidelines and cautions for using the proposed measure are provided. Additional advantages of the new recommended method in enhancing the efficiency of the conventional measures of lessening spudcan extraction difficulties are discussed. Key words: breakout force, centrifuge tests, clays, pullout, sand, suction, spudcan foundations. On rencontre des difficultes lors de l'extraction des caissons de fondation des plateformes de forage autoelevatrices mobiles installes dans l'argile molle et dans des sediments de limon. Cet article presente les resultats des essais sur modeles en centrifugeuse realises dans le but de proposer un moyen simple, efficace et pratique visant a faciliter l'extraction de caissons de fondation reposant dans l'argile molle normalement consolidee et legerement surconsolidee. Les essais ont ete effectues sur des caissons experimentaux de deux geometries differentes couramment utilises sur le terrain. Les caissons de fondation ont ete extraits apres avoir ete enfonces a des profondeurs de 2 a 3 fois leur diametre et apres une duree d'exploitation d'environ 19 mois. La pression interstitielle mesuree au niveau de la face inferieure du caisson au cours du processus d'extraction a permis de comprendre l'augmentation de la succion a l'interface de la base du caisson et du sol sous-jacent. Les essais ont ete realises en presence et en l'absence d'une couche de sable placee localement entre le caisson et la surface de la boue, au niveau du point de penetration. Sur le terrain, une couche de gravier peut etre introduite a l'aide d'un systeme de tuyaux de descente flexible. La presence d'une couche de sable (dont l'epaisseur est inferieure a la longueur de l'extremite du caisson) sous le caisson enfonce a permis de diminuer les difficultes liees a l'extraction des piliers de plateforme et de reduire de 73 a 83% la succion a la base du caisson, ce qui a eu pour effet de diminuer de 31 a 32% la force d'arrachement. En outre, pour une profondeur de penetration donnee, la resistance d'installation a augmente de 24 a 26%, ce qui a entraine une reduction de 15 a 17% de la profondeur de penetration du caisson pour une precharge donnee et, en consequence, une diminution de la force d'arrachement atteignant 50%. Le present article fournit des recommandations et des mises en garde d'utilisation de la mesure proposee. Il expose egalement les avantages supplementaires de la nouvelle methode recommandee en permettant d'ameliorer l'efficacite des mesures traditionnelles visant a attenuer les difficultes liees a l'extraction du caisson. [Traduit par la Redaction] Mots-cles: force d'arrachement, essais en centrifugeuse, argiles, arrachement, sable, succion, caissons de fondation., Introduction 'Mobile' jack-up rig and spudcan foundation In the offshore oil and gas industry, 'mobile' independent-leg jack-up rigs are widely used in performing most offshore drilling in water depths up [...]

Spudcan foundations for offshore mobile drilling rigs are large saucer-shaped foundations that can penetrate several tens of meters into soft sediments. The penetration depth is typically predicted by considering a wished-in-place foundation at different depths and following traditional bearing capacity approaches to assess the depth at which the estimated capacity matches the applied loading. However, the geometry of the spudcan and its progressive mode of penetration lead to soil failure mechanisms that differ markedly from those relevant to onshore practice. This paper presents a new rational design approach for assessing spudcan penetration in single layer clays based on a study combining centrifuge model testing and large deformation finite-element (FE) analysis. The design approach takes account of the evolving failure mechanisms in the soil, which start with cavity formation and surface heave at shallow penetration, gradually transforming to backflow of soil over the spudcan. A detailed FE parametric study has explored the relevant range of normalized strength, strength nonhomogeneity, and spudcan base roughness, with results validated against centrifuge model test data. The penetration response curves are presented in terms of profiles of bearing capacity factors, forming nondimensional design charts along with simplified expressions for convenient use in practice. Comparisons with approaches suggested in the SNAME design code suggest an urgent need to update current practice. DOI: 10.1061/(ASCE)GT.1943-5606.0000054 CE Database subject headings: Load bearing capacity; Centrifuge models; Clays; Finite element method; Heterogeneity; Foundation design.

This paper reports results from a series of model tests undertaken to provide insight into the behavior of a stiffened caisson anchor during installation and monotonic pullout in lightly overconsolidated calcareous silt. The tests were carried out in a beam centrifuge, varying the installation process (jacked in or jacked in followed by suction-assisted installation), and the consolidation period before anchor pullout. The mudline load inclination was also varied to encompass various mooring configurations. The centrifuge data were used to calibrate analytical installation and vertical pullout capacity models, based on conventional bearing and frictional resistance factors but with strain rate-dependent and strain-softened undrained shear strength for the soil. Piezocone-based direct design approaches were also proposed, deriving end-bearing and frictional resistance from cone tip resistance and sleeve friction, respectively. Holding capacity of suction caisson under inclined loading was presented as failure envelopes expressed in terms of dimensionless vertical and horizontal components of caisson net resistance, which agreed well with a plastic limit analysis-based envelope developed for suction caissons in clay. The regain of anchor capacity was found to be in good agreement with predictions based on the cavity expansion framework. [ABSTRACT FROM AUTHOR]

