水平荷载下导管架平台桩基础的非线性有限元分析

导管架平台桩基础的控制荷载主要为风荷载、波浪荷载、地震荷载等水平荷载,为研究水平荷载下导管架平台桩基础的承载特性,采用非线性有限元分析方法对水平荷载下桩-土之间的相互作用进行研究,提出了有效模拟桩基水平承载特性的有限元模型,分析了模型桩的刚度、直径、土质参数中水平土压力系数、剪胀角对桩基承载特性的影响及水平荷载下群桩承载特性,并将有限元计算结果与API规范及模型试验结果进行对比。研究结果表明,非线性有限元分析方法分析水平荷载下桩-土相互作用是可行的,计算结果可为导管架平台的桩基设计提供参考。     

Experimental study on the mechanical behaviour of a heat exchanger pile using physical modelling

This study aims to provide knowledge on the thermo-mechanical behaviour of heat exchanger piles, through a laboratory scale model. The model pile (20 mm in external diameter) was embedded in dry sand. The behaviour of the axially loaded pile under thermal cycles was investigated. After applying the axial load on the pile head, the pile temperature was varied between 5 and 30 °C. Seven tests, corresponding to various axial loads ranging from 0 to 70 % of the pile estimated bearing capacity, were performed. The results on pile head displacement show that heating under low axial load induced heave and cooling induced settlement; the pile temperature-displacement curve was found to be reversible and compatible with the thermal expansion curve of the pile. However, at higher axial loads, irreversible settlement of the pile head was observed after a few thermal cycles. The axial load profile measured by the strain gauges evidenced that the pile head load was mainly transferred to the pile toe. Nevertheless, thermal cycles modified significantly the mobilised skin friction along the pile. The total pressure measured at various locations in the soil mass was also slightly influenced by the thermal cycles.     

轴向荷载对斜桩水平承载特性影响试验及理论研究

斜群桩受水平荷载作用时,群桩中的基桩受到径向荷载、轴向荷载和弯矩的共同作用。为研究轴向荷载对斜桩水平承载特性的影响,完成了3根单桩以及1组1×2斜桩的大尺寸模型试验。试验结果表明：轴向拉力作用会降低斜桩的水平刚度和极限承载力;而轴向压力作用则会使其水平刚度和极限承载力提高。基于桩侧浅层土体楔形破坏假定,推导了考虑轴向荷载影响的斜桩水平极限土抗力计算公式,提出了桩侧土抗力的p-y曲线方法,并通过模型试验及现场试验验证其合理性。     

Static Equilibrium of Screw Anchor Pile Under Lateral Load in Sands

The screw anchor piles are installed in ground by screwing which is done with the help of torque motors. In this paper, the lateral load capacity of screw anchor piles is examined through an experimental investigation carried on model piles embedded in dry sand. The tests were carried on screw anchor piles with different number of helices provided in continuation. Lateral loads were applied at different height above the soil surface. The embedment length of screw anchor piles was also varied to study the behaviour of screw anchor piles under lateral loads. Some tests were conducted on plain shaft pile to compare the lateral load capacity of screw anchor piles with that of plain shaft piles. An empirical equation for computation of lateral loads has been developed considering lateral resistance, bearing resistance, uplift resistance and lateral resistance offered by soil in pile on the basis of experimental results. A theoretical model for predicting lateral load capacity of screw anchor piles in dry sand, consistent with the experimental findings has been developed in this study.     

Depth of Fixity of Piles in Clay Under Dynamic Lateral Load

Dynamic experiments were carried out on instrumented model aluminium single piles embedded in clay of different consistencies to study its bending behaviour under lateral loads. Piles with different length to diameter ratios were used. Dynamic lateral load of different magnitudes ranging from 7 to 30 N at wide range of frequencies from 2 to 50 Hz were applied. The load transferred to the pile, pile head displacement and the strain variation along the pile length were measured using a dedicated data acquisition system. Static lateral load tests were also performed to investigate the magnification of dynamic response of piles in clay. It is found that the maximum bending moment due to dynamic load is magnified by about 1.5–4 times in comparison to the static load for short piles but about 9 times for long piles. Depth of fixity and effective pile length is also largely amplified under dynamic loads, thus indicating that a pile which behaves as a flexible pile under static load, may not exhibit flexible behaviour under dynamic load.     

