Behaviour of rigid piles in marine clays under lateral cyclic loading

In the field of ocean engineering, pile foundations are extensively used in supporting several structures. In many cases, piles are subjected to significant lateral loads. The environment prevalent in the ocean necessitates the piles to be designed for cyclic wave loading. In this investigation, the behaviour of rigid piles under cyclic lateral loading has been studied through an experimental programme carried out on model piles embedded in a soft marine clay. Static tests were also conducted on piles embedded in a clay bed prepared at different consistencies suitable to field situations. Cyclic load was applied by using a specially designed pneumatic controlled loading system. Tests were conducted on model piles made of mild steel (MS), aluminium and PVC with wide variation in pile soil relative stiffness. For cyclic load levels less than 50% of static lateral capacity, the deflections are observed to increase with number of cycles and cyclic load level and stabilise after a certain number of cycles. For cyclic load levels greater than 50% of static lateral capacity, the deflections are observed to increase enormously with number of cycles. The results of post-cyclic load tests indicate that the behaviour under static load can improve for cyclic load levels less than 40% of the static lateral capacity. The variations in the load capacity due to cyclic loading are explained in terms of the changes in strength behaviour of soil.     

Design Recommendations for Pile Subjected to Cyclic Load

Pile foundations subjected to cyclic load is an age-old problem dealt with for decades by geotechnical engineers. The ocean environment necessitates the piles supporting offshore structures to be designed against lateral cyclic loading initiated by wave action. Substantial experimental and analytical investigations have already been conducted by the author and other researchers. The quasi-static load reversal induces deterioration in the strength and stiffness of the soil-pile system introducing progressive reduction in the bearing capacity as well as settlement of the pile foundation, the degree of such degradation has been observed to be a function of the cyclic load parameters and the type of soil. Based on these observations, a design recommendation has been attempted in this paper for piles subjected to cyclic load in cohesive soil.     

Development of the cyclic p–y curve for a single pile in sandy soil

Pile foundations that support transmission towers or offshore structures are dominantly subjected to cyclic lateral load induced by wind and waves. For a successful design, it is crucial to investigate the effect of cyclic lateral loads on the pile behavior that is loaded laterally. Although the p–y curve method is generally utilized to design the cyclic laterally loaded pile foundations, the effect of cyclic lateral loads on the pile has not been properly implemented with the p–y curve. This reflects a lack of consideration of the overall stiffness change in soil–pile interaction. To address this, a series of model pile tests were conducted in this study on a preinstalled aluminum flexible pile under various sandy soil conditions. The test results were used to investigate the effect of cyclic lateral loads on the p–y behavior. The cyclic p–y curve, which properly takes into account this effect, was developed as a hyperbolic function. Pseudo-static analysis was also conducted with the proposed cyclic p–y curve, which showed that it was able to properly simulate cyclic laterally loaded pile behavior in sandy soil.     

Centrifuge modeling of single pile response due to lateral cyclic loading in kaolin clay

In offshore engineering, pile foundations are commonly constructed in marine deposits to support various structures such as offshore platforms. These piles are subjected to lateral cyclic loading due to wind, wave action, and drag load from ships. In this paper, centrifuge model tests are conducted to investigate the response of the existing single piles due to lateral cyclic loading. The cyclic loading was simulated by a hydraulic actuator. It is found that the residual lateral movement and bending strain are induced in the existing pile after each loading–unloading cycle. This is because plastic deformation is induced in the soil surrounding the existing pile during each loading–unloading cycle. By increasing the applied loads during cyclic loading–unloading process, the lateral movements and bending strains induced in the pile head increase simultaneously. As the cyclic loading varies from 10 to 50 kN, the residual pile head movement increases from 40 to 154?mm, and the residual bending strain of the existing pile varies from 100 to 260 με. The ratio of residual to the maximum pile head movements varies from 0.17 to 0.22, while the ratio of residual to the maximum bending strains is in a range of 0.12–0.55.     

