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Drag anchors are often employed in offshore floating facility moorings. The standard drag anchor design approach is based on a deterministic load and resistance factor design (LRFD) framework that considers characteristic design "low" and "high" estimates of soil strength and other geotechnical parameters, combined with code-specified partial factors. A disadvantage of this approach is that the resulting anchor designs may not achieve a consistent level of reliability. This paper describes a study that addresses this limitation by developing and demonstrating a generalised drag anchor probability of failure analysis framework for inclusion in a reliability-based assessment (RBA) of a mooring. A feature of the study is the inclusion of consolidation and cyclic loading effects in the anchor analysis. The study highlights the benefits an RBA approach can offer to the drag anchor design process, including reduced anchor size and preloading requirements, and increased confidence in the anchor design and estimate of anchor performance. For temporarily moored facilities, this approach offers the potential to exploit expanded weather windows for operations. For permanently moored floating offshore wind developments, this approach may allow adoption of reduced levels of target reliability, thereby reducing costs for systems with a large number of anchors. [ABSTRACT FROM AUTHOR]

Grillage foundations may be chosen as an alternative to standard shallow offshore foundations owing to their improved hydrodynamic performance during installation. They consist of a mesh of vertical grilles that penetrate the seabed. During vertical loading, the capacity of a conventional surface foundation may be obtained from grillage foundations if sufficient penetration is permissible. In this paper, a series of laboratory tests is reported to investigate the performance of grillage foundations in sand with different spacing, grille thickness, and numbers of grilles. The load-displacement results are examined in detail and compared with a previously proposed analytical solution. It is shown that the analytical solution captures the key aspects of the foundation's performance if minor improvements are made to the analysis. It is proposed that the improved analytical solution may form the basis of future design. In addition, intergrille plugging behaviour similar to that observed for open-ended piles has been examined photographically. Key words: footings-foundations, offshore engineering, laboratory testing, sand. Des fondations en grillage peuvent etre selectionnees comme alternative aux fondations en mer peu profonde typiques, en raison de leur meilleure performance hydrodynamique lors de l'installation. Ces fondations consistent en un grillage vertical penetrant le fond marin. Avec suffisamment de penetration, les fondations en grillage peuvent atteindre la capacite en chargement vertical des fondations de surface conventionnelles. Dans cet article, les resultats d'une serie d'essais en laboratoire sont presentes. L'etude visait a evaluer la performance de fondations en grillage dans du sable en variant l'espacement, l'epaisseur des grilles et le nombre de grilles. Les resultats en termes de charge-deplacement sont examines en details et compares a une solution analytique proposee precedemment. Il est demontre que la solution analytique reproduit les aspects cles de la performance de la fondation suite a des ajustements mineurs effectues sur l'analyse. Il est suggere d'utiliser la solution analytique amelioree comme base pour les conceptions futures. De plus, le colmatage inter-grille, similaire au comportement observe pour des pieux a bout ouvert, a ete examine a l'aide de photographies. Mots-cles : semelles-fondations, ingenierie en mer, essai en laboratoire, sable. [Traduit par la Redaction], Introduction Shallow foundations are popular for installing offshore infrastructure on the seabed. Bransby et al. (2011) recently described a novel type of shallow foundation--a grillage--that may offer substantial benefits for [...]