Estimation of bearing capacity of open-ended model piles in sand

The plugging of pipe piles is an important phenomenon, which is not adequately accounted for in the current design recommendations. An open-ended pipe pile is said to be plugged when the soil inside the pile moves down with the pile, resulting in the pile becoming effectively closed-ended. Plugging is believed to result in an increase in the horizontal stresses between the pile and the surrounding soil, which results in an increase in skin friction. A total number of 60 model pile tests are carried out to investigate the behavior of plugs on the pile load capacity and the effects of plug removal. Different parameters are considered, such as pile diameter–to–length ratio, types of installation in sands of different densities, and removal of the plug in three stages (50, 75, and 100 %) with respect to the length of plug. The changes in the soil plug length and incremental filling ratio (IFR) with the penetration depth during pile driving show that the open-ended piles are partially plugged from the outset of the pile driving. The pile reached a fully plugged state for pressed piles in loose and medium sand and partially plugged (IFR = 10 %) in dense sand. For driven piles, the IFR is about 30 % in loose sand, 20 % in medium sand, and 30 % in dense sand. The pile load capacity increases with increases in the length of the plug length ratio (PLR). The rate of increase in the value of the pile load capacity with PLR is greater in dense sand than in medium and loose sand. Based on test results, new empirical relation for the estimation of the load carrying capacity of open-ended piles based on the IFR is proposed.     

Buckling behaviour of single pile and pile bent in the Scotch Road Bridge

The buckling behaviour of the 360 × 152 steel H-piles supporting the integral abutments of the Scotch Road Bridge, located in Trenton, New Jersey, has been studied for the cases of single pile and pile bent. Three-dimensional finite-element models for single pile and pile bent have been developed to study the behaviour of these fully embedded piles under axial and lateral loading. An iterative analysis based on extracting the eigenvalues and eigenvectors (mode shapes) that correspond to the pile(s) critical buckling loads has been adopted. The pile(s) and the surrounding sand were modelled using solid continuum elements in the finite-element model. Material non-linearity is accounted for in both the piles and the soil in the base state of the model. A parametric study has been utilized to determine the effect of the geometric and material properties of the pile and the surrounding sand on the predicted critical buckling loads of the piles. The effects of four parameters have been studied: soil stiffness, pile length, type of connection, and combining vertical and lateral loads. The results from the parametric study showed that the variation of the percentage change in the sand stiffness, pile length, and combining vertical and lateral loads with the critical buckling loads of the 360 × 152 H-piles is nonlinear. Furthermore, the parameters studied are more influential in affecting the critical buckling load of a single pile than a pile bent, with the exception of the ‘type of connection’ parameter.     

On the influence of vertical loads on the lateral response of pile foundation

The influence of vertical loads on the lateral response of group piles installed in sandy soil and connected together by a concrete cap is studied through finite elements analyses. The analyses focus on the five piles in the middle row of 3 × 5 pile groups. The vertical load is applied by enforcing a vertical displacement equivalent to 2% of the pile diameter through the pile cap prior to the application of the lateral loads. The results have shown that the lateral resistance of the leading pile (pile 1) does not appear to vary considerably with the vertical load. However, the vertical load leads to 23%, 36%, 64%, and 82% increase in the lateral resistance of piles 2–5, respectively. The increase in the lateral pressures in the sand deposit is the major driving factor to contribute the change in the lateral resistance of piles, depending on the position of the pile in the group. The distribution of lateral loads among piles in the group tends to be more uniform when vertical loads were considered leading to a more economical pile foundation design.     

