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The volume of a submarine landslide is likely to be amplified by shear band propagation along a basal surface or spreading failure extension in the sliding layer. Although the mechanism of translational submarine landslides has been understood using the interpretation of shear band propagation, assessment of the limits of failure extension, either through shear band propagation or retrogressive spreading, remains a challenge due to the diversity of post failure mechanisms. The paper aims to explore all post failure possibilities and quantify failure initiation and extension accordingly, focusing on the upslope conditions. The different failure mechanisms are explored with the aid of the large deformation finite element modeling. Original criteria for scarp position, shear band length at slab failure, retrogression conditions and ultimate spreading limits are proposed, following the process zone approach considering force equilibrium of the sliding mass. Simplified analytical predictions are shown to agree well with the numerical observations. The findings and criteria presented in the study are expected to help assess the ultimate scale of upslope failure in submarine landslides, which is important in determining geohazard zonation of submarine landslides. Plain Language Summary: Submarine landslides are often huge compared to onshore counterparts, but why and how huge? Answering these questions will increase our understanding of submarine landslide evolution and help to assess risks of submarine landslide recurrence. For example, the Storegga Slide offshore Norway, which occurred ∼8,200 years ago, covered an area as large as UK and triggered tsunami waves seriously impacting nearby islands. A large submarine landslide might be initiated from localized failure within an embedded weak layer, followed by failure propagation along the weak layer and extensive slab failure in the overlying sliding layer. These two post‐failure phenomena may greatly increase the scale of submarine landslides. Here we simulate submarine landslides in different conditions and explore all possible upslope post‐failure mechanisms, using a large deformation numerical modeling method. Based on the observed mechanisms, we develop a new model and original criteria to predict the scarp position and failure extension limit of submarine landslides. Altogether, findings here are expected to help assess the ultimate scale of submarine landslides, which is important in determining geohazard zonation of submarine landslides. Key Points: We explore and quantify all upslope failure possibilities of submarine landslides through numerical modeling and theoretical analysesWe establish a framework for assessing failure initiation, covering different failure mechanismsWe propose simple predictions for scarp position and retrogression limit, which allows rapid assessment of scale of submarine landslides [ABSTRACT FROM AUTHOR]

Gigantic submarine landslides pose a significant threat to offshore infrastructures and may be initiated by an inconspicuous weakened zone, followed by catastrophic shear band propagation under the effect of strain softening. Much attention has been paid over the last two decades to unidirectional (parallel to the travel direction of sliding mass) shear band propagation (SBP) related exclusively to an initiation zone that is wide relative to the SBP direction. The paper investigates for the first time the mechanisms of more realistic planar shear band propagation via numerical modelling including finite element and finite difference analyses. Rather than propagating only along the travel direction for a 1D shear band in a plane-strain slope, the multidirectional propagation behaviour of a planar shear band depends significantly on the aspect ratio of the initiation zone. New criteria for catastrophic propagation of a planar shear band are proposed and are shown to be in good agreement with the numerical results. The criteria with respect to the initiation zone area can be incorporated easily within a GIS-based stability analysis of catastrophic submarine landslides, facilitating robust hazard analysis for offshore developments. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Pile tip damage has become of particular concern for the offshore wind industry due to the extensive use of large-diameter, thin-walled piles for monopiles and jacket structures. The potential for damage has increased in recent developments due to the occurrence of embedded boulders within the seabed sediments (e.g. glacial tills) or partially weathered soft rocks (e.g. limestones, chalk). To assess the risk of pile tip damage, this paper presents a simple schematic approach to estimate: i) whether pile tip damage might occur following impact with embedded boulders; and ii) what soil strength (e.g. as reflected in the cone tip resistance q c) might initiate progressive distortion of an initial minor dent (extrusion buckling). The methodology was developed based on results of centrifuge model tests and large displacement finite element analysis, encompassing: i) undamaged thin-walled piles installed through embedded layers of different sized boulders; and ii) initially 'pre-dented' piles driven through homogeneous sand triggering progressive distortion. Numerical analysis was used to develop predictive guidelines for conditions that might cause significant pile tip damage. The guidelines were validated by comparison with results from model tests and also through well documented field cases where extrusion buckling occurred. The paper provides guidance for choosing the critical parameters involved in the calculation processes to enable direct application in design, allowing assessment of whether more detailed investigation and analysis should be undertaken. [ABSTRACT FROM AUTHOR]

