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Many offshore geotechnical problems--such as cyclic T-bar penetration and lateral buckling of pipelines--are affected by the tendency of fine-grained soils to strain-soften due to remoulding. Careful calibration of the constitutive model parameters that control strain softening is essential for accurate simulation of these processes. This is usually achieved by matching constitutive model response with standard element test data, which implicitly assumes that homogeneous stress/strain fields exist within the sample. However, popular element test protocols (e.g., triaxial, simple shear) cannot apply sufficient deformation-- at least as measured at the boundaries--to achieve fully remoulded conditions. This work explores the potential to determine strain-softening parameters experimentally, directly from image-based full-field deformation measurements and external loading data. Artificial data generated from finite element simulations of biaxial compression and T-bar penetration tests, using a non-locally regularised strain-softening constitutive model based on Modified Cam Clay, are then used to demonstrate the potential of the proposed technique. The results demonstrate significant potential for the application of the technique to identify constitutive parameters from full-field measurements even when polluted with modest measurement noise. Key words: large deformation modelling, particle image velocimetry, full-field deformation measurement, constitutive parameter calibration, 1. Introduction Geotechnical problems--particularly in the field of offshore geomechanics----often involve significant remoulding of soil due to large movements of structural elements. Selected examples of such problems are cyclic T-bar [...]

The risk posed by seabed instability is of increasing significance as offshore activities including oil and gas developments continue to expand in this century. Many studies have considered runout of debris flows resulting from submarine landslides and potential impact on offshore infrastructure. However, initiation of slab failure resulting from shear band propagation (SBP) at the onset of a landslide has received less attention, although it is of key importance for estimating the scale and consequences of landslides. The present paper explores arrest of SBP as well as global slab failure above the shear band. The complete evolution of a submarine landslide from shear band initiation, propagation, slab failure, and arrest of SBP is observed through large-deformation finite element (LDFE) modelling. Governing equations for both dynamic and quasi-static SBP are established and solved, with results showing good agreement with the LDFE results. Lower limits for the final shear band length and for global slab failure are proposed for cases with high strain-rate dependency of shear strength, where shear band propagation is quasi-static; upper limits are proposed for cases without rate effects where the behaviour is dynamic. Key words: failure, landslides, shear strength, slopes, offshore engineering. Les risques poses par l'instabilite des fonds marins sont de plus en plus importants, car les activites d'exploitation de mer, y compris les developpements petroliers et gaziers, continuent de prendre de l'ampleur au cours de ce siecle. De nombreuses etudes ont pris en compte le debordement des coulees de debris resultant des glissements sous-marins et l'impact potentiel sur les infrastructures d'exploitation de mer. Cependant, l'initiation de la rupture de la plaque resultant de la propagation de la bande de cisaillement (PBC) au debut d'un glissement a recu moins d'attention, bien qu'elle soit d'une importance capitale pour estimer l'echelle et les consequences des glissements. Le present article explore l'arret de la PBC ainsi que la rupture de la plaque globale au-dessus de la bande de cisaillement. L'evolution complete d'un glissement sous-marin a partir de l'initiation de la bande de cisaillement, de la propagation, de la rupture de la plaque et de l'arret de la SBP est observee grace a une modelisation par elements finis de grandes deformations (<< LDFE >>). Les equations determinantes pour la PBC dynamiques et quasi statiques sont etablies et resolues, les resultats montrant un bon accord avec les resultats par LDFE. Des limites inferieures pour la longueur finale de la bande de cisaillement et pour la rupture globale de la plaque sont proposees pour les cas ou la vitesse de deformation depend fortement de la resistance au cisaillement, ou la propagation de la bande de cisaillement est quasi statique; des limites superieures sont proposees pour les cas sans effets de taux lorsque le comportement est dynamique. [Traduit par la Redaction] Mots-cles: echec, glissements, resistance au cisaillement, pentes, ingenierie d'exploitation de mer., Introduction Instability of submarine or subaqueous slopes is ubiquitous throughout the world, including much studied examples such as the giant Storegga landslide that occurred 8200 years ago in the North [...]