A simplified analysis method for piled raft and pile group foundations with batter piles

A simplified method of numerical analysis has been developed to estimate the deformation and load distribution of piled raft foundations subjected to vertical, lateral, and moment loads, using a hybrid model in which the flexible raft is modelled as thin plates and the piles as elastic beams and the soil is treated as springs. Both the vertical and lateral resistances of the piles as well as the raft base are incorporated into the model. Pile–soil–pile, pile–soil–raft and raft–soil–raft interactions are taken into account based on Mindlin's solutions for both vertical and lateral forces. The validity of the proposed method is verified through comparisons with several existing methods for single piles, pile groups and piled rafts. Workable design charts are given for the estimation of the lateral displacement and the load distribution of piled rafts from the stiffnesses of the raft alone and the pile group alone. Additionally, parametric studies were carried out concerning batter pile foundations. It was found that the use of batter piles can efficiently improve the deformation characteristics of pile foundations subjected to lateral loads. Copyright © 2002 John Wiley & Sons, Ltd.     

Numerical analysis of the response of battered piles to inclined pullout loads

This paper presents a three‐dimensional finite element analysis of the response of battered piles to the combined lateral and vertical pullout loads. Analyses are carried out using an elastoplastic constitutive law based on the non‐associated Mohr–Coulomb criterion. The influence of the contact condition at the pile–soil interface is also investigated. Analyses show that the load's inclination with regard to the pile's axis affects both the lateral and axial response of the battered piles. Analyses also show that the pullout capacity of battered piles is affected by the pile's inclination regarding the vertical axis as well as the load's inclination regarding the pile's axis. The investigation of the influence of the contact condition at the soil–pile interface shows that the possibility of sliding at the soil–pile interface affects the response of battered piles subjected to loads with low inclination regarding the pile's axis. Copyright © 2008 John Wiley & Sons, Ltd.     

直接加固软弱下卧土层的地基工作性状模型试验研究

高压喷射注浆法可以在软弱下卧土层中直接形成加固桩体对软土地基进行加固。对这种加固方法形成的地基进行了一组模型试验,其中包含了天然地基和加固地基,加固地基中设置了一组3×3的加固桩体,通过对比研究了直接加固软弱下卧土层的地基处理方法对地基工作性状的影响。结果表明：直接加固软弱下卧土层的方法能很好地改善地基的沉降特性,提高地基的承载能力。加固桩体能有效地传递荷载,使加固区软弱土体的压缩量大大减小。随着荷载的增加,加固桩体顶端承担的荷载大体呈线性增加,桩顶轴力表现为角桩最大,边桩次之,中心桩最小。加固桩体上部受负摩阻力作用,下部受正摩阻力作用,桩身最大轴力位于距桩顶25 cm处。加固区的平均桩-土应力比随荷载的增加而减小。     

Vertical loads effect on the lateral pile group resistance in sand

Vertical loads effect on the lateral response of a 3×5 pile group embedded in sand is studied through a two-dimensional finite element analysis. The soil-pile interaction in three-dimensional type is idealized in the two-dimensional analysis using soil-pile interaction springs with a hysteretic nonlinear load displacement relationship. Vertical loads inducing a vertical pile head displacement of 0.1-pile diameter increase the lateral resistance of the single pile at a 60 mm lateral deflection by 8%. Vertical loads inducing the same vertical displacement applied to a pile group spaced at 3.92-pile diameter increase the overall lateral resistance by 9%. The effect on individual piles, however, depends on the pile position. The vertical load decreases the lateral resistance of the leading pile (pile 1) by 10% and increases the lateral resistances of piles 2, 3, 4, and 5 by 9%, 14%, 17%, and 35%, respectively. Vertical loads applied to the pile group increase the confining pressures in the sand deposit confined by the piles but the rate of increase in those outside the group is relatively small, resulting in the difference in a balance of lateral soil pressures acting at the back of and in front of the individual pile.     

