H-M Bearing Capacity of A Modified Suction Caisson Determined by Using Load-/Displacement-Controlled Methods

This paper presents a series of monotonically combined lateral loading tests to investigate the bearing capacity of the MSCs(modified suction caissons) in the saturated marine fine sand. The lateral loads were applied under load- and displacement-controlled methods at the loading eccentricity ratios of 1.5, 2.0 and 2.5. Results show that, in the displacement-controlled test, the deflection-softening behavior of load-deflection curves for MSCs was observed, and the softening degree of the load-deflection response increased with the increasing external skirt length or the decreasing loading eccentricity. It was also found that the rotation center of the MSC at failure determined by the load-controlled method is slightly lower than that by the displacement-controlled method. The calculated MSC capacity based on the rotation center position in serviceability limit state is relatively conservative, compared with the calculated capacity based on the rotation center position in the ultimate limit state. In the limit state, the passive earth pressures opposite the loading direction under load- and displacement-controlled methods decrease by 46% and 74% corresponding to peak values, respectively; however, the passive earth pressures in the loading direction at failure only decrease by approximately 3% and 7%, compared with their peak values.     

Response of skirted suction caissons to monotonic lateral loading in saturated medium sand

Monotonic lateral load model tests were carried out on steel skirted suction caissons embedded in the saturated medium sand to study the bearing capacity. A three-dimensional continuum finite element model was developed with Z_SOIL software. The numerical model was calibrated against experimental results. Soil deformation and earth pressures on skirted caissons were investigated by using the finite element model to extend the model tests. It shows that the ＂skirted＂ structure can significantly increase the lateral capacity and limit the deflection, especially suitable for offshore wind turbines, compared with regular suction caissons without the ＂skirted＂ at the same load level. In addition, appropriate determination of rotation centers plays a crucial role in calculating the lateral capacity by using the analytical method. It was also found that the rotation center is related to dimensions of skirted suction caissons and loading process, i.e. the rotation center moves upwards with the increase of the ＂skirted＂ width and length; moreover, the rotation center moves downwards with the increase of loading and keeps constant when all the sand along the caisson＇s wall yields. It is so complex that we cannot simply determine its position like the regular suction caisson commonly with a specified position to the length ratio of the caisson.     

Earth Pressure Distribution and Sand Deformation Around Modified Suction Caissons (MSCs) Under Monotonic Lateral Loading

The modified suction caisson(MSC) is a novel type of foundation for ocean engineering, consisting of a short external closed-top cylinder-shaped structure surrounding the upper part of the regular suction caisson(RSC). The MSC can provide larger lateral bearing capacity and limit the deflection compared with the RSC. Therefore, the MSC can be much more appropriate to use as an offshore wind turbine foundation. Model tests on the MSC in saturated sand subjected to monotonic lateral loading were carried out to investigate the effects of external structure sizes on the sand surface deformation and the earth pressure distribution along the embedded depth. Test results show that the deformation range of the sand surface increases with the increasing width and length of the external structure. The magnitude of sand upheaval around the MSC is smaller than that of the RSC and the sand upheaval value around the MSC in the loading direction decreases with the increasing external structure dimensions. The net earth pressure in the loading direction acting on the internal compartment of the MSC is smaller than that of the RSC at the same embedded depth. The maximum net earth pressure acting on the external structure outer wall in the loading direction is larger than that of the internal compartment, indicating that a considerable amount of the lateral load and moment is resisted by the external skirt structure.     

Accumulated Rotation of A Modified Suction Caisson and Soil Deformation Induced by Cyclic Loading

Modified suction caissons (MSCs) acting as offshore wind turbine foundations will generate the accumulated rotation under cyclic loading resulted from waves. The accumulated rotation and the range of soil deformation around the MSC under long-term cyclic wave loading were studied using 3-D numerical simulations. The Morison equation was adopted to calculate the wave loadings. It was found that the MSC accumulated rotation increases linearly with the increase of the logarithm of cyclic number. The normalized expression was proposed to reflect the relationship between the accumulated rotation and cyclic number. The soil deformation range around the MSC increases when increasing the cyclic number and loading amplitude. It can also be concluded that the accumulated rotation increases rapidly with this change of excess pore pressure in the first 4000 cycles. The responses of the MSC to wave and wind loads were also investigated. Results show that the accumulated rotation of the MSC under both wave and wind loadings is larger than that under the wave loading only.     

