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Wind turbines are typically designed based on fatigue and serviceability limit states, but still require an accurate assessment of bearing capacity. Overconsolidated clay deposits in Canada often have a thin layer of crust with a relatively high undrained shear strength developed from weathering, desiccation, and geochemical processes. However, existing design methods only assess the bearing capacity using effective area and inclination factor without consideration of surficial crusts. This paper studies the undrained VHMT (vertical, horizontal, moment, and torsional) failure envelope of circular foundations founded on a surficial crust underlain by a uniform soil with a zero-tension interface condition using finite element analysis. An analytical expression for the VHMT failure envelope is derived. [ABSTRACT FROM AUTHOR]

Suction caissons supporting offshore wind turbines are exposed to great horizontal loading above the soil surface, which may lead to overturning failure of the caisson. This paper presents a modified three-dimensional failure mechanism to analyze the anti-overturning bearing capacity of suction caissons. The modified failure mechanism is composed of a meniscus-conical wedge with a meniscus shape at the soil surface and a scooped shape. The analytical solution to the anti-overturning bearing capacity of the suction caisson is deduced in terms of the limit equilibrium method, followed by a parametric study of the wedge depth ratio (c) to optimize the critical failure mechanism that satisfies both the force and moment equilibriums. Thus, the methodologies are relatively easy to implement in traditional spreadsheets, and the analyses tend to perform very quickly. The effects of gap formation at the rear side of the caisson, loading eccentricity, and adhesion factor at the caisson–soil interface on the anti-overturning bearing capacity are also investigated. Compared with the finite element limit analysis results, experimental data, and existing theoretical solutions, it is proved that the presented limit equilibrium analysis can satisfactorily predict the anti-overturning bearing capacity of suction caissons with low aspect ratios for offshore wind turbines in uniform and linearly increasing strength clays. [ABSTRACT FROM AUTHOR]

This paper describes a program of centrifuge model tests on the uplift behaviour of suction caisson foundations. The parameters considered were the loading rate, caisson diameter (D), soil strength profile, and type of footing (i.e., mono-caisson and tetra-caissons group). The loading responses were examined in terms of total uplift resistance, suction beneath the caisson lid, and the vertical displacements of the caisson and at the soil surface. There exists a critical uplift displacement, approximately 0.02D and 0.01D for the mono-caisson and the tetra-caissons groups, respectively, at which a turning point can be identified in the load–displacement curve. This was found to be attributed to the adhesion on the caisson–soil interface reaching a peak response and then dropping. Of interest is that the tetra-caissons group exhibits much greater normalized uplift resistance than the mono-caisson group before reaching an uplift displacement of about 0.02D, suggesting superiority of the former in term of serviceability. However, a reversed trend was observed at greater displacement, and accordingly an empirical model was derived to quantify the shadowing effect of caisson groups. Regarding the cyclic response, several cycles of large-amplitude loading are sufficient to reduce the ultimate bearing capacity of caisson(s) to below the self-weight of the inner soil plug(s), indicating a transition of failure mechanism. [ABSTRACT FROM AUTHOR]

This paper presents an efficient, practical, and automated strategy for deriving modified Cam clay parameters from undrained self-boring pressuremeter (SBPM) data. A mixed approach involving a parametric sweep and numerical optimization is used, with a focus on parameter groups that dominate the modified Cam clay response in undrained cavity expansion. The proposed technique is illustrated using data from SBPM tests carried out in soft estuarine clay. The resulting parameters are used to back-analyse large-scale foundation load tests and are shown to provide an excellent match to the measured foundation response. [ABSTRACT FROM AUTHOR]

A stiffened deep mixed (SDM) column can significantly increase the bearing capacity, reduce settlement, and enhance the slope stability of soft clays as compared with a conventional deep mixed (DM) column. This technique involves inserting plain concrete, reinforced concrete or a steel pile into the center of the DM column after the DM column is constructed. In this paper, a series of centrifugal modeling tests were conducted to investigate the performance of an SDM column-supported embankment over soft clay. A model embankment supported only by DM columns was constructed for comparison. Two ideal numerical models of column-reinforced soil under equal stress and equal strain conditions were established to explore the role the column played in accelerating soil consolidation. A parametric study was conducted to investigate the influence factors of the length of the core pile, column spacing, thickness of the underlying soil, modulus and thickness of the cushion, and modulus of the slab on the load transfer of the system, and some recommendations were proposed for its application. The load-transfer mechanism of an SDM column-supported embankment system with a slab was established based on the development of the volumetric strains and the principal stresses in the numerical models. [ABSTRACT FROM AUTHOR]

This paper summarizes 239 static load tests to evaluate the performance of four static design methods for axial resistance of driven piles in clay. The methods are ISO 19901-4:2016, SHANSEP, ICP-05, and NGI-05. The database is categorized into four groups depending on the load type (compression or uplift) and pile tip condition (open or closed end). The model uncertainty in resistance prediction is quantified as a ratio between measured and calculated resistance, which is called a model factor. The measured resistance is interpreted as a load producing a settlement level of 10% pile diameter. Database studies show that the four methods present a similar accuracy, where the mean and coefficient of variation (COV) of the model factor are around 1 and 0.3, respectively. The COV values are smaller than those for driven piles in sand available in literature. The model statistics determined from the database are applicable to a simplified or full probabilistic form of reliability-based design (RBD) of driven piles in clay. As an illustration, the resistance factors in load and resistance factor design (LRFD, a simplified form of RBD) are calibrated by Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