Cyclic and seismic response of single piles in improved and unimproved soft clays

Presented in this paper are results of two centrifuge tests on single piles installed in unimproved and improved soft clay (a total of 14 piles), with the relative pile–soil stiffness values varying nearly two orders of magnitude, and subjected to cyclic lateral loading and seismic loading. This research was motivated by the need for better understanding of lateral load behavior of piles in soft clays that are improved using cement deep soil mixing (CDSM). Cyclic test results showed that improving the ground around a pile foundation using CDSM is an effective way to improve the lateral load behavior of that foundation. Depending on the extent of ground improvement, elastic lateral stiffness and ultimate resistance of a pile foundation in improved soil increased by 2–8 times and 4–5 times, respectively, from those of a pile in the unimproved soil. While maximum bending moments and shear forces within piles in unimproved soil occurred at larger depths, those in improved soil occurred at much shallower depths and within the improved zone. The seismic tests revealed that, in general, ground improvement around a pile is an effective method to reduce accelerations and dynamic lateral displacements during earthquakes, provided that the ground is improved at least to a size of 13D × 13D × 9D (length × width × depth), where D is the outside diameter of the pile, for the pile–soil systems tested in this study. The smallest ground improvement used in these tests (9D × 9D × 6D), however, proved ineffective in improving the seismic behavior of the piles. The ground improvement around a pile reduces the fundamental period of the pile–soil system, and therefore, the improved system may produce larger pile top accelerations and/or displacements than the unimproved system depending on the frequency content of the earthquake motion.     

Experimental investigation of the behavior of pile groups in sand under different loading rates

The behavior of pile groups in sand under different loading rates is investigated. A total of 60 tests were conducted in the laboratory using model steel piles embedded in a medium dense sand. The model piles have an outside diameter of 25 mm and embedment length of 500 mm. Five different configurations of pile groups (2 × 1, 3 × 1, 2 × 2, 2 × 3, 3 × 3) with center to center spacing between the piles of 3d, 6d and 9d (d is the pile diameter) were tested. The piles were subjected to axial compressive loads under four different loading rates: 1.0, 0.5, 0.1 and 0.05 mm/min. Test results indicated that the axial compressive capacity of pile group increases with the loading rate such that the pile capacity versus logarithm of loading rate data plot approximately along a straight line. The slope of this line increases as the number of piles in a group increases and it decreases by increasing the spacing between piles in a group.     

Centrifuge shaking table tests on 4 × 4 pile groups in liquefiable ground

A series of centrifuge shaking table model tests are conducted on 4?×?4 pile groups in liquefiable ground in this study, achieving horizontal–vertical bidirectional shaking in centrifuge tests on piles for the first time. The dynamic distribution of forces on piles within the pile groups is analysed, showing the internal piles to be subjected to greater bending moment compared with external piles, the mechanism of which is discussed. The roles of superstructure–pile inertial interaction and soil–pile kinematic interaction in the seismic response of the piles within the pile groups are investigated through cross-correlation analysis between pile bending moment, soil displacement, and structure acceleration time histories and by comparing the test results on pile groups with and without superstructures. Soil–pile kinematic interaction is shown to have a dominant effect on the seismic response of pile groups in liquefiable ground. Comparison of the pile response in two tests with and without vertical input ground motion shows that the vertical ground motion does not significantly influence the pile bending moment in liquefiable ground, as the dynamic vertical total stress increment is mainly carried by the excess pore water pressure. The influence of previous liquefaction history during a sequence of seismic events is also analysed, suggesting that liquefaction history could in certain cases lead to an increase in liquefaction susceptibility of sand and also an increase in dynamic forces on the piles.     