Current site investigation practice for offshore pipeline design relies on soil parameters gathered from boreholes or in situ test soundings to depths of 1-2 m below the mudline. At these depths, the fine-grained seabed is very soft and possesses low undrained strength, which can be difficult to measure. This paper describes a centrifuge test programme undertaken to evaluate the feasibility and performance of a novel penetrometer designed to assess the shallow strength of soft seabed over continuous horizontal profiles. This device is termed the vertically oriented penetrometer (VOP). Tests were performed on a normally consolidated kaolin sample, with the VOP translated horizontally at velocities ranging from 1 to 30 mm/s, after embedding the VOP at 30 and 45 mm depths. All tests involved many cycles of VOP forward and backward movement to assess its potential to derive the ratio of intact to fully remoulded strength. Strength determination is achieved by dragging the VOP at a specified embedment depth along the soil surface, and deriving the soil strength from the measured resistance as if the VOP were a laterally loaded pile. The VOP is shown to yield comparable strength measurements to that of a T-bar penetrometer. The VOP is a potentially valuable addition to the range of tools used to characterize soil strength, both in small-scale centrifuge models and, following practical development, potentially also in the field. Key words: clays, penetrometer, pipeline, shear strength, site investigation. Pour concevoir des pipelines en mer, on se base habituellement sur les parametres du sol mesures dans les trous de forage ou lors de sondages d'essai sur le terrain realises a des profondeurs comprises entre 1 et 2 m sous le fond marin. A ces profondeurs, le sol finement granuleux est tres mou et presente une faible resistance non drainee, qui peut etre difficile a mesurer. Le present article decrit un programme d'essais en centrifugeuse visant a evaluer la faisabilite et les performances d'un nouveau type de penetrometre permettant d'estimer la resistance superficielle d'un fond marin mou sur des profils horizontaux continus. Cet appareil est appele penetrometre oriente verticalement (POV). Les essais ont ete effectues sur un echantillon de kaolin normalement consolide, le POV etant soumis a une translation a des vitesses variant entre 1 et 30 mm/s, apres enfouissement du POV a des profondeurs de 30 et 45 mm. Tous les essais ont consiste en nombreux cycles de mouvements du POV en avant et en arriere, afin que l'on puisse evaluer sa capacite a mesurer le quotient de la resistance au cisaillement a l'etat intact par la resistance au cisaillement a l'etat totalement remanie. On a pu determiner la resistance en tramant le POV a une profondeur d'enfouissement specifique le long de la surface du sol et en deduisant la resistance du sol a partir de la resistance mesuree, comme si le POV etait un pieu charge lateralement. On constate que le POV fournit des mesures de resistance comparables a celles obtenues a l'aide d'un penetrometre a barre en T. Le POV est donc tres utile et pourrait s'ajouter a l'eventail des outils servant a caracteriser la resistance du sol dans le cadre d'essais en centrifugeuse sur des modeles reduits et possiblement sur le terrain apres une phase de developpement pratique. [Traduit par la Redaction] Mots-cles: argiles, penetrometre, pipeline, resistance au cisaillement, etude sur le terrain., Introduction This paper describes the development of a novel device for measuring the near-surface soil undrained strength, named the vertically oriented penetrometer (VOP). This device is a short rigid cylinder, [...]

This paper reports advanced three-dimensional large-deformation finite element analyses investigating the effect of the padeye offset on the behaviour of plate anchors. The analyses varied the normalized padeye offset from 0 to 0.5 and identified two counterbalancing effects on the anchor bearing capacity induced by the padeye offset. These are the change in anchor inclination with respect to the direction of loading (detrimental due to the reduction of bearing area) and the reduction of loss of embedment (beneficial, as the anchor mobilizes stronger soil). The results provide insights to optimize the design of plate anchors. The underlying concepts also have applications for other types of anchors. Key words: plate anchor, large-deformation finite element analysis, clays, offshore engineering, loss of embedment, bearing capacity. Cet article presente des analyses par elements finis avancees en trois dimensions et en grande deformation servant a etudier l'effet de l'espacement des mains de fer sur le comportement des ancrages fi plaque. Les analyses ont fait varier l'espacement normalise des mains de fer de 0 a 0,5 et a identifie deux effets contre-balanciers sur la capacite portante des ancrages causes par l'espacement des mains de fer. Il s'agit de la variation de l'inclinaison de l'ancrage selon la direction de la charge (effet negatif du a la reduction de la surface portante) et de la reduction des pertes d'enfouissement (benefique puisque l'ancrage mobilise un sol plus fort). Ces resultats offre des informations pour optimiser la conception des ancrages a plaque. Les concepts de base ont aussi des applications pour d'autres types d'ancrage. [Traduit par la Redaction] Mots-cles: ancrage fi plaque, analyse par elements finis en grande deformation, argiles, ingenierie en mer, perte d'enfouissement, capacite portante., Introduction Plate anchors are an economic and efficient deep-water mooring solution, which have been used increasingly in recent years (Dove et al. 1998; Murff et al. 2005; Yang et al. [...]

