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Failure envelope formulations are typically employed to assess the ultimate capacity of foundations under combined loading and for incorporation in macro-element models. However, the complex interaction between each load component, especially for six degree of freedom (6DoF) loading, means that determining satisfactory formulations is often a complex process. Previous researchers have identified this difficulty as an obstacle to the adoption of the failure envelope approach in foundation engineering applications. To address this issue, the paper describes a systematic procedure for deriving globally convex failure envelope formulations; the procedure is applied to a circular surface foundation, bearing on undrained clay, in 6DoF load space. The formulations are shown to closely represent the failure load combinations determined from finite element analyses for a variety of loading conditions, including non-planar horizontal-moment loading. An example macro-element model based on the proposed formulation is described; the macro-element model provides a close representation of the foundation behaviour determined from a separate finite element analysis. A key aspect of the paper is that it demonstrates an automated process to determine well-behaved failure envelope formulations. The automated nature of the process has considerable advantages over the manual procedures that have previously been employed to determine failure envelope formulations.

Content uploaded by Stephen Suryasentana

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All content in this area was uploaded by Stephen Suryasentana on Jul 01, 2021

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... The appropriate η for any soil strength heterogeneity ratio and relative scour depth can be selected from X. Guo et al. Applied Ocean Research 118 (2022) 103007 It should be noted that in practice, the combined action of wind and waves may result in non-planar horizontal-moment loading, and a fully 6 degrees of freedom (6DoF) failure envelope is more accurate for these non-planar loading situations Suryasentana et al., 2021). But due to expensive computing costs required for the 6DoF failure envelope, the 3DoF failure envelope (i.e., F V − F H − M failure envelope) as a simplified idealization was considered in majority of the literature (e.g., Bransby and Randolph, 1998;Vulpe, 2015;Zhao et al., 2020;Suryasentana et al., 2020aSuryasentana et al., , 2020b and appropriate for situations when horizontal-moment loading is approximately planar. ...

Local scour may induce bearing capacity loss of the foundation significantly by removing soil around the foundation and it has become one of the pivotal factors causing the failure of structures in offshore engineering. Therefore, it is indispensable to investigate the effect of local scour on bearing capacity behaviour of foundations. This paper focuses on the effects of dimensions of local scour hole on both ultimate bearing capacities and failure envelopes of the monopile and the caisson in undrained soil with different soil strength heterogeneity ratios using three-dimensional finite element method. The results show that the scour depth has a more significant effect on the ultimate bearing capacity than the scour hole angle. Meanwhile, the scour depth has a more significant effect on the lateral bearing capacity (i.e., horizontal bearing capacity and moment) than on the vertical bearing capacity. For given dimensions of local scour hole, the effect of local scour on the bearing capacity decreases as soil strength heterogeneity ratio increases. Moreover, local scour affects not only the size of the failure envelopes of the monopile and the caisson, but also their shape. Based on the above findings, a procedure of designing the monopile and the caisson to consider local scour effect is proposed, which could be helpful to the design of the foundations in practical engineering.

In this study, a PFC2D model based on the XRD component distribution of the granite specimen was built by the discrete element method, and the model parameters were calibrated with reference to uniaxial compression tests. Subsequently, numerical simulations of the granite heat conduction, the evolution of thermal stress and thermal cracking characteristics, and the compressive failure of the heated specimen were completed with reference to the specimen heating process and the uniaxial compression test of the heated specimen. The simulation results show that the micro-parameters in the discrete model calibrated with k and b values could reflect the specimen deformation characteristic in the mechanical test. During heat conduction below 300 °C, the excursions of the stress tensor were insignificant, the abrupt changes of the stress ratio were little, the thermal cracks were less, and the uniaxial failure pattern of the heated specimen was similar to that at room temperature. Above 300 °C, the excursion of the stress tensor presented an apparent elliptical shape, with significant abrupt values of the stress ratio, and a greater amount of thermal cracking. The uniaxial failure pattern of the heated specimen exhibited clear end and edge failure.