Large-deformation finite-element analysis has been used to model the undrained response of skirted shallow foundations in uplift and compression. Large-deformation effects involve changes in embedment ratio and operative local soil shear strength with increasing foundation displacement--either in tension or compression. Centrifuge model testing has shown that these changes in geometry affect the mobilized bearing capacity and the kinematic mechanisms governing failure in undrained uplift and compression. Small-strain finite-element analysis cannot by definition capture the effects of changing foundation embedment ratio and variation in local soil strength with foundation displacement. In this paper, load-displacement relationships, ultimate capacities, and kinematic mechanisms governing failure from large-deformation finite-element analyses are compared with centrifuge model test results for circular skirted foundations with a range of embedment between 10 and 50% of the foundation diameter. The results show that the large-deformation finite-element method can replicate the load-displacement response of the foundations over large displacements, pre- and postyield, and also capture differences in the soil deformation patterns in uplift and compression. The findings from this study increase confidence in using advanced numerical methods for determining shallow skirted foundation behavior, particularly for load paths involving uplift. [ABSTRACT FROM AUTHOR]

Estimation of soil behavior type from cone penetration testing, and the interpretation of dissipation tests, is complicated in intermediate soil types, such as silty sands, sandy silts, etc., where partial consolidation occurs during penetration. This issue is investigated in this paper using results from cavity expansion and finite element analyses as well as field and centrifuge piezocone data. The implications for soil classification are examined using analytical expressions to explore the effect of normalized shear strength, rigidity index, and overconsolidation ratio relative to the influence of partial consolidation and viscous effects under fully undrained conditions. It is shown that partial drainage conditions can affect where data plots on soil behavior charts, thus complicating soil classification. The effect on dissipation tests following partial consolidation during cone penetration is shown to create errors in interpretation using experimental and numerical data. A new approach is developed based in part on manipulation of solutions for pore pressure dissipation (following undrained penetration) to account for these errors when interpreting dissipation tests. Errors can become significant during standard cone penetration testing when the t50 dissipation time is less than about 50 s. Guidelines, including equations and a chart, are presented for practical use. Finally, implementation of this approach is demonstrated in a brief case study. [ABSTRACT FROM AUTHOR]

The loss in anchor embedment during keying, as it rotates to become normal to the cable load, reduces the uplift capacity of anchors in normally consolidated clay. The keying behavior of plate anchors has been studied previously by using centrifuge and field model tests. In this paper, a large deformation finite-element approach incorporating frequent mesh regeneration and allowing for evolution of the anchor-chain profile, was developed to simulate the keying process of rectangular and strip plate anchors. A parametric study was undertaken to quantify the loss in anchor embedment during keying in terms of the anchor geometry, soil properties, loading eccentricity, and inclination. The embedment loss decreased dramatically with increasing loading eccentricity and decreasing chain angle at the mudline to the horizontal. The loss in anchor embedment during keying increased as the local soil strength increased relative to the weight of the anchor, up to a limit determined by the eccentricity of loading. In contrast, the effect of the soil strength gradient was minimal. Findings of this study yield a recommendation that the most important factor in anchor keying, loading eccentricity, should not be less than half the anchor width for an effective design. [ABSTRACT FROM AUTHOR]

Abstract: Geo-hazard assessment of the potential damage to a pipeline caused by a submarine landslide requires a quantitative model to evaluate the impact forces on the pipeline. In contrast with typical geotechnical problems, the strain rate within the fast moving, flow-like submarine landslide is typically far higher, which will lead to enhancement of the soil strength and therefore result in larger impact forces. Generally, there are two possible predictive frameworks for strain-rate dependence: a fluid dynamics framework and a geotechnical framework. By comparison of common rheological models adopted in these two different approaches, a unified additive power-law model, a normalised form of the Herschel–Bulkley model from fluid mechanics, is explored in this paper. This model has been used in conjunction with a large deformation finite element approach to investigate the undrained limiting loads on a cylinder moving steadily through inertia-less soft rate-dependent material, in order to quantify the strain-rate effects. The flow mechanism and the effects of the shear-thinning index and Oldroyd number on the shear zones are explored. The calculated resistance factors are compared with the drag coefficients obtained from computational fluid dynamics analysis. The average rate of strain experienced by the soil flowing past the cylinder is estimated for a given flow velocity and an expression in the form of a conventional bearing capacity equation, but with shear strength linked directly to the normalised flow velocity, is proposed to predict the magnitude of the viscous force exerted by the debris flow. [Copyright &y& Elsevier]