Shallow skirted foundations can mobilize uplift resistance from end bearing in the short to medium term. However, uncertainty exists over the magnitude of reverse end bearing resistance compared with resistance in compression, and how this might be affected by a gap between the external face of the foundation skirt and the adjacent soil. The study presented in this paper explores this problem through centrifuge model tests, investigating the effect of skirt embedment ratio on (i) the magnitude of reverse end bearing capacity compared with compression capacity, (ii) the uplift displacement associated with spontaneous loss of suction during uplift, and (iii) the existence of a vertical gap along the external skirt-soil interface. The results show that (i) peak uplift capacity equivalent to compression capacity can be mobilized for a fully sealed foundation with an intact skirt-soil interface, (ii) suction required for reverse end bearing can be maintained through considerable foundation displacement, even for a low skirt embedment ratio, and (iii) the presence of a vertical gap along the external skirt-soil interface causes abrupt loss of suction beneath the top plate after minimal foundation displacement, with subsequent uplift capacity being markedly reduced. Key words: bearing capacity, centrifuge modelling, clays, footings-foundations, offshore engineering, suction. Les fondations chemisees peu profondes peuvent mobiliser de la resistance au soulevement a une extremite sur une periode de temps courte et moyenne. Cependant, il y a des incertitudes en lien avec la magnitude de la resistance a l'extremite inverse comparativement a la resistance en compression, ainsi que la facon dont cette magnitude est affectee par l'espace entre la face externe de la chemise de la fondation et le sol adjacent. L'etude presentee dans cet article explore cette problematique a l'aide d'essais de modelisation par centrifugeuse servant a evaluer l'effet du ratio d'ancrage de la chemise sur (i) la magnitude de la capacite portante de l'extremite inverse comparativement a la capacite en compression, (ii) le deplacement en soulevement associe a la perte de succion spontanee durant le soulevement et (iii) l'existence d'un espace vertical sur l'interface externe entre la chemise et le sol. Les resultats demontrent que (i) une capacite de soulevement maximale equivalente a la capacite en compression peut etre mobilisee dans le cas d'une fondation completement scellee avec un interface chemise-sol intact, (ii)la succion necessaire pour la capacite portante de l'extremite inverse peut etre maintenue malgre des deplacements considerables de la fondation, meme pour un faible ratio d'ancrage de la chemise et (iii) la presence d'un espace vertical le long de l'interface externe chemise-sol entrame une perte de succion abrupte sous la plaque superieure suite a un deplacement minimal de la fondation, et la capacite de soulevement est reduite significativement par la suite. [Traduit par la Redaction] Mots-cles: capacite portante, modelisation par centrifugeuse, argiles, semelles/fondations, ingenierie en mer, succion., Introduction Skirted foundations Skirted foundations comprise a top plate with a peripheral 'skirt'. The skirt penetrates the seabed until the top plate comes into contact with the seabed, confining a [...]

Current design practice for pipe-seabed interaction in soft soils is generally based on the assumption of undrained behaviour throughout laying and subsequent operation. In reality, drainage and consolidation around a partially embedded pipe can have a marked effect on the vertical penetration and horizontal breakout resistance. In this paper, a large-deformation finite element methodology coupled with the 'modified Cam clay' plasticity soil model has been developed to study the coupled consolidation behaviour of soil around partially embedded seabed pipelines. Simulations of penetration show that after laying, subsequent consolidation leads to further embedment by an amount dependent on the level of drainage that occurred during laying. Also, if the pipe is embedded under undrained conditions, the waiting period between laying and operation allows the soil around the pipe to consolidate under the pipe self-weight. The consolidation process results in an increase in the strength of the soil. The lateral breakout resistance and the direction of pipe movement on breakout thus depend on the consolidated strength of the soil around the pipe, as well as the applied loading. The envelopes of vertical-lateral combined loading bearing capacity differ markedly from those predicted assuming undrained behaviour throughout. Key words: pipeline, offshore engineering, clay, consolidation, penetration, finite element method. La pratique courante de conception de l'interaction tuyau-fond marin est generalement basee sur l'hypothese d'un comportement non draine durant le placement et durant l'operation subsequente. En realite, le drainage et la consolidation autour d'un tuyau partiellement enfoui peut avoir un effet marque sur la penetration verticale et la resistance horizontale a la separation. Dans cet article, une methodologie par elements finis a grande deformation couplee avec le modele de plasticite du sol << Cam clay modifie >> a ete developpee pour etudier le comportement couple en consolidation du sol autour de tuyaux partiellement enfouis dans le fond marin. Des simulations de penetration montrent que suite a la deposition, la consolidation subsequente entrame un enfouissement supplementaire dependamment du niveau de drainage s'etant produit durant la deposition. De plus, si le tuyau est enfoui en conditions non drainees, la periode d'attente entre la deposition et l'operation permet au sol autour du tuyau de consolider sous le poids du tuyau. Le processus de consolidation resulte en une augmentation de la resistance du sol. La resistance a la separation laterale et la direction du mouvement du tuyau lors de la separation dependent ainsi de la resistance a la consolidation du sol autour du tuyau, de meme que sur la charge appliquee. Les enveloppes de capacite portante en sollicitations verticales-laterales combinees different de faon marquee de celles predites en supposant un comportement non draine. [Traduit par la Redaction] Mots-cles: tuyau, ingenierie en mer, argile, consolidation, penetration, methode par elements finis., Introduction When offshore oil and gas reserves are exploited in deep-water, long pipelines are required within each field and are often used to transport the hydrocarbons to shore. In shallow [...]