Lateral cyclic response of an aluminum model pile in sand

Lateral cyclic load tests were performed on an aluminum model pile in dry sand. Two levels of loading were adopted to represent different service load conditions. The maximum number of loading cycles was 1,000. From the test results, it was found that the even though in the service load condition, the pile response was still affected by cyclic effects and a larger load level would produce more significant influence. In a global point of view, the lateral displacement and maximum moment increased with loading cycles, while the secant stiffness within a cycle decreased with cycles. The cyclic effect was more significant on the lateral displacement than on the moment. In a local point of view, cyclic loading would degrade the equivalent subgrade stiffness for the soil shallower than about seven times diameter. In addition, the secant subgrade stiffness within a cycle increased with loading cycles. Some experimental relationships of lateral pile response and loading cycles were built and compared with those in the literature.     

海洋工程中桩基水平向承载力可靠指标的影响因素分析

桩基础水平向承载力的计算是海洋工程中桩基设计的重要组成部分。论文在搜集了大量平台建设资料的基础上,以现有的桩基水平向承载力的设计计算方法为依据,进行了水平向承载力的可靠度研究。对影响可靠指标的各个因素进行了灵敏度分析。     

Centrifuge modelling of lateral loading behaviour of a “semi-rigid” Mono-pile in soft clay

Abstract

Mono-pile foundations have been widely used for offshore wind turbines principally due to their convenient construction and cost-effective nature. So far, little attention has been paid to large diameter “semi-rigid” piles that have distinct behaviours from flexible or ideally rigid piles. This paper presents a series of centrifuge model tests to study the deforming and bearing characteristics of a 5.9 dia. semi-rigid pile under lateral loadings in kaolin clay. For monotonic loading, a modified p–y curve analysis model considering rotational soil flow near the rotation centre of pile was proposed, highlighting the limitation of classic plane-strain based plasticity models to evaluate the ultimate lateral pile-soil resistance. For cyclic loading, a strong correlation between the degree of soil degradation and cyclic load amplitude was identified. Besides, a degradation factor model, accounting for various cyclic stress levels and soil depths, was proposed, which can be used to assess the accumulative displacement of semi-rigid piles under cyclic loadings in soft clay.     

Behavior of Pile Group with Elevated Cap Subjected to Cyclic Lateral Loads

The pile group with elevated cap is widely used as foundation of offshore structures such as turbines, power transmission towers and bridge piers, and understanding its behavior under cyclic lateral loads induced by waves, tide water and winds, is of great importance to designing. A large-scale model test on 3×3 pile group with elevated cap subjected to cyclic lateral loads was performed in saturated silts. The preparation and implementation of the test is presented. Steel pipes with the outer diameter of 114 mm, thickness of 4.5 mm, and length of 6 m were employed as model piles. The pile group was cyclic loaded in a multi-stage sequence with the lateral displacement controlled. In addition, a single pile test was also conducted at the same site for comparison. The displacement of the pile cap, the internal forces of individual piles, and the horizontal stiffness of the pile group are presented and discussed in detail. The results indicate that the lateral cyclic loads have a greater impact on pile group than that on a single pile, and give rise to the significant plastic strain in the soil around piles. The lateral loads carried by each row of piles within the group would be redistributed with loading cycles. The lateral stiffness of the pile group decreases gradually with cycles and broadly presents three different degradation patterns in the test. Significant axial forces were measured out in some piles within the group, owing to the strong restraint provided by the cap, and finally lead to a large settlement of the pile group. These findings can be referred for foundation designing of offshore structures.     