Lateral Bearing Capacity of Modified Suction Caissons Determined by Using the Limit Equilibrium Method

The modified suction caisson (MSC) adds a short-skirted structure around the regular suction caissons to increase the lateral bearing capacity and limit the deflection. The MSC is suitable for acting as the offshore wind turbine foundation subjected to larger lateral loads compared with the imposed vertical loads. Determination of the lateral bearing capacity is a key issue for the MSC design. The formula estimating the lateral bearing capacity of the MSC was proposed in terms of the limit equilibrium method and was verified by the test results. Parametric studies on the lateral bearing capacity were also carried out. It was found that the lateral bearing capacity of the MSC increases with the increasing length and radius of the external skirt, and the lateral bearing capacity increases linearly with the increasing coefficient of subgrade reaction. The maximum lateral bearing capacity of the MSC is attained when the ratio of the radii of the internal compartment to the external skirt equals 0.82 and the ratio of the lengths of the external skirt to the internal compartment equals 0.48, provided that the steel usage of the MSC is kept constant.     

Earth Pressure on Caissons in Marine Clay Under Cyclic Loading

Coastal protection is proposed to be made out of a contiguous caisson type of wall. These caissons can be designed to resist both lateral static and cyclic loading. With adequate depth of embedment, the walls can be designed to offer significant lateral passive resistance to counteract the lateral static and cyclic loading arising out of wave action. This article describes a set of laboratory tests on model caissons embedded into soft marine clay with different embedment depths. Specially designed earth pressure cells are embedded into the caisson at different depths. A pneumatic system was used to apply lateral static and cyclic loading. Test beds were prepared conforming to soft clay conditions in a test tank of appropriate size. The test results reveal that with this type of arrangement the variation in earth pressure with depth can be conveniently established. The earth pressure developed is related to the lateral load applied. The depth at which the maximum earth pressure occurs is same for both static and cyclic loading. Further, under cyclic loading there is no failure encountered even under cyclic loading level corresponding to 0.9 times the ultimate static lateral capacity.     

Rotation Center and Horizontal Bearing Capacity of the Bucket Foundation in Soft Ground

In the last 20 years, the bucket foundation has been developed as a new type of offshore platform structure. Because of its short period of application to engineering practices, the theoretical decision on the rotation center and horizontal bearing capacity of the bucket foundation has yet to be agreed. A limit analysis method is used to determine the updated rotation center position and horizontal bearing capacity to evaluate the failure mechanisms of the bucket foundations. The results are compared with numerical simulation and experiments, and also with other theoretical methods. The proposed method can satisfactorily consider the engineering conditions and the result is accurate in determining the rotation center and horizontal bearing capacity.     

Study on Failure Mechanism and Bearing Capacity of Three-Dimensional Rectangular Footing Subjected to Combined Loading

This paper presents two kinematic failure mechanisms of threc-dimensional rectangular footing resting on homogeneous undrained clay foundation under uniaxial vertical loading and uniaxial moment loading. The failure mechanism under vertical loading comprises a plane strain Prandti-type mechanism over the central part of the longer side, and the size of the mechanism gradually reduces at the ends of the longer side and over the shorter side as the corner of rectangular footing is being approached where the direction of soil motion remains normal to each corresponding side respectively. The failure mechanism under moment loading comprises a plane strain scoop sliding mechanism over the central part of the longer side, and the radius of scoop sliding mechanism increases linearly at the ends of the longer side. On the basis of the kinematic failure mechanisms mentioned above, the vertical ultimate bearing capacity and the ultimate bearing capacity against moment or moment ultimate bearing capacity are obtained by use of upper bound limit analysis theory. At the same time, numerical analysis results, Skempton＇ s results and Salgado et al. ＇s results are compared with this upper bound solution. It shows that the presented failure mechanisms and plastic limit analysis predictions are validated. In order to investigate the behaviors of undrained clay foundation beneath the rectangular footing subjected to the combined loadings, numerical analysis is adopted by virtue of the general-purpose FEM software ABAQUS, where the clay is assumed to obey the Mohr-Coulomb yielding criterion. The failure envelope and the ultimate bearing capacity are achieved by the numerical analysis results with the varying aspect ratios from length L to breadth B of the rectangular footing. The failure mechanisms of rectangular footing which are subjected to the combined vertical loading V and horizontal loading H （Vertical loading V and moment loading M, and horizontal loading H and moment loading M respectively are observed in the finite e     