Submarine landslides represent one of the most significant geohazards on the continental slope in respect of the risk they pose to infrastructure such as deep water pipelines. A numerical approach, based on the finite-element method but using remeshing, was established in this paper to simulate large flow deformation of debris from a landslide and to quantify the loads and displacements imposed on pipelines embedded in the seabed. A simple two-dimensional elastic perfectly plastic soil model with plane strain conditions was employed in this analysis. The pipeline was restrained by a set of springs so that the load on the pipeline built up to a stable value, representing the limiting load at which the debris flowed over the pipeline. A parametric study was undertaken by varying the pipeline embedment and the relative strengths of the debris and seabed. The analysis results show that the various combinations of soil strength and embedment depth lead to different debris-pipeline movement patterns and consequently lead to rather different magnitudes of the loads imposed on pipelines. The pipeline is subjected to the largest load (an equivalent pressure of 11.5 times debris strength) from the landslide when it rests on the weakest seabed. The pressure is proportional to the debris material strength but varies inversely with the seabed strength for partially embedded pipelines. For all strength combinations, there is a critical embedment depth beyond which the force on the pipeline reduces to a very small magnitude. [ABSTRACT FROM AUTHOR]

Three-dimensional large deformation finite-element (FE) analyses were performed to investigate plate anchor capacity during vertical pullout. The remeshing and interpolation technique with small strain approach was expanded from two-dimensional to three-dimensional conditions and coupled with the FE software, ABAQUS. A modified recovery of equilibrium in patches technique was developed to map stresses after each remeshing. Continuous pullout of rectangular plate anchors was simulated and the large deformation results for strip, circular, and rectangular anchors were compared with model test data, small strain FE results, and plastic limit solutions. Interface conditions of no breakaway (bonded) and immediate breakaway (no tension) were considered at the anchor base. The effects of anchor roughness, aspect ratio, soil properties, and soil overburden pressure were investigated. It was found that the anchor roughness had minimal effect on anchor performance. For square and circular deep anchors under immediate breakaway conditions, the maximum uplift capacity increased with soil elastic modulus, which suggests that lower bound limit analysis and small strain FE analysis may overestimate the capacity. The soil beneath the anchor base separates from the anchor at a certain embedment depth near the mudline, once tensile stresses were generated. The ratio of separation depth to anchor width was found to increase linearly with the ratio of soil undrained shear strength to the product of soil effective unit weight and anchor width and was independent of the initial anchor embedment depth. [ABSTRACT FROM AUTHOR]

The effect of foundation embedment on consolidation has not been considered previously in a systematic manner, although this is of particular interest for offshore foundations, where embedment is provided by skirts that enclose a compressible soil plug. For skirted foundations, critical uncertainties include what to assume in terms of the degree of drainage at skirt tip level, and the relative time scales of consolidation within the soil plug and beneath the foundation. In this paper, results from small strain finite-element analyses are used to quantify the immediate and time-dependent response of circular skirted foundations to uniaxial vertical loading. Foundations with frictionless and fully rough skirt-soil interfaces with varying ratio of embedment depth to foundation diameter are considered and the responses compared with those for surface foundations. The findings show that both skirt-soil interface roughness and embedment ratio have a significant effect on the consolidation response. [ABSTRACT FROM AUTHOR]

Vertically installed plate anchors have been investigated in this paper by numerical analysis and centrifuge modeling. In the numerical analysis, the large deformation finite-element method (remeshing and interpolation technique with small strain) was used to simulate strip plate anchor rotation. In the centrifuge model tests, transparent soils were used to observe square anchor rotation. The loss in anchor embedment during anchor keying was assessed for anchors in uniform and normally consolidated soils with anchor pullout angle varying from 30° to 90° to the horizontal. It was found that the loss in anchor embedment during anchor keying may be expressed in terms of a nondimensional anchor geometry factor, which is a function of the eccentricity of the padeye, angle of loading, and the net moment applied to the anchor at the stage where the applied load balances the anchor weight. However, once the anchor geometry factor reaches a certain value, the loss in anchor embedment stabilizes at 0.25–0.5 times the anchor width. The loss in anchor embedment decreases linearly with decreasing pullout angle. Simple formulae and design procedures have been proposed to estimate the loss in anchor embedment during keying. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In practice, natural soils exhibit strain-rate dependency and also soften as they are sheared and remolded. Traditional design approaches, however, have been developed based on idealized rate-independent perfectly plastic soil. This paper presents results for deep penetration of spudcan foundations in strain-softening, rate-dependent, soft clays quantifying the effects relative to results for ideal soil. The analysis was carried out using a large deformation finite-element (LDFE) approach, modifying the simple elastic-perfectly plastic Tresca soil model to allow strain softening, and incorporate strain-rate dependency of the shear strength. Parametric analyses were undertaken varying the strain-rate parameter, the sensitivity and ductility of the soil, the normalized penetration rate, and the soil strength nonhomogeneity. Overall, penetration resistance for rate-dependent, strain-softening clays lay below that for ideal soil. Increased brittleness of the soil led to marked reduction in penetration resistance, only partly compensated by strain-rate dependency. Key results have been presented in the form of design charts, fitted by simple expressions to estimate the limiting cavity depth above the advancing spudcan and the limiting bearing capacity factor at depth. [ABSTRACT FROM AUTHOR]