This paper reports a series of geotechnical centrifuge model tests conducted to investigate the mechanical reinforcement of slopes by vegetation. Some of the model slopes contained young willow trees, which were grown in controlled conditions to provide different root distributions and mechanical properties. Slopes were brought to failure in the centrifuge by increasing water pressures. The failure mechanisms were investigated photographically and using post-test excavation. By measuring the soil properties and pore pressures in each test when failure occurred, slope stability calculations could be performed for each slope failure. These back-calculations of stability suggest that only a small amount of reinforcement was provided by the root system even when it was grown for 290 days before testing. In contrast, the use of the measured root properties and a commonly used root reinforcement model suggests that significant reinforcement should have been provided by the roots. This disparity is probably due to either inappropriate assumptions made in the root reinforcement model or soil alteration produced by root growth. Such disparities may exist in the application of root reinforcement models to full-scale slopes and therefore require additional study. The modelling technique outlined in this paper is suitable for further investigation of root mechanical interactions with slopes. Key words: centrifuge modelling, landslides, root reinforcement, slope stabilization. Resume : Cet article presente une seerie d'essais modeliseesa l'aide d'une centrifuge geotechnique dans le but d'evaluer le renforcement meecanique des pentes par la vegetation. Quelques pentes des modeles ont des jeunes saules qui ont grandi dans des conditions controls afin d'obtenir diffeerentes distributions de racines et diffeerentes proprieteesmecaniques. Les pentes ont ete ameneees jusqu'a la rupture dans la centrifuge en augmentant les pressions d'eau. Les mecanismes de rupture ont ete investiguesa l'aide de photographies et d'excavations suite aux essais. Des calculs de stabilite de pente ont pu etre effectues pour chaque rupture de pente a partir des mesures des proprietes des sols et des pressions interstitielles lors de la rupture. Ces calculs de stabilite; suggerent que les racines fournissent seulement un leger renforcement lorsque la vegetation a pousse durant 290 jours avant l'essai. Cependant, lorsqu'on utilise un modele traditionnel de renforcement par les racines, avec les proprietes des racines mesurees, les resultats indiquent qu'un renforcement significatif aurait dfl etre fourni par les racines. De telles differences peuvent se produire lors de l'application des modeles aux pentes a l'eechelle reelle et ainsi des etudes suppleementaires sont requises. La technique de modelisation presentee dans cet article est appropriee pour l'investigation plus poussee des interactions mecaniques des racines avec les pentes. Mots-cles: modelisation par centrifuge, glissements de terrain, renforcement par les racines, stabilisation de pentes. [Traduit par la Redaction], Introduction Vegetation within natural and engineered slopes can alter mechanical performance considerably through the reinforcing effects of roots and altered hydrology (Mickovski et al. 2009). In common engineering design, however, [...]

Plant root systems frequently permeate both natural and engineered soil slopes, influencing slope stability via mechanical reinforcement and soil drying. These root systems are often loaded by external forces during slope movements and when plant stems are subject to animal foraging or wind gusts. A series or physical model tests were conducted to examine how root geometries, root properties, and soil effective stress stales affect the pullout capacity of simple unbranched model roots. Lengths of wood, rubber, and real roots were pulled from dry and partially saturated sand. The tests revealed the importance of the root to soil stiffness ratio during progressive failure, the mechanical properties of soil (and interfaces) at low effective stresses, the root diameter, and the tortuosity of the root material. Scaling issues due to shear banding are more important, and effective stresses under wet conditions are smaller than in conventional geotechnical practice because roots have a relatively small diameter (typically [10.sup.-4] [10.sup.-1] m) and are at shallow depth (typically 0-2 m). After careful consideration of these effects, predictions were made for the pullout capacity of small root sections based on material data for willow roots (Salix viminalis) in sand. Key words: interface shear, physical modelling, scale effects, root anchorage, root pullout. Les systemes de racines des plantes penetrent frequemment a travers des pentes de sol naturelles ou construites, ce qui influence la stabilite des pentes par renforcement mecanique el assechement du sol. Ces systemes de racines subissent souvent des forces externes durant les mouvements de sol et lorsque les tiges sont soumises au paturage des animaux et aux coups de vents. Une serie d'essais a l'aide d'un modele physique ont ete effectues afin d'examiner l'influence de la geometrie des racines, des proprietes des racines et des etats de contraintes effectives du sol sur la capacite d'arrachement d'un modele simple de racines sans branche. Des longueurs de bois, de caoutchouc et de vraies racines ont ete arrachees de sols composes de sable sec et partiellement sature. Les essais ont revele l'importance du ratio de rigidite racine/sol durant la rapture progressive, des proprietes mecaniques du sol (et de l'interface) pour des contraintes effectives faibles, du diametre des racines, et de la tortuosite des racines. Les effets d'echelle sont plus importants en raison des bandes de cisaillement, et les contraintes effectives en conditions humides sont plus faibles qu'en pratique conventionnelle geotechnique, puisque les racines ont un diametre relativement petit (typiquement entre [10.sup.-4] et [10.sup.-1] m) et sont peu profondes (typiquement de 0 a 2 m). En tenant compte de ces effets, des predictions ont ete etablies pour determiner la capacite d'arrachement de petites sections de racines basees sur des donnees de racines de saule (Salix viminalis) dans du sable. Mots-cles : cisaillement a l'interface, modelisation physique, effets d'echelle, ancrage des racines, arrachement des racines. [Traduit par la Redaction], Introduction Plant roots are natural soil anchors that have evolved to resist external stresses from wind loading and animal foraging and also act as inclusions that reinforce soil. There is [...]