Failure Loci of Suction Caisson Foundations Under CombinedLoading Conditions

Suction caissons are widely used to support offshore fixed platforms in coastal areas. The loadings transferred to suction caissons include the eccentric lateral force induced by waves and self weight of the platform structure. However, under this kind of combined loading conditions, the failure mechanism of caissons with shallow embedment depths is quite different from conventional deep foundations or onshore shallow footings. The behaviour of caissons subjected to combined loadings may be described with the "failure locus" in force resultant spaces. Here the failure loci of smooth caissons are studied by use of finite element approach, with the embedment ratio of caissons varying in the range of 0.25～1.0 and eccentricity ratio of horizontal loadings in 0～10. The platform settlement and tilt limits are involved into determination of failure loci, thus the platforms can avoid significant displacements for the combined loadings located inside the failure locus. Three families of loading paths are used to map out the locus. It is found that the shape of failure loci depends on 3 non-dimensional parameters, and the failure locus of a given caisson changes gradually from the elliptical curve to hooked curve with increasing shear strength of soil. The lateral capacity of short caissons may be enhanced by vertical forces, compared with the maximum lateral capacity of long caissons occurring at the vertical force being zero. The critical embedment ratios partitioning elliptical and hooked loci are proposed.     

Numerical Analyses of Bearing Capacity of Deep-Embedded Large-Diameter Cylindrical Structure on Soft Ground Against Lateral Loads

1 .IntroductionThe large-diameter cylindrical structure for coastal and offshore engineering has been widely usedin China .Thistype of structureis composed of a steel or reinforced concrete cylindrical thin-wall shellplaced partlyintothe ground by a speci…     

Assessment of cyclic response to suction caisson in clay using a three-dimensional displacement approach

An investigation was made to present analytical solutions of cyclic response to suction caisson subjected to inclined cyclic loadings in clay using a three-dimensional displacement approach. A model representing the relationship between vertical load and vertical displacement and that between lateral load and lateral displacement along the skirt of suction caisson subjected to cyclic loadings is proposed for overconsolidated clay. For the effect of vertical load on cyclic load capacity of suction caisson, using the Mindlin solution in the case of a vertical point load, the vertical stress of soil under the base of suction caisson is presented. For the stress state of soil beneath the base of suction caisson subjected to cyclic loading, the Mohr–Coulomb failure line and critical state line are presented and the relationship between total stress, effective mean principal stress, stress difference, and pore-pressure is elucidated. The comparison of results predicted by the present method for a suction caisson subjected to cyclic loadings in clay has shown good agreement with those obtained from field tests. Cyclic behavior of clay up to failure is made clear from the relationship between cyclic tensile load, vertical and lateral displacements, and rotation and that between depth, vertical, and lateral pressures.     

Determination of Shaft Capacity for Pile under Cyclic Axial Loading in Clay

For load-controlled and displacement-controlled test data of piles cyclically axially loaded in clay, the displacement conditions are suggested for determining the shaft capacity. According to the suggested displacement conditions, not only the results of shaft capacity from laboratory model piles are close to those from in-situ piles, but also the results of load-controlled tests are in satisfactory agreement with those of displacement-controlled tests. Moreover, based on the test data of laboratory model piles in combination with the test data published, the paper suggests the values of the normalized shaft capacity of piles under a variety of static and cyclic loading combinations.     