Offshore developments for hydrocarbon resources have now progressed to water depths approaching 3000 m, with geotechnical design increasingly focused on soft sediments in the upper 30 m or so of the seabed. Due to the difficulties and high cost in recovering high-quality samples from deepwater sites, there is increasing dependence on in situ testing techniques for determining the geotechnical design parameters. This paper summarizes the findings from a joint industry project, undertaken by the Norwegian Geotechnical Institute and the Centre for Offshore Foundation Systems at The University of Western Australia, on the use of in situ testing for the characterization of deepwater soft soils. The project focused on theoretical and empirical studies for the interpretation of piezocone, T-bar, and ball penetration test data, linking the penetration resistance to shear strengths determined from in situ vane tests and laboratory tests on high-quality samples. Guidelines are summarized for interpreting in situ test data, particularly for estimating intact and remoulded undrained shear strengths from the penetration resistance measured by each type of penetrometer. Suggestions are also given for future development of in situ testing techniques to maximize the potential of in situ testing in characterization of deepwater soft soils. Key words: offshore soil investigations, soft clay, full flow penetrations, undrained shear strength. Les developpements des ressources d'hydrocarbures en mer atteignent maintenant des profondeurs de pres de 3000 m, et la conception geotechnique est de plus en plus centree [s.sub.ur] les sediments mous des premiers 30 m du fond marin. En raison des difficultes et des couts eleves associes a la recuperation d'echantillons de qualite des sites profonds, les techniques in situ voient leur utilisation augmenter lors de la determination des parametres geotechniques de conception. Cet article resume les resultats obtenus lors d'un projet industriel conjoint, entrepris par l'Institut geotechnique norvegien et par le Centre for Offshore Foundation Systems a l'Universite de Western Australia, [s.sub.ur] l'utilisation de methodes in situ pour la caracterisation des sols marins mous et profonds. Le projet s'est penche [s.sub.ur] les etudes theoriques et empiriques de l'interpretation des donnees provenant d' essais au piezocone, au T-bar et au penetrometre a bille, reliant la resistance a la penetration aux resistances en cisaillement determinees par des essais scissometriques in situ et en laboratoire effectues [s.sub.ur] des echantillons de qualite elevee. Des directives pour interpreter les donnees d'essais in situ sont resumees, en particulier pour l'estimation de la resistance au cisaillement non drainee intacte et remaniee a partir des resistances a la penetration obtenues pour chaque type de penetrometre. Des suggestions sont aussi fournies pour le developpement futur des techniques d'essai in situ afin de maximiser le potentiel de la caracterisation in situ des sols mous et profonds. Mots-cles : etudes des sols marins, argile molle, penetration en ecoulement complet, resistance au cisaillement non drainee. [Traduit par la Redaction], Introduction Geotechnical properties of near-surface seabed sediments are of increasing importance for deepwater hydrocarbon field developments, where offshore installations will typically comprise wellheads and subsea completions, pipelines, and shallow anchoring [...]