Observations after earthquakes where surface fault ruptures crossed engineering facilities reveal that some structures survived the rupture almost unscathed. In some cases, the rupture path appears to divert, 'avoiding' the structure. Such observations point to an interaction between the propagating rupture, the soil, and the foundation. This paper (i) develops a two-step nonlinear finite-element methodology to study rupture propagation and its interaction with strip foundations; (ii) provides validation through successful Class 'A' predictions of centrifuge model tests; and (iii) conducts a parameter study on the interaction of strip foundations with normal fault ruptures. It is shown that a heavily loaded foundation can substantially divert the rupture path, which may avoid outcropping underneath the foundation. The latter undergoes rigid body rotation, often detaching from the soil. Its distress arises mainly from the ensuing loss of support that takes place either at the edges or around its center. The average pressure q on the foundation largely dictates the width of such unsupported spans. Increasing q decreases the unsupported width, reducing foundation distress. The role of q i s dual: (1) it compresses the soil, 'flattening' fault-induced surface 'anomalies'; and (2) it changes the stress field underneath the foundation, facilitating rupture diversion. However, even if the rupture is diverted, the foundation may undergo significant stressing, depending on its position relative to the fault outcrop. CE Database subject headings: Geological faults; Soil-structure interaction; Finite element method; Centrifuge models; Seismic effects; Foundations.

The three notorious earthquakes of 1999 in Turkey (Kocaeli and Duzce) and Taiwan (Chi-Chi), having offered numerous examples of surface fault rupturing underneath civil engineering structures, prompted increased interest in the subject. This paper develops a nonlinear finite-element methodology to study dip-slip ('normal' and 'reverse') fault rupture propagation through sand. The procedure is verified through successful Class A predictions of four centrifuge model tests. The validated methodology is then utilized in a parametric study of fault rupture propagation through sand. Emphasis is given to results of engineering significance, such as: (1) the location of fault outcropping; (2) the vertical displacement profile of the ground surface; and (3) the minimum fault offset at bedrock necessary for the rupture to reach the ground surface. The analysis shows that dip-slip faults refract at the soil-rock interface, initially increasing in dip. Normal faults may keep increasing their dip as they approach the ground surface, as a function of the peak friction angle [[phi].sub.p] and the angle of dilation [[psi].sub.p]. In contrast, reverse faults tend to decrease in dip, as they emerge on the ground surface. For small values of the base fault offset, h, relative to the soil thickness, H, a dip-slip rupture cannot propagate all the way to the surface. The h/H ratio required for outcropping is an increasing function of soil 'ductility.' Reverse faults require significantly higher h/H to outcrop, compared to normal faults. When the rupture outcrops, the height of the fault scrap, s, also depends on soil ductility. CE Database subject headings: Finite element methodology; Predictions; Centrifuge model; Earthquakes; Shear deformation; Scale effect.

The effect of pore fluid viscosity on the stiffness, damping, and liquefaction characteristics of sands was investigated to assess the potential contributions to a centrifuge model seismic response for soils saturated with high-viscosity pore fluid. Resonant column tests with cyclic loading frequencies in the range of 20-45 Hz were performed on a variety of fluids and sand sizes. At a strain level less than 2 x 10(super -4), the damping increased with pore fluid viscosity and shear strain amplitude, and it decreased with sand particle size. However, at shear strain greater than about 2 x 10(super -4), the increased skeleton damping tended to mask any effect of additional damping due to fluid viscosity. The liquefaction tests on fine silica sand revealed that the increase in total energy dissipation was not more than 10% for 100 cS oil when compared with water at a driving frequency of 25 Hz. Based on the experimental results, a simple model is proposed to examine the dependency of viscous damping on pore fluid viscosity, loading frequency, particle size, and shear strain amplitude.