Seismic Passive Earth Pressure with Seepage for Cohesionless Soil

The authors deal with the computing seismic passive earth pressure acting on a vertical rigid wall. The wall is provided with a drainage system along soil-structure interface and retains the cohesionless backfill subjected to water seepage. A general solution for the seismic passive earth pressure is presented. The solution is based on Coulomb's theory wherein seismic forces are assumed to be pseudostatic. The solution considers the pore water pressures induced by water seepage and earthquake shaking. Some important parameters are included in the solution. The parameters are the soil effective internal friction angle, wall friction, soil unit weight, and horizontal and vertical seismic acceleration coefficients. The comparison of the total seismic passive earth pressure in horizontal direction from the present method with published works indicates that the present method may be reasonable. The variations of the passive earth pressure coefficient with the soil effective internal friction angle are investigated for different wall friction angles and seismic forces. The effect of the water seepage on the seismic passive earth pressure is also investigated.     

Ultimate Lateral Capacity of Rigid Pile in <Emphasis Type="Italic">c</Emphasis>–<Emphasis Type="Italic">φ</Emphasis> Soil

To date no analytical solution of the pile ultimate lateral capacity for the general c–φ soil has been obtained. In the present study, a new dimensionless embedded ratio was proposed and the analytical solutions of ultimate lateral capacity and rotation center of rigid pile in c–φ soils were obtained. The results showed that both the dimensionless ultimate lateral capacity and dimensionless rotation center were the univariate functions of the embedded ratio. Also, the ultimate lateral capacity in the c–φ soil was the combination of the ultimate lateral capacity (f _c ) in the clay, and the ultimate lateral capacity (f _φ ) in the sand. Therefore, the Broms chart for clay, solution for clay (φ=0) put forward by Poulos and Davis, solution for sand (c=0) obtained by Petrasovits and Awad, and Kondner’s ultimate bending moment were all proven to be the special cases of the general solution in the present study. A comparison of the field and laboratory tests in 93 cases showed that the average ratios of the theoretical values to the experimental value ranged from 0.85 to 1.15. Also, the theoretical values displayed a good agreement with the test values.     

An investigation into the lateral loading response of shallow bucket foundations for offshore wind turbines through centrifuge modeling in sand

A shallow suction bucket is a new foundation type for offshore wind turbines. Due to its large size and bulky shape, the bucket and the soil within the bucket do not necessarily deform as a whole. Moreover, limited research has been conducted on the hydrodynamic wave influence on the shallow bucket bearing response. These factors pose great challenges to the shallow bucket foundation design. This paper presents a set of centrifuge tests of a shallow bucket model subjected to monotonic and dynamic lateral loads to study the lateral bearing response of shallow bucket foundations in the field under combined loads induced by wind, waves, etc. In addition to the routine measurements (e.g., load-displacement), the soil pressures on the bucket and the distribution and evolution of the excess pore pressures in the surrounding soils are also obtained. The deformation pattern of the bucket (e.g., rotation center) is revealed through displacement measurements. Finally, the proposed easy-to-use analytical equations using the limit equilibrium to assess the bearing capacity of bucket foundations, taking into account the influence of the soil strength degradation caused by hydrodynamic wave loadings, are found to yield good results upon comparison with the centrifuge data, providing useful guidelines for the design of shallow bucket foundations.     

Discrete element method simulations of offshore plate anchor keying behavior in granular soils

Abstract

The keying mechanism of plate anchors (PLA) embedded into granular sandy soils is investigated in this work using the discrete element method (DEM) modeling to simulate microscale response and to observe the emergent macroscale behavior. Parameters (e.g., padeye eccentricity, loading direction) that influence anchor keying are analyzed in the simulations. The load-displacement response and embedment losses during keying are evaluated and compared to published experimental results from the literature. The keying mechanism of the PLA for different padeye eccentricities and loading directions are investigated. The DEM results are found to have the same trends as published experimental results. The embedment loss has a bilinear response with the padeye eccentricity which is in accordance with the experimental results reported in the literature. Embedment losses increase linearly with increasing loading directions. Microscale observations of mobilized particle friction, particle rotation, contact force network, and steering coefficient during keying are used to provide insights into the keying mechanisms. The potential for particle mobilization is reached more quickly for the larger padeye eccentricities. The particle rotation is the major keying mechanism for all the cases in the simulations. Finally, the granular assembly adjacent to the PLA is steering from horizontal to vertical for all padeye eccentricities.     