This paper presents a theoretical framework termed the convex modular modelling (CMM) framework, which provides a convenient and expedient approach for constructing thermodynamically consistent constitutive models. This paper demonstrates how the CMM framework can be used to build increasingly complex constitutive models by mixing and matching re-usable components from a library of convex base functions in a systematic manner. It also describes the use of the modified LogSumExp (MLSE) function as a general and smooth approximation to the pointwise maximum function for any yield function (e.g. the Mohr-Coulomb/Tresca yield function). The MLSE function is then used to develop several new yield functions such as a convex and smooth approximation of the Matsuoka-Nakai yield function, a generalised polygonal yield function and a ‘Reuleaux triangle’-shaped yield function. As CMM is simple to use, it potentially offers a more accessible path for constitutive modellers to take advantage of the hyperplasticity framework to develop robust constitutive models.

Suction caisson foundations provide options for new foundation systems for offshore structures, particularly for wind turbine applications. During the foundation design process, it is necessary to make reliable predictions of the stiffness of the foundation, since this has an important influence on the dynamic performance of the overall support structure. The dynamic characteristics of the structure, in turn, influence its fatigue life. This paper describes a thermodynamically-consistent Winkler model, called OxCaisson, that delivers computationally efficient estimates of foundation stiffness for caissons installed in homogeneous and non-homogeneous linear elastic soil, for general six degrees-of-freedom loading. OxCaisson is capable of delivering stiffness predictions that are comparable to those computed with three-dimensional finite element analysis, but at a much lower computational cost. Therefore, the proposed model is suited to design applications where both speed and accuracy are essential, such as large-scale fatigue assessments of offshore wind farm structures. The paper demonstrates that the OxCaisson model can also be applied to short rigid monopile foundations.

The failure envelope approach is widely used to assess the ultimate capacity of shallow foundations for combined loading, and to develop foundation macro-element models. Failure envelopes are typically determined by fitting appropriate functions to a set of discrete failure load data, determined either experimentally or numerically. However, current procedures to formulate failure envelopes tend to be ad hoc, and the resulting failure envelopes may not have the desirable features of being convex and well-behaved for the entire domain of interest. This paper describes a new systematic framework to determine failure envelopes - based on the use of sum of squares convex polynomials - that are guaranteed to be convex and well-behaved. The framework is demonstrated by applying it to three data sets for failure load combinations (vertical load, horizontal load and moment) for shallow foundations on clay. An example foundation macro-element model based on the proposed framework is also described.

The failure envelope approach is commonly used to assess the capacity of shallow foundations under combined loading, but there is limited published work that compares the performance of various numerical procedures for determining failure envelopes. This paper addresses this issue by carrying out a detailed numerical study to evaluate the accuracy, computational efficiency and resolution of these numerical procedures. The procedures evaluated are the displacement probe test, the load probe test, the swipe test (referred to in this paper as the single swipe test) and a less widely used procedure called the sequential swipe test. Each procedure is used to determine failure envelopes for a circular surface foundation and a circular suction caisson foundation under planar vertical, horizontal and moment (VHM) loading for a linear elastic, perfectly plastic von Mises soil. The calculations use conventional, incremental-iterative finite element analysis (FEA) except for the load probe tests, which are performed using finite element limit analysis (FELA). The results demonstrate that the procedures are similarly accurate, except for the single swipe test, which gives a load path that underpredicts the failure envelope in many of the examples considered. For determining a complete VHM failure envelope, the FEA-based sequential swipe test is shown to be more efficient and provide better resolution than the displacement probe test, while the FELA-based load probe test is found to offer a good balance of efficiency and accuracy.

A generalised framework is presented for predicting the consolidated undrained capacity of rectangular mat foundations on normally consolidated soft clay under combined loading in six degrees of freedom as a function of relative preload and degree of consolidation. Consolidated undrained response is investigated by coupled small-strain finite-element analysis using the modified Cam Clay plasticity constitutive model. Increases in the load-carrying capacity of a foundation under combined loading in six degrees of freedom following vertical preload with subsequent consolidation are demonstrated and quantified. The results are presented as failure envelopes in multi-directional load space and are shown to expand proportionally as a function of degree of consolidation for a given relative preload. A methodology and a set of expressions are provided to predict the shape and size of failure envelopes for rectangular mat foundations for any degree of preloading and consolidation.

Submarine sediments in many deep-water regions exhibit a thin crust overlying geologically normally consolidated clay. Load-carrying capacity of mudmat foundations for supporting subsea infrastructure installed on seabeds with a surficial crust is of great interest to foundation designers. Finite-element analyses have been performed to investigate the undrained response of mudmats under combined six degree-of-freedom loading in terms of the effect of crust thickness, foundation embedment and relative shear strengths of the underlying soft clay and crust. Results are presented as failure envelopes and expressions are presented to enable calculation of the uniaxial and combined load capacities under fully three-dimensional loading.