This paper presents a numerical investigation of the effect of foundation size on the response of shallow circular foundations on siliceous and calcareous sands. The study is based on the predictive capabilities of the MIT-S1 soil model for simulating both the compression and shear behaviors of natural sands over a wide range of densities, K0 values and confining pressures. The paper highlights the variations in the deformation mechanisms for the siliceous and calcareous sands cases. The assessment of the bearing capacity factor, Nγ, is examined, showing a dramatic decrease in the values with increasing foundation size for the case of footings on calcareous sands, eventually converging to a terminal Nγ value. At this stage the sand resistance is insensitive to variations in initial density and foundation size because the sand tends to loose its initial characteristics due to grain crushing, leading the material rapidly toward ultimate conditions. In the silicious sand case, it is found that, eventually, for extremely large footing diameters, the deformation mechanism progresses toward a punching shear mechanism, rather than the classical rapture pattern accompanied by surface heave as employed in current bearing capacity equations. A dimensional transition between the failure mechanisms can clearly be defined, referred to as a “critical size” in the Nγ–D relationship. [ABSTRACT FROM AUTHOR]

The behavior of strip and circular plate anchors during vertical pullout in uniform and normally consolidated clays was studied in this paper by means of small strain and large deformation finite-element analyses. Both fully bonded (attached), and “vented” (no suction on rear face), anchors were considered. The current numerical results were compared with existing laboratory test data, finite-element results, and analytical solutions. This study showed that, in small strain analysis, the scatter of existing data was mainly due to the effect of soil stiffness. In large deformation analysis, when soil and anchor base were attached with suction, the pullout capacity factor formed a unique curve independent of the soil strength (su), soil effective unit weight (γ′) and anchor size (B=width of strip anchor and D=diameter of circular anchor). The transitional embedment depth ratio, HSD/B or HSD/D, (where HSD=transition depth between shallow and deep embedment) was 1.4 for a strip anchor and 0.75 for a circular anchor. The ultimate pullout capacity factors (Nc) for deep embedment were 11.6 and 11.7 for smooth and rough strip anchors and 13.1 and 13.7 for smooth and rough circular anchors, respectively. However, when the anchor base was vented, the soil stayed attached to the anchor base for deep embedment, and the pullout capacity was therefore the same as for the attached anchor. The separation depth ratio, Hs/B or Hs/D, (where Hs=embedment depth at which the soil and anchor base separated) was found to increase linearly with the normalized strength ratio, su/γ′B or su/γ′D. [ABSTRACT FROM AUTHOR]

This paper addresses numerical simulation of deep penetration of full-flow penetrometers in strain-softening, rate-dependent, cohesive soil, and the observed phenomenon of periodic shear bands. The analysis was conducted using a large deformation finite element approach, modifying the simple elastic–perfectly plastic Tresca soil model to allow strain-softening, with strain-rate dependency being incorporated in order to avoid spurious mesh dependency. Parametric analyses were carried out varying the strain-softening parameters (hence the relative brittleness of the soil), the rigidity index of the soil, and the strain-rate parameter. Increased brittleness of the soil led to reduction in the penetration resistance, but also to increasingly significant oscillations in the resistance–penetration responses. The oscillation was found to result from periodic shear bands evolving cyclically ahead of the advancing cylindrical and spherical penetrometers. Analyses with different values of rigidity index confirmed further that the periodic shear bands were a real material phenomenon, rather than due to errors in numerical simulation. Similar phenomena have been observed for continuous flow problems in granular materials. However, rising strain-rate dependency tended to suppress the oscillations. [ABSTRACT FROM AUTHOR]