Stability analysis of suction bucket foundations under wave cyclic loading and scouring

Suction bucket foundation is a typical type for offshore turbines. Scour caused by wave and current can reduce the stability of foundation and then endanger the whole structure. This paper details a series of suction bucket model tests performed in sand under wave cyclic loading. The model tests investigate the effect of scour on stability of bucket foundation by artificially excavated scour hole around the foundation. It is revealed that the behavior of foundation bearing capacity can be divided into two stages: the initial cyclic stage and the final stage (showing either cyclic stability or cyclic failure). When the wave circulation is stable, the sand on the front and back sides of the foundation is suspected to be liquefied. With the increase in scour depth, the stability of foundation is gradually reduced, the behavior of foundation gradually changes from a state of cyclic stability to cyclic failure, and the number of waves that can be withstood is drastically reduced. Finally, the height of the center of rotation of the suction bucket was observed to descend with the increase in scour depth.     

Behavior of Pile Groups under Lateral Load

Based on investigation and model tests, and in combination with the research work on group effect for pile groups under lateral loads relating to the code of fixed offshore platforms, a series of studies have been performed on the behavior and failure mechanism of laterally loaded pile groups, critical pile spacing inducing group effect, lateral bearing capacity of pile groups and its main influence factors, the stress-strain relationship for single piles and pile groups and so on. Some new laws about non-uniformity of load distribution in the longitudinal direction of pile groups and load-deflection (p - y) curves for pile groups have been discovered, and an empirical formula is presented in order to remedy the defect of current calculating methods at home and abroad. These results can be used for reference in the design of pile foundation under lateral loads.     

Analysis Procedure of the Cyclic Bearing Capacity for Suction Anchors in Soft Clays

This article presents a procedure to calculate the bearing capacity of suction anchors subjected to inclined average and cyclic loads at the optimal load attachment point using the undrained cyclic shear strength of soft clays based on the failure model of anchors proposed by Andersen et al. The constant average shear stress of each failure zone around an anchor is assumed and determined based on the static equilibrium condition for the procedure. The cyclic shear strength of each failure zone is determined based on the average shear stress. The cyclic bearing capacity is finally determined by limiting equilibrium analyses. Thirty-six model tests of suction anchors subjected to inclined average and cyclic loads were conducted, which include vertical and lateral failure modes. Model test results were predicted using the procedure to verify its feasibility. The average relative error between predicted and test results is 1.7%, which shows that the procedure can be used to calculate the cyclic bearing capacity of anchors with optimal loading. Test results also showed that the anchor was still in vertical failure mode under combined average and cyclic loads if an anchor was in vertical failure mode under static loads. The anchor failure would depend on the vertical resistance degradation under cyclic loads if an anchor was in lateral failure mode under static loads. Cyclic bearing capacities associated with the number of load cycles to failure of 1000 were about 75% and 80% of the static bearing capacity for vertical failure anchors and lateral failure anchors, respectively.     

Analysis and Design of Monopile Foundations for Offshore Wind-Turbine Structures

Offshore wind turbines (OWTs) are generally supported by large-diameter monopiles, with the combination of axial forces, lateral forces, bending moments, and torsional moments generated by the OWT structure and various environmental factors resisted by earth pressures mobilized in the soil foundation. The lateral loading on the monopile foundation is essentially cyclic in nature and typically of low amplitude. This state-of-the-art review paper presents details on the geometric design, nominal size, and structural and environmental loading for existing and planned OWT structures supported by monopile foundations. Pertinent ocean-environment loading conditions, including methods of calculation using site-specific data, are described along with wave particle kinematics, focusing on correlations between the loading frequency and natural vibration frequency of the OWT structure. Existing methods for modeling soil under cyclic loading are reviewed, focusing in particular on strain accumulation models that consider pile–soil interaction under cyclic lateral loading. Inherent limitations/shortcomings of these models for the analysis and design of existing and planned OWT monopile foundations are discussed. A design example of an OWT support structure having a monopile foundation system is presented. Target areas for further research by the wind-energy sector, which would facilitate the development of improved analyses/design methods for offshore monopiles, are identified.