Seabed pipelines must be designed to accommodate thermal expansion--which is commonly achieved through controlled lateral buckling--and to resist damage from submarine slides. In both cases, the pipe moves laterally by a significant distance and the overall pipeline response is strongly influenced by the lateral pipe-soil resistance. Here, the process of pipe penetration and lateral displacement is investigated using a large-deformation finite element method, with a softening rate-dependent soil model being incorporated. The calculated soil flow mechanisms, pipe resistances, and trajectories agree well with plasticity solutions and centrifuge test data. It was found that the lateral resistance is strongly influenced by soil heave during penetration and the berm formed ahead of the pipe during lateral displacement. For 'light' pipes, the pipe rises to the soil surface and the soil failure mechanism involves sliding at the base of the berm. In contrast, 'heavy' pipes dive downwards and a deep shearing zone is mobilized, expanding with continuing lateral movement. The different responses are reconciled by defining an 'effective embedment' that includes the effect of the soil berm or wall ahead of the pipe. The relationship between normalized lateral resistance and effective embedment is well fitted using a power law. Key words: pipeline, clay, finite element method, buckling, penetration. Les tuyaux placees au fond de la mer doivent etre concjus de maniere a contenir l'expansion thermique--ce qui est normalement accompli par flambement lateral controle--et a resister aux dommages causes par les glissements sousmarins. Dans les deux cas, le tuyau se deeplace lateralement sur une distance significative, et le comportement global du tuyau est fortement influence par la resistance laterale entre le tuyau et le sol. Dans cet article, le processus de penetration du tuyau et de deeplacement lateral est evaluee a l'aide d'une meethode par elements finis pour les grands deplacements, a laquelle un modele de sol avec l'adoucissement des deformations dependant d'un taux est ajoute. Les mecanismes d'ecoulement, les resistances des tuyaux et les trajectoires calcules correspondent bien avec les solutions de la theorie de la plasticite et les donneees des essais a la centrifuge. Il a ete determine que la resistance laterale est fortement influencee par le soulevement du sol durant la penetration et aussi par la berme formeee au devant du tuyau durant le deplacement lateral. Dans le cas de tuyaux << leegers >>, le tuyau monte a la surface du sol et un glissement a la base de la berme est implique; dans le mecanisme de rupture du sol. A l'oppose, les tuyaux << lourds >> se deeplacent vers le bas et une zone de cisaillement profonde est mobiliseee et s'etend a mesure que le mouvement lateral s'avance. Ces comportements diffeerents sont combines dans le parametre << enfouissement effectif >> qui inclus l'effet de la berme ou du mur en avant du tuyau. La relation entre la re;sistance late;rale normalise;e et l'enfouissement effectif est bien repre;sente;e par une loi de puissance. Mots-cles: tuyau, argile, meethode d'eelements finis, flambement, penetration. [Traduit par la Redaction], Introduction Seabed pipelines are an increasingly significant component of offshore hydrocarbon developments. As exploration moves into deeper waters, more distant from shore, the design and construction of the seabed pipelines [...]

This paper presents results and analyses of the dynamic behavior of a small-diameter pile driven in very soft clay, with its tip embedded to a depth of 4.5 m with a clear distance of 1-1.5 m above a stiffer substratum. The research program included static and dynamic load tests performed over a period of 15 days, with blows from three different drop heights. The dynamic signals were analyzed using two different pile-soil interaction models. The performance of the dynamic models in predicting the shaft and base behavior of the model pile, as well as the change in bearing capacity and in some model parameters the dynamic with time, are addressed in particular. Key words: dynamic pile tests, soft clay, soil models, set up. Cet article presente les resultats et analyses du comportement dynamique d'un pieu de faible diametre enfonce dans de l'argile tres molle, avec sa point a une profondeur de 4,5 m et a une distance libre de 1 a 1,5 m au-dessus d'une fondation plus dure. Le programme de recherche comportait des essais de chargement statiques et dynamiques effectues sur une periode 15 jours, et consistaient de coups de trois elevations de chute differentes. Les signaux dynamiques ont ete analyses a l'aide de deux modeles differents d'interaction pieu-sol. L'article discute de facon particuliere de la performance des modeles dynamiques pour predire le comportement de l'arbre et de la base du pieu modelise, ainsi que de la dynamique dans le temps de la capacite portante et d'autres parametres du modele. Mots-cles : essais dynamiques de pieux, argile molle, modeles de sol, installation. [Traduit par la Redaction], Introduction Fine-grained soils exhibit time-dependent behavior, affecting their stress--strain response and shear strength. The most common processes are associated with water migration (consolidation), but other phenomena are also recognized, such [...]

Abstract: Large deformation finite element (LDFE) analyses were performed to study the installation of caissons by suction and jacking in normally consolidated clay. The penetration of the caisson wall was [...]

Ref.: Can. Geotech. J. 52(6): 682-693 (2015) dx.doi.org/10.1139/cgj-2014-0120. On page 689, right column, eighth line of the second paragraph, replace 'Mmax/B2Lsu = 4.17' with '[M.sub.max]/[B2Ls.sub.u] = 2.05'. Received 20 July [...]