海上风电单桩基础土相互作用特性影响因素分析

采用有限元软件ABAQUS建立了海上风电单桩基础与土相互作用数值计算模型,将波浪、洋流及风荷载等效成双向对称循环荷载,研究了水平循环荷载作用下不同因素对桩身水平位移、剪力和弯矩的影响规律。研究表明,随着循环荷载比的增加,桩身位移零点和桩身剪力反弯点沿埋深逐渐下移,桩身弯矩最大值点位于浅层土体;不同荷载频率时桩身位移在零点以上变化较大,桩身弯矩随着频率的增加逐渐增大;单向循环荷载作用下桩身位移最大,双向对称循环荷载作用下桩身位移最小;壁厚较小时对桩身水平位移影响较大;在位移零点之上范围内可以考虑设计"上厚下薄"的钢管桩,以减小桩身水平位移;不同桩壁厚时桩身剪力曲线在埋深约6D处出现交点,且泥面处桩身弯矩变化不明显。     

Offshore pile foundations in sand under earthquake loading

A new approach to the analysis of pile foundations, developed recently for the analysis of a pile supported offshore structure, is described. The method uses a coupled soil-pile analysis which takes into account the non-linear resistance of the pile to lateral deformation and the effect of progressively increasing pore water pressures on that resistance. The analysis allows also for radiation of energy away from the site. Typical of results given in the paper are: (1) the effect of pore water pressure increases on the API cyclic loading curves, (2) the degradation in lateral stiffness due to pore pressure increases for piles with fixed and free heads, (3) variations in deflections and moments with depth due to pore water pressure from those predicted using current API procedures.     

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.     

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.     

Lateral loading of a pile using strain wedge model and its application under scouring

The scour hole around a pile will reduce the capacity of a laterally loaded pile. The strain wedge model is capable to derive a p–y curve for the analysis of a lateral loaded pile on a nonlinear Winkler foundation. To improve and extend the ability of the strain wedge method, a modified strain wedge (MSW) method is developed, in which a nonlinear lateral deflection of the pile is assumed to describe the varied soil strain distribution in the passive wedge. And then by treating the soil weight involved in the strain wedge as a vertical load at the bottom of the scour hole, an equivalent wedge depth is obtained to consider the effect of scour hole dimensions on the response of laterally loaded piles in sand. The validity of the MSW model is proved by comparisons with a centrifuge test without scour. And its applicability in the problem of a pile with scour is performed by a comparison with a model test and a FE analysis. The analysis shows the pile displacement at the pile head with scour can be obtained by multiplying the corresponding deflection without scour with an amplification factor related to scour depth at large load level.     

Effect of Scour on the Behavior of Laterally Loaded Single Piles in Marine Clay

Most offshore and coastal structures are supported by pile foundations, which are subjected to large lateral loads due to wind, wave, and water currents. Water currents can induce scouring around piles that reduces lateral capacity and increases lateral deflection of a pile. Current design methods mostly consider the complete removal of soil layers around piles by scouring. In reality, however, scouring creates scour holes at different shapes, sizes, and depths. Their effects on the behavior of laterally loaded piles are not well investigated. A numerical model of a single pile in soft marine clay was first calibrated against field test data without scour. Then several key factors of scour were analyzed, such as the depth, width, and slope of the scour hole and the diameter and head fixity of the pile. The relationships of the ultimate lateral capacity of the single pile with the depth, width, and slope angle of the scour hole were obtained. The numerical results show that the scour depth had more significant influence on the pile lateral capacity than the scour width. In addition, the pile with a free head was more sensitive to scour than the pile with a fixed head.     

Bearing Behavior of Cast-in-Place Expansive Concrete Pile in Coral Sand Under Vertical Loading

The low side friction of piles in coral sand results in the low bearing capacity of foundations. In this paper, expansive concrete pile is utilized to improve the bearing capacity of pile foundations in coral sand. Both model tests and numerical simulation are performed to reveal the bearing mechanism of expansive concrete pile in coral sand.Results showed that the lateral earth pressure near pile increases obviously and the side friction of piles is improved,after adding expansion agent to the concrete. The horizontal linear expansion is 1.11% and the bearing capacity increased 41% for the pile, when 25% expansion agent is added. Results in finite element numerical simulation also show that ultimate bearing capacity increases with the increase of the linear expansion ratio. Besides, the area for obvious increase in side friction is below the surface of soil about three times the pile diameter, and the expansion leads to a high side friction sharing of the pile. Therefore, the cast-in-place expansive concrete pile is effective in improving the bearing capacity of piles in coral sand.     