Yield and plastic potential surfaces are often affected by problems related to con-vexity. One such problem is encountered when the yield surface that bounds the elastic domain is itself convex; however, convexity is lost when the surface expands to pass through stress points outside the current elastic domain. In this paper, a technique is proposed, which effectively corrects this problem by providing linear homothetic expansion with respect to the centre of the yield surface. A very compact implicit integration scheme is also presented, which is of general applicability for isotropic constitutive models, provided that their yield and plastic potential functions are based on a separate mathematical definition of the meridional and deviatoric sections and that stress invariants are adopted as mechanical quantities. The elastic predictor-plastic corrector algorithm is based on the solution of a system of 2 equations in 2 unknowns only. This further reduces to a single equation and unknown in the case of yield and plastic potential surfaces with a linear meridional section. The effectiveness of the proposed convexification technique and the efficiency and stability of the integration scheme are investigated by running numerical analyses of a notoriously demanding boundary value problem.

This paper presents a new yield function, defined in terms of stress invariants and suitable for isotropic geomaterials. It is a generalization of that of the Modified Cam-Clay model and as such it retains all the mathematical advantages of the original formulation which are particularly convenient for the numerical integration of the constitutive law. In addition the proposed function is capable of providing a wide range of shapes and it is therefore suitable for defining both the yield and the plastic potential surfaces. As compared to the original MCC ellipse, one additional parameter is introduced for defining the shape of the meridional section, which conveniently controls also the relative position of the Normal Compression and Critical State lines. In the deviatoric plane the function not only provides the exact shape of classical failure criteria, such as von Mises, Drucker-Prager, Matsuoka-Nakai, Lade-Duncan, Tresca and Mohr-Coulomb, but it is also capable of rounding the hexagons of the last two criteria with a continuity of class at least C2 required for achieving a quadratic convergence of the integration scheme. The new function has an unrestricted domain of definition, expands/shrinks homothetically with respect both to the origin of the stress space and to its centre and is characterized by convexity for any level set. The last two important features were obtained by applying the convexification technique proposed by Panteghini and Lagioia (2017).

Circular foundations are widely employed in offshore engineering to support facilities and are generally subjected to fully three-dimensional loading due to the harsh offshore environmental load and complex operational loads. The undrained capacity of surface circular foundations on soil with varying strength profiles and under fully three-dimensional loading is investigated and presented in the form of failure envelopes that obtained from finite element analyses. The combined ultimate limit state of circular foundations is defined as the two-dimensional failure envelopes in resultant H-M loading space accounting for the vertical load and torsion mobilisation. The effects of vertical load and torsion mobilisation, soil shear strength heterogeneity and loading angle from moment to horizontal load on the shape of normalised H-M failure envelopes are explored. A series of expressions are proposed to describe the shape of failure envelopes obtained numerically, enabling essentially instantaneous generation of failure envelopes and optimisation of a circular foundation design based on constraint of any input variable through implementation in an automated calculation tool. An example application is ultimately provided to illustrate how the proposed expressions may be used in practice.

This paper demonstrates that the torsional-effect of horizontal loads on a subsea structure can represent a governing load-case for the foundation. It is intended to alert potential designers to this aspect as there is currently not much specific guidance in the public domain. Codes of practice for foundation design of offshore platforms are routinely used for the design of subsea structures, and the design is often performed by engineers whose technical-speciality is not necessarily geotechnics. The design can be performed strictly according to such a code but the underlying intention of the code can be overlooked; specifically, that that stability for all credible load-cases be considered in order to achieve a design that is fit-for-purpose. The importance of torque loading is initially demonstrated through examples of simple shallow foundation limit-equilibrium, and expanded with discussion of more realistic cases. It is shown that the sliding capacity could quite conceivably be about half that which could be determined using a traditional code approach, albeit inappropriately. The influence of torque on piled foundations is also discussed, with particular reference to well-head protection frames that are supported on the well's grouted conductor. Relationships between axial and torsional capacity are explained and it is shown that latter is limited by the number of high-torque connectors. It is also shown that it may be inappropriate to directly apply conventional t-z design methods to determine the distribution of torque. Copyright © 2004 by The International Society of Offshore and Polar Engineers.