砂卵石地层中单、双排钢板桩围堰现场水平载荷试验研究

针对松花江砂卵石地层上的钢板桩围堰进行现场模型的水平载荷试验,介绍了模型的施工及试验方法,研究了双排钢板桩通过拉杆连接这种结构在水平载荷作用下的变形特性,同时进行了单排钢板桩水平载荷试验作为对比分析;通过测量其深层水平位移及桩顶位移,分析其桩顶位移预警值为及水平荷载的影响深度;通过测量拉杆轴力,分析水平力的传递,对类似工程设计有一定的参考作用。     

Thermal analysis of energy piles in sand

The present work investigates the behaviour of geothermal energy piles in sand subjected to thermal loading and the resulting soil-structure interaction, numerically using the finite element software Abaqus and user-defined material subroutines for soil. The stress-strain response of sand has been simulated using CASM constitutive model based on critical-state soil mechanics. Detailed parametric sensitivity studies have been carried out to understand the effects of different end conditions of the pile, relative densities of the soil, coefficients of lateral earth pressure of the ground, lengths and diameters of the pile, thermal loads, coefficients of friction at the pile-soil interface, critical-state friction angles of soil, thermal conductivity of soil, specific heat of soil and thermal conductivity of the pile on the stress response of soil, deformation of the pile and soil, and strains in the pile. The results show that negative shear stress is generated in the soil at the pile-soil interface. In the pile with both ends restrained the lateral earth pressure coefficient in soil increases due to high radial strain generation. Moreover, the lateral earth pressure coefficient in soil increases with the increase in the thermal load, the coefficient of friction at the pile-soil interface and the critical-state friction angle of the soil.     

Comparison of the load response of closed-ended and open-ended pipe piles driven in gravelly sand

Slow-maintained static load tests were performed on closed-ended and open-ended steel pipe piles driven side by side in a gravelly sand soil profile. The site investigation consisted of multiple cone penetration tests (CPTs) and standard penetration tests (SPTs), as well as laboratory tests on soil samples collected at various depths from the test site to determine basic soil properties. The test piles were densely instrumented with a combination of electrical-resistance and vibrating-wire strain gauges. The open-ended test pile was a specially fabricated double-wall, fully-instrumented pile, allowing for separation of the measurements of the inner and outer shaft resistances. Detailed comparison of the load test results, in terms of driving resistance, load response and profiles of unit shaft and base resistances for the two test piles, is presented and discussed. The applicability of three CPT-based pile design methods is assessed through a layer-by-layer comparison of the estimated resistances with those measured in the static load tests.

     

Approximate computer analysis of pile groups subjected to loads and ground movements

This paper describes the development of an approximate approach for the analysis and design of piles subjected to axial and lateral loading and also to vertical and horizontal ground movements. The analysis involves a number of simplifications in order to make it feasible to implement. For example, it considers the behaviour of a ‘representative’ pile in a group to characterize the behaviour of all piles in the group, and adopts approximations to derive free-field interaction factors from the conventional interaction factors for direct loading. The analysis has been implemented via a computer program called EMbankment PIle Group (EMPIG) and has the ability to incorporate the following features:

• 1. single piles or pile groups,
• 2. applied vertical, lateral and moment loading on the pile cap,
• 3. the effects of axial and lateral soil movements caused by embankment construction,
• 4. a layered soil profile,
• 5. non-linear axial and lateral response of the piles.

Comparisons between solutions from EMPIG and other independent programs suggest that it is capable of providing results of adequate accuracy for practical design purposes. The analysis has been used to investigate the effects of pile rake on a typical bridge abutment group. The presence of raked piles can have a detrimental effect on group behaviour, especially in the presence of ground movements. Large lateral deflections can be generated and axial forces and moments in the piles are increased. Comparisons are also made with the results of centrifuge model tests on abutment pile groups. Copyright © 1999 John Wiley & Sons, Ltd.     

Piles under axial and torsional loads

An analysis of axially and torsionally loaded piles is presented in which the pile is treated as an elastic bar supported on two series of interacting non-linear axial and torsional springs. The characteristics of these springs depend on the soil properties and the diameter of the pile as well as on the interaction between the axial and torsional response. Predicted pile responses are compared to the results of model tests conducted on piles installed in a soft clay bed and loaded with combined axial and torsional loads. Both experimental results and theoretical predictions show that a torque applied to the pile head affects significantly the settlement and the axial bearing capacity of the pile.