The penetration resistance of a cylindrical T-bar penetrometer in soft clay is affected by features such as anisotropy, high strain rates, and gradual strain-softening during passage of the T-bar. In order to evaluate these effects, a detailed numerical study has been undertaken, comprising: (1) finite-element analysis; and (2) a strain path approach within the upper bound plasticity mechanism. These studies showed that the T-bar factor is relatively insensitive to the degree of strength anisotropy, provided the penetration resistance is normalized by the average shear strength. Strain rates were found to be six or seven orders of magnitude greater than typical laboratory testing rates, and about three orders of magnitude higher than in a standard vane test. However, the effect of high strain rates is partly compensated by remolding of the soil, where average strains of 400% are imposed on the soil. Charts are presented showing how the separate effects of high strain rates and partial softening may be combined to derive a T-bar factor for a given soil. The paper concludes with a discussion of the measurement of remolded shear strength using cyclic T-bar tests, and interpretation of the T-bar resistance in fully remolded soil. [ABSTRACT FROM AUTHOR]

The uniaxial vertical bearing capacity of square and rectangular footings resting on homogeneous undrained clay is investigated with finite element analyses, using both Tresca and von Mises soil models. Results are compared with predictions from conventional bearing capacity theory and available analytical and numerical solutions. By calibrating the finite element results against known exact solutions, best estimates of bearing capacity for rough-based rectangular footings are derived, with the shape factor fitted by a simple quadratic function of the footing aspect ratio. For a square footing, the bearing capacity is approximately 5% lower than that based on Skempton’s shape factor of 1.2. [ABSTRACT FROM AUTHOR]

The piled raft is a geotechnical composite construction, consisting of the three elements piles, raft, and soil, which is applied for the foundation of tall buildings in an increasing number. In a parametric study, 259 different piled raft configurations have been analyzed by means of three-dimensional elastoplastic finite element analyses. In the study, the pile positions, the pile number, the pile length, and the raft-soil stiffness ratio as well as the load distribution on the raft has been varied. In the scope of this paper, the results of the parametric study are presented and design strategies for an optimized design of piled rafts subjected to nonuniform vertical loading are discussed. [ABSTRACT FROM AUTHOR]

New analytical models have recently been presented for evaluating the peak bearing capacity of circular footings penetrating sand-over-clay and the subsequent capacity once embedded in the underlying clay. However, in lieu of any analytical model, current methods of analysis use an inaccurate linear interpolation for the bearing capacity between the peak and the clay surface. This paper proposes an analytical model for calculating the bearing capacity of circular footings after the peak failure in sand-over-clay soils and before the footing enters the clay. Based on observations from visualisation experiments conducted in a geotechnical centrifuge, and consistent with the silo analysis approach used in predicting the peak capacity, the failure mechanism is a vertical frustum of sand progressively being forced into the underlying clay. Analysis of the capability of the new model to retrospectively simulate a database of 66 geotechnical centrifuge tests is provided. Motivation for improved prediction methods comes from the offshore mobile jack-up industry where concerns remain over the punch-through of large 20 m diameter spudcan footings in sand-over-clay. Accurate prediction of not just the peak bearing capacity but the subsequent load-displacement behaviour is required in order to assess the effectiveness of operational methods used to mitigate punch-through incidents. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The material point method (MPM), which is a combination of the finite element and meshfree methods, suffers from significant computational workload due to the fine mesh that is required in spite of its advantages in simulating large deformations. This paper presents a parallel computing strategy for the MPM on the graphics processing unit (GPU) to boost the method’s computational efficiency. The interaction between a structural element and soil is investigated to validate the applicability of the parallelisation strategy. Two techniques are developed to parallelise the interpolation from soil particles to nodes to avoid a data race; the technique that is based on workload parallelisation across threads over the nodes has a higher computational efficiency. Benchmark problems of surface footing penetration and a submarine landslide are analysed to quantify the speedup of GPU parallel computing over sequential simulations on the central processing unit. The maximum speedup with the GPU used is ∼30 for single-precision calculations and decreases to ∼20 for double-precision calculations. [ABSTRACT FROM AUTHOR]