A full-flow ball penetrometer is now routinely used in centrifuge testing for characterizing single-layer clay, silt, sand, and stratified soil samples. The behavior of the standard ball penetrometer (including the shaft) used in the field, penetrating through single and two-layer uniform clays, has recently been investigated. However, the ball penetrometers used in centrifuge testing has either a similar or higher area ratio (of the shaft to the ball) and significantly greater equivalent prototype diameter compared with the standard one used in the field. The thickness of the soil layers, however, are scaled accurately mimicking strength profiles in the field, leading to lower relative thickness of the soil layers. Large deformation finite element (LDFE) analyses were, therefore, carried out for the centrifuge ball penetrometers to investigate its performance in characterization of single- and double-layer clays. The results were validated against plasticity solutions and other previously published finite element (FE) results prior to undertaking a detailed parametric study, exploring a range of normalized soil properties and layer thickness. Three commonly used centrifuge balls were studied. For single-layer uniform clay, the area ratio of shaft to ball was shown to have significant influence on the deep bearing factors and the critical penetration depth of attaining that factor. For two-layer uniform clays, the undrained shear strength of the top layer was mostly underestimated because of the lower thickness of that layer relative to the centrifuge ball diameter. In addition, the shear strength of the bottom layer was mostly overestimated because of trapping of stiff clay at the base of the ball, advancing in stiff-over-soft clay deposits. To interpret a reference undrained strength profile, taking into account the scale effects of the penetrometer relative to layer thickness, a framework has been proposed accounting for these effects, with the LDFE data used to calibrate the framework. [ABSTRACT FROM AUTHOR]

A significant difference between predicted and measured installation resistance of stiffened suction caissons was identified due to the existing uncertainty regarding the mobilized soil-flow mechanisms. This paper describes an extensive investigation of stiffened-caisson penetration in nonhomogeneous clays undertaken through large deformation finite-element (LDFE) analysis to provide insight into the soil behavior during installation of a caisson. The soil-flow mechanisms around and between stiffeners, and inside and outside of the caisson, and the corresponding penetration resistances were presented from a parametric study, exploring a range of dimensionless parameters related to stiffened-caisson geometry, caisson roughness, and soil strength nonhomogeneity. The LDFE results were compared with centrifuge test data in terms of the soil-flow mechanisms and penetration resistance profile, with good agreement obtained. Three interesting features in the mobilized soil-flow mechanisms inside the caisson were observed: (1) soil started to infill the gap between the bottom two stiffeners when the bottom stiffener penetration reached Hr (Hr is the critical depth of rotational soil flow around bottom stiffener); (2) soil started to infill the gap between the second and third bottom stiffeners when the second stiffener penetration reached Hc (Hc is the critical depth of soil backflow into gaps above the 2nd stiffener); (3) profound soil heave due to significant inward flow and the presence of stiffeners and gap between stiffeners. Both Hr and Hc were shown to be a function of the stiffener-width to caisson-diameter ratio, normalized soil strength at the mudline, and soil strength nonhomogeneity. To predict the penetration resistance profile in the field, a rational analytical penetration model, based on the revealed soil-flow mechanisms, was proposed. The LDFE data were used to calibrate the model. Very good agreements were obtained when the proposed model was validated against measured data from field installation and centrifuge tests. [ABSTRACT FROM AUTHOR]

This paper describes a new approach for interpreting cone penetrometer data in soft-stiff-soft clay deposits. The identification of layer boundaries and interpretation of shear strength profile were of particular interest. The proposed approach was based on an extensive parametric study using large deformation finite-element (LDFE) analyses, with a standard cone penetrometer penetrated continuously from the soil surface. The LDFE model has been validated against existing theoretical solutions and numerical results, with good agreement obtained. Regardless of strength ratio between two successive layers, the interface of soft-stiff layers can be identified at 0.8D (D is the cone diameter) below the kink in penetration resistance curve in the top soft layer. The interface of stiff-soft layers can be demarked at 1.3D above the kink in the penetration resistance profile in the bottom soft layer. The undrained shear strength of a soft clay layer can be interpreted using a single-layer approach and the resistance profile without the influence of the adjacent stiff layer. The interpretation for the interbedded stiff layer necessitates implementing a correction factor, which is shown to be a function of the thickness and rigidity index of the stiff layer and the strength ratio between that layer and the bottom layer. The proposed design framework is illustrated through a flowchart to be used in practice. [ABSTRACT FROM AUTHOR]