Estimation of the bearing capacity of shallow foundations on layered soil profiles, such as a sand layer of finite thickness over clay, is mainly based on empirical models resulting from the interpretation of experimental test results. While it is generally accepted that such models may be applicable to soil properties and footing geometries outside the range tested experimentally, they offer limited insights on how the assumed failure mechanism affects their range of application. In particular, the contribution of the sand layer to the overall capacity is accounted for via simple considerations, which are valid only for a specific range of problem parameters. This paper addresses the estimation of the undrained bearing capacity of a rigid strip footing resting on the surface of a sand layer of finite thickness overlying clay, using finite element limit analysis (FELA). The rigorous upper and lower bound theorems of plasticity are employed to bracket the true bearing capacity of the footing, and identify the geometry of possible failure mechanisms. Insights gained from FELA simulations are used to develop a new simple bearing capacity model, which captures the variation in shear resistance from the sand layer with the dimensionless undrained strength of the clay layer. The proposed model provides results that are in close agreement with published experimental studies, and allows treating simple problems, such as the design of working platforms, without having to resort to numerical simulations. [ABSTRACT FROM AUTHOR]

As a type of shallow foundation, a mudmat serves as the seabed support structure for subsea wells, pipeline manifolds, and pipeline terminations. The shallow foundations are usually designed with perforations to facilitate installation and removal, but the influence of these perforations has not been fully understood. This paper presents a method to analyze the bearing capacities of both two-dimensional (2D) and three-dimensional (3D) perforated shallow foundations using finite element analysis. The soil was idealized as a Tresca material, with the undrained strength increasing linearly with depth. The outcome indicates that perforations have nonnegligible effects on the bearing capacity of shallow foundations. The bearing capacity decreases with increasing perforation ratio, R, and the degree of reduction increases with the increase of the dimensionless ratio kB/Suo, where k is the shear strength gradient, B is the width of the foundation, and Suo is the shear strength at the mudline. For 2D shallow foundations, there exists a critical perforation ratio, Rc; when the perforation ratio is lower than the critical perforation ratio, the perforated foundation does not lose its bearing capacity. For 3D shallow foundations, the bearing capacity decreases directly with the increase of perforation ratio, R. [ABSTRACT FROM AUTHOR]

The presence of a thin soft clay layer inside a bed of sand may significantly reduce the bearing capacity of the sand layer, imposing a risk of punch-through failure. In this paper, finite element (FE) simulations are reported using a hardening soil (HS) model for sand. The FE model has been verified against centrifuge tests involving loose and dense sand layers overlying clay soil. The effects of sand stiffness, foundation roughness, sand friction angle, undrained clay strength, clay strength nonhomogeneity, and sand and clay layer geometries on the foundation peak capacities have been studied. Punch-through failure is initiated with an inclined sand plug being sheared and pushed into the underlying soft clay. During punch-through, the clay layer fails due to significant radial squeezing. Existing analytical models do not capture the combined failure mechanism of sand shearing and clay radial squeezing. A new analytical model is developed to estimate the peak punch-through capacity of a spudcan in sand with an interbedded clay layer, showing improved performance over the current industry guidelines. [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 capacity of dynamically installed anchors in soft normally consolidated clay was examined experimentally through a series of field tests on a 1:20 reduced-scale anchor. The anchors were installed through free fall in water, achieving tip embedment of 1.5-2.6 times the anchor length, before being loaded under undrained conditions at various load inclinations. Vertical anchor capacities were between 2.4 and 4.1 times the anchor dry weight and were satisfactorily predicted using the American Petroleum Institute approach for driven piles. Anchor capacity under inclined loading increased as the load inclination approached horizontal; the field data indicated this increase to be up to 30% for the minimum achievable inclination of about 20° to the horizontal. Corresponding large-deformation finite element analyses showed a similar response, with the maximum capacity occurring at a load inclination between 30° and 45° to the horizontal. The finite element results demonstrate that, for the anchor geometry considered, an inclined load at the anchor padeye could be decomposed into ultimate vertical and moment loading at the anchor centroid. The establishment of a vertical and moment loading yield envelope for the geometry investigated forms the basis of a simple design procedure presented in the paper. [ABSTRACT FROM AUTHOR]

The design of shallow foundations is dominated by issues of settlement rather than bearing capacity per se. The ability to predict the settlement of foundations at a given factor of safety is hence of key importance in design. In this paper, the energy method for a linear-elastic, perfectly plastic method utilizing the von Mises' yield criterion with associated flow developed and reported by McMahon et al. in 2013 is extended to consider the nonlinear behaviour of soil. The energy method is used to investigate the load-settlement behaviour of shallow foundations by utilizing an ellipsoidal cavity-expansion mechanism and deformation fields within the boundaries of the classical Hill and Prandtl mechanisms. An elastic mechanism obtained from an analysis in ABAQUS was also investigated using this energy method. The upper-bound approach demonstrates that the cavity-expansion mechanism produces a better solution at small values of settlement, whereas at greater settlements the Prandtl mechanism is shown to produce a more optimal upper-bound solution. [ABSTRACT FROM AUTHOR]