Abstract: Large deformation finite element (LDFE) analysis is being applied increasingly in geomechanics as it allows numerical interpretation of problems in which the structural element moves a relatively long distance through the soil. The ‘remeshing and interpolation technique with small strain’ (RITSS) method for LDFE analysis, in which the soil domain is periodically remeshed with the stress and material properties interpolated from the old to the new within the standard Lagrangian finite element framework, has been successfully applied to a number of practical applications. It allows any standard finite element theory to be used in the Lagrangian analysis, and because the mesh topography and connectivity are not influenced by the previous deforming increment, large deformations are possible. The major barrier of the RITSS method is that the remeshing and interpolation requires specialised and user-dependent computer code. This has limited its application to specialists and hindered its routine application in engineering practice. This paper proposes a simpler, more practical method to implement RITSS for geotechnical applications. By utilising the ABAQUS in-built procedures for interpolation and remeshing, it avoids any need for user-defined code (although a piece of Python script can be used to automate the iteration instead of operating the ABAQUS user interface). A series of four example problems benchmarking this new approach show it to be robust and numerically accurate. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Monopiles supporting offshore wind turbines are generally of relatively low slenderness ratio and are subjected to lateral loads that emanate from high above the seabed at an eccentricity that in most cases exceeds the embedded length of the pile. The rotational stiffness is a critical aspect of their design, owing to tight tolerances on the cumulative rotation that is permitted by industry guidelines. From a practical perspective, most design calculations for the lateral response are performed using load transfer analysis. However, the low slenderness ratio and high moment loading relative to the horizontal load have prompted closer scrutiny of the load transfer formulations that have been used historically in the offshore oil and gas industry, since these were calibrated from the response of much longer piles under very different loading conditions. Recent industry-led studies have led to new recommendations, including the addition of lateral and rotational springs at the pile base and rotational load transfer springs distributed down the pile shaft. As a first step to evaluate the initial stiffness of such springs, this paper presents results from three-dimensional finite element analyses of a homogenous elastic soil, with the aim of relating values of spring stiffness to the elastic shear modulus of the soil. Effects of non-uniform soil stiffness are discussed and the gradually increasing benefits of including the additional spring terms are quantified. [ABSTRACT FROM AUTHOR]

Lateral buckling analysis of on-bottom submarine pipelines is of particular interest in the offshore industry due to the complexities involved in the analysis, and the potential design efficiencies that can be unlocked. Classical buckling theories by previous researchers and recent joint industry projects provide a basis for estimation of the critical buckling load of a straight, or in some cases imperfect, pipe on either a rigid or elastic seabed. However, systematic solutions for the combined effects of nonlinear soil properties and the as-laid geometry – specifically the out-of-straightness – on the buckle initiation behaviour have not been developed previously. This paper reports an investigation of the buckling problem of an imperfect (non-straight) on-bottom pipeline subjected to axial compressive loading. The seabed was modelled with lateral and axial nonlinear, springs to idealise the load-displacement behaviour of the soil and the pipe was modelled with pipe elements. Buckling was performed by a displacement controlled finite element method with the modified Riks algorithm that is available in the commercial software ABAQUS. This numerical tool was used to develop a parametric solution for the present problem in terms of the various pipe material and geometry parameters and the lateral and axial pipe-soil interaction parameters. In particular, the influence of the magnitude and stiffness of the lateral pipe-soil response was investigated, highlighting the sensitivity of the pipeline response to the geotechnical inputs. The results have been synthesised in a generic non-dimensionalised design chart to estimate the buckling load, valid for the range of inputs covered by the parametric study. [ABSTRACT FROM AUTHOR]

Full-flow penetrometer testing in fine-grained soils indicates that undrained shear strength decreases due to remoulding induced generation of excess pore pressures and increases due to strain rate effects and dissipation of excess pore pressures. This paper presents a viscoplastic strain-softening–hardening constitutive model that captures this duality of behaviour via non-local regularisation, strain rate dependency, and consolidation induced recovery of sensitivity. The model is based on Structured Modified Cam Clay and Perzyna's overstress framework. Validation of the implementation is first demonstrated through simulation of constant rate of strain triaxial and oedometer compression tests performed on different clays. The model is then applied in large deformation finite element analyses of monotonic and variable rate T-bar penetration to demonstrate successful numerical implementation and its ability to simulate strain rate effects. Finally, the model is applied in the simulation of undrained cycles of penetration of a T-bar penetrometer in kaolin clay and a carbonate silt and compared with data from geotechnical centrifuge experiments. The results show that the model without sensitivity recovery significantly underestimates the consolidated-undrained resistance in both soils, suggesting that recoverable sensitivity is an aspect of behaviour that ought to be considered in the development of constitutive models for soft sensitive soils. [Display omitted] [ABSTRACT FROM AUTHOR]