A series of centrifuge model tests were conducted to investigate the contribution of root reinforcement to slope stability. A compacted sandy clay slope, inclined at 45°, was reinforced with model roots. The model roots were varied in material, architecture, and numbers. They had stiffness values corresponding to upper and lower values found for plant roots. The architecture included taproots and branched roots. Slope collapse was triggered by raising the water table while soil displacements, pore-water pressures, and root strains were measured. The mode of failure was changed by the presence of roots from a progressive block failure to translational failure. The tests revealed how axial strains and bending strains were mobilized in the roots and how the roots influenced the slope failure mechanism. Different limit equilibrium slope stability calculations were performed at slope failure conditions to quantify the amount of reinforcement provided by different root types. These measured root reinforcement contributions were compared with those predicted according to common root reinforcement models. A reinforcement calculation method allowing for root pull-out was found to give the best agreement. Key words: centrifuge modelling, slopes, reinforced soils, model roots. Une serie d'essais par centrifuge a ete realisee pour etudier la contribution du renforcement par les racines sur la stabilite de la pente. Une pente faite de sable argileux compacte, inclinee a 45°, a ete renforcee avec des racines modelisees. Les racines modelisees variaient en termes de materiaux, architecture et nombre. Elles avaient des valeurs de rigidite correspondant aux valeurs minimales et maximales pour des racines de plantes. L' architecture incluait des racines pivotantes et des racines ramifiees. L' effondrement de la pente etait declenche par le rehaussement de la nappe phreatique pendant que le sol est en deplacement; les pressions interstitielles et les deplacements des racines ont ete mesures. Le mode de rupture est modifie par la presence des racines d' une rupture progressive en bloc a une rupture en translation. Les essais ont revele comment les deformations axiales et en torsion sont mobilisees dans les racines, et comment les racines influencent le mecanisme de rupture de la pente. Differents calculs d' equilibre limite de la stabilite de la pente ont ete realises aux conditions de rupture de la pente afin d' evaluer la quantite de renforcement fournie par les differents types de racines. Ces contributions au renforcement par les racines ont ete comparees a celles predites par des modeles communs de renforcement par les racines. Une methode de calcul du renforcement permettant le retrait des racines a ete determinee comme etant celle offrant la meilleure concordance. Mots-cles : modelisation par centrifuge, pentes, sols renforces, racines modelisees. [Traduit par la Redaction], Introduction Vegetation affects soil slope stability in two ways: (i) through mechanical reinforcement by the roots and (ii) by modifying the hydraulic conditions. Neither of these effects is traditionally considered [...]

Grillage foundations are an alternative to solid surface mudmats for supporting seabed infrastructure, offering improved hydrodynamic performance and savings in foundation material. Recent research has demonstrated that grillages can be designed to have similar vertical bearing capacity to a mudmat with the same footprint. This is extended herein by: (a) determining grillage performance under horizontal loading at constant vertical load (V-H); (b) the application and development of existing plasticity-based models for predicting performance; (c) comparing the V-H behaviour with surface mudmats; and (d) discussing the implications for design. Experimental tests were conducted in sands over a range of densities and in two different modes, representing different installation procedures. In over-penetrated tests, the foundations were installed to achieve a vertical bearing capacity , followed by horizontal loading at a constant vertical load with < . In normally penetrated tests, foundations were installed to before horizontal loading at constant vertical load with = . Both normalised V-H yield surfaces and a plasticity-based simulation model are presented for use in design. Laboratory-scale grillages offer improved horizontal capacity in loose and medium-dense sands and similar horizontal capacity in very dense sand, compared with surface mudmats. [ABSTRACT FROM AUTHOR]

Presents a study that used finite element analysis to investigate the interaction between anchor flukes and undrained soil at failure. Behavior of a rectangular and a wedge-shaped anchor; Presentation of the results in terms of a plasticity framework to produce an associated plastic failure locus for combined loading conditions; Introduction of a design method that aims to predict drag anchor embedment trajectory and holding capacity through examination of the fluke-soil interaction.

Studies undrained soil deformation around piles displaced laterally through soil. Use of two-dimensional finite-element analysis; Passive lateral loading; Considerations of both the local and global soil behavior and the pile-group geometry and loading; Disparity between load-transfer curves.