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.     

Modelling the drained response of bucket foundations for offshore wind turbines under general monotonic and cyclic loading

The response of bucket foundations on sand subjected to planar monotonic and cyclic loading is investigated in the paper. Thirteen monotonic and cyclic laboratory tests on a skirted footing model having a 0.3 m diameter and embedment ratio equal to 1 are presented. The loading regime reproduces the typical conditions of offshore wind turbines: very large cyclic overturning moment, large cyclic horizontal load and comparatively little, self-weight induced, vertical load. The experimental soil-foundation response is interpreted within the macro-element approach, using an existing analytical model, suitably modified to accommodate the footing embedment and the application of cyclic load. Details of the proposed model are provided together with evidences of its ability to reproduce the essential features of the experimentally observed behaviour. The results of the study aim at increasing the confidence in the use of the macro-element approach to predict the response of bucket foundations for offshore wind turbines, notably as the long-term accumulated displacements are concerned.     

Small- and large-scale model tests on the buckling property of slender pile

Slender piles embedded in soft ground or liquefied soil may buckle under vertical load. In this paper, both small- and large-scale model tests are conducted to investigate the buckling mechanisms of a slender pile and the lateral earth pressure acting on the pile. To observe the buckling of a slender pile, the strain-controlled loading method is adopted to apply a vertical load. When the two ends of a slender pile are hinged, the buckling mechanisms of small- and large-scale model tests are same. It should be noted that this applies only to a system with a small ratio of pile bending stiffness to soil bending stiffness. An applied vertical load increases with an increasing pile head settlement until it reaches the critical buckling load. By further increasing the pile head settlement, the measured load approaches the critical buckling load. In the large-scale model test, the measured lateral earth pressure (i.e., active and passive) acting on the slender pile varies linearly with the lateral pile displacement when the measured range is 3–5?m beneath the ground. A critical buckling calculation method has been adopted to compare with the conventional “m” method. The two-sided earth pressure calculation method can achieve more approximate results with the model test.     

水平循环荷载下海上风电楔形单桩土体变形研究

海上风电工程主要受到风、波浪及洋流等产生的水平循环荷载作用,本文研究楔形单桩基础在水平循环荷载作用下的变形规律,并探讨不同循环荷载对变形规律产生的影响,以确保风电设施正常运行。通过数值模拟建立海上风电单桩-海床模型,考虑土体超孔隙水压力的演变规律及土体致密规律,土体采用UBC3D-PLM本构模型。本文重点讨论并分析在不同水平循环荷载作用下楔形单桩基础与等截面单桩基础的桩周土体位移、塑性应变及桩基累计转角位移之间的差异。研究结果表明：楔形结构会降低桩周土体位移及塑性应变,使得楔形单桩基础旋转中心位置更低,产生倾覆的可能更小,当循环荷载比为0.7时,累计转角位移能减少41.86%;循环荷载越大,楔形单桩基础水平受荷特性越好,累计位移减少量的增长率越高。研究成果可为今后海上风电基础的选择与设计提供参考。     

Numerical study on long-term monopile foundation response

This paper analyzes the long-term monopile foundation that undergoes numerous mechanical cycles. The semiempirical scheme is adopted to involve a mechanical constitutive model to extract stress and strains at the first cycle and polynomial-type strain accumulation functions to track the progressive plastic deformation. In particular, the strain function contains the fundamental features that require simulating the long-term response of geomaterials: volumetric strain (terminal void ratio) and shear strain (shakedown or ratcheting), the strain accumulation rate, and stress obliquity. The numerical simulation shows evolution of displacements, pile rotation, and stress redistribution along the embedded pile as the number of load cycles increases. The analysis highlights that the pile rigidity affects the pattern of horizontal stress and displacement. The repetitive lateral load enhances the lateral load resistance due to soil densification along the pile.