Geotechnical applications often involve large displacements of structural elements, such as penetrometers or footings, in soil. Three numerical analysis approaches capable of accounting for large deformations are investigated here: the implicit remeshing and interpolation technique by small strain (RITSS), an efficient Arbitrary Lagrangian–Eulerian (EALE) implicit method and the Coupled Eulerian–Lagrangian (CEL) approach available as part of commercial software. The theoretical basis and implementation of the methods are discussed before their relative performance is evaluated through four benchmark cases covering static, dynamic and coupled problems in geotechnical engineering. Available established analytical and numerical results are also provided for comparison purpose. The advantages and limitation of the different approaches are highlighted. The RITSS and EALE predict comparable results in all cases, demonstrating the robustness of both in-house codes. Employing implicit integration scheme, RITSS and EALE have stable convergence although their computational efficiency may be low for high-speed problems. The CEL is commercially available, but user expertise on element size, critical step time and critical velocity for quasi-static analysis is required. Additionally, mesh-independency is not satisfactorily achieved in the CEL analysis for the dynamic case. [ABSTRACT FROM AUTHOR]

Abstract: Spudcan installation in sand overlying clay carries a risk of potential punch-through failure. This paper investigates the penetration resistance of spudcans on loose sand overlying clay soils using large deformation finite element analysis. Comparison with existing experimental results shows that the Mohr–Coulomb yield criterion with unchanging shear strength parameters simulates well the behaviour of loose sand, for which strain softening is not significant. The paper quantifies the effects on the punch-through behaviour of the spudcan of the main input parameters, including the undrained shear strength of the underlying clay, and the thickness, friction angle and stiffness of the upper sand. The current design guideline, SNAME , significantly underestimates the peak bearing capacity of the spudcan in comparison with the numerical results. [Copyright &y& Elsevier]

[Display omitted] Large lateral displacement of trenched subsea pipelines may be induced by ground movement, landslides, ice gouging, etc. Pre-excavated seabed soil is usually used for backfilling as a cost-effective solution. The large difference between the shear strength of the highly remoulded backfill material and the native soil may significantly affect the pipeline-backfill-trench interaction, the failure mechanisms, and consequently the lateral soil resistance. However, this challenging and less-explored aspect has not been covered thoroughly in design codes. In this paper, the influences of pipeline-backfill-trench interaction on the failure mechanism and resultant lateral soil resistance were investigated by large deformation finite element analyses. Two different methods were considered and compared, including the remeshing and interpolation technique with small strain (RITSS) and coupled Eulerian-Lagrangian (CEL) methods, to assess their relative merits for this problem. A modified Tresca model considering strain-softening effects was incorporated and a parametric study was conducted to investigate the influences of key factors including the trench geometry, stiffness of backfill material, native seabed soil properties, burial depth, and intensity of pipeline-trench bed interaction. The study showed that the ignorance of the pipeline-backfill-trench interaction by using uniform soil may result in underestimation and overestimation of the lateral soil resistance against the pipeline moving inside the backfill and into the trench wall, respectively. [ABSTRACT FROM AUTHOR]

[Display omitted] This paper presents a methodology to simulate the strain-softening-hardening response of sensitive clays when subjected to extensive remoulding. A non-local strain-softening technique is implemented into the Abaqus Finite Element Analysis (FEA) software through its user defined material subroutine UMAT, in the form of a critical state based strain-softening constitutive model. For the comparison of conventional and non-local formulations, a series of analyses have been performed investigating mesh dependency issues and the validity of a simple softening-scaling rule for practical applications. The non-local model significantly reduces solution mesh-dependency for small-strain analyses of biaxial shear and updated Lagrangian analyses of buried pipe uplift. Finally, the non-local model is applied to simulate undrained cycles of penetration and the consolidated-undrained capacity of a T-bar in kaolin clay. This analysis highlights the limitations of the use of standard element tests (e.g. triaxial tests) to calibrate the model input parameters that control the strain-softening aspect of the constitutive law, and the need for a constitutive law that captures the partial recovery of sensitivity during consolidation. [ABSTRACT FROM AUTHOR]