Pseudostatic Seismic Response Analysis of a Pile Group in a Soil Slope

This paper presents a simple approximate pseudostatic method for estimating the maximum internal forces and horizontal displacements of a pile group located in a soil slope. The method is extension of an existing similar method developed by the authors for the case of a horizontal ground surface. The method employed for horizontal ground case involves two main steps: first, the free-field soil movements caused by the earthquake are computed; Then, the response of the pile group is analyzed based on the maximum free-field soil movements as static movements, as well as a static loading at the pile head, which depends on the computed spectral acceleration of the structure being supported. This newly developed methodology takes into account the effects of group interaction and soil yielding. Simple modifications are applied to take into account the effect of slope on seismic deformations of the pile group, making use of the Newmark sliding block method. The applicability of the approach and the developed program is verified by comparisons made with both experimental shaking table tests and the results of a more refined analysis of a pile-supported wharf. It is demonstrated that the proposed method yields reasonable estimates of the pile maximum moment and horizontal displacement for many practical cases, despite its relative simplicity. The simplifying assumptions and the limitations as well as reliability of the methodology are discussed, and some practical conclusions on the performance of the proposed approach are suggested.     

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.     

Analysis of pile behaviour in liquefying sloping ground

In this paper, a numerical procedure based on the finite element method is outlined to investigate pile behaviour in sloping ground, which involves two main steps. First a free-field ground response analysis is carried out using an effective stress based stress path model to obtain the ground displacements, and the degraded soil stiffness and strength over the depth of the soil deposit. Next a dynamic analysis is carried out for the pile. The interaction coefficients and ultimate lateral pressure of soil at the pile–soil interface are calculated using degraded soil stiffness and strength due to build-up of pore pressures, and the soil in the far field is represented by the displacements calculated from the free-field ground response analysis. Pore pressure generation and liquefaction strength of the soil predicted by the stress path model used in the free-field ground response analysis are compared with a series of simple shear tests performed on loose sand with and without an initial static shear stress simulating sloping and level ground conditions, respectively. Also the numerical procedure utilised for the analysis of pile behaviour has been verified using centrifuge data, where soil liquefaction has been observed in laterally spreading sloping ground. It is demonstrated that the new method gives good estimate of pile behaviour, despite its relative simplicity.     

A simplified analysis method for piled raft foundations subjected to ground movements induced by tunnelling

A simplified method of numerical analysis has been developed to estimate the deformation and load distribution of piled raft foundations subjected to ground movements induced by tunnelling and incorporated into a computer program ‘PRAB’. In this method, a hybrid model is employed in which the flexible raft is modelled as thin plates, the piles as elastic beams, and the soil is treated as interactive springs. The interactions between structural members, pile–soil–pile, pile–soil–raft and raft–soil–raft interactions, are modelled based on Mindlin's solutions for both vertical and lateral forces. The validity of the proposed method is verified through comparisons with some published solutions for single piles and pile groups subjected to ground movements induced by tunnelling. Thereafter, the solutions from this approach for the analysis of a pile group and a piled raft subjected to ground movements induced by tunnelling are compared with those from three‐dimensional finite difference program. Good agreements between these solutions are demonstrated. The method is then used for a parametric study of single piles, pile groups and piled rafts subjected to ground movements induced by tunnelling. Copyright © 2005 John Wiley & Sons, Ltd.     

浅埋隧道施工对建筑物桩基的影响分析

浅埋隧道穿越上部桩基础的问题值得关注。在概述目前研究成果的基础上,采取两阶段分析方案,利用分层土体的剪切位移法,着重分析了浅埋隧道开挖引起的土体移动对隧道顶桩位移及轴力的影响,并与整体有限元计算结果进行了比较,结果表明该计算方法是合理的,且计算量小,无需建立复杂的计算模型,在工程领域有较好的实用性。     

基于两阶段法的堆载对公路桥梁桩基础影响分析

首先基于布西奈斯克解,将表面作用有集中荷载时半无限弹性体的应力变形解在荷载作用区域利用复合辛普森公式进行数值积分,得到了表面处于不同形式的分布荷载作用下土体自由场水平及竖直方向应力和变形的计算公式。结合有限差分法及Mindlin位移解分别推导出弹性地基中桩顶作用有集中荷载时单桩桩、土单元的位移,两者结合便得到主动桩的分析方法。通过将堆载作用下的土体自由场位移施加于桩上把上述自由场分析及主动桩分析结合起来,推导出堆载作用下被动单桩的竖向和水平向承载特性的分析方法。选取地表附近作用有矩形均布荷载的计算工况,通过与有限元计算结果的对比验证了所提出的分析方法的正确性;将所提出的方法计算结果与某处工程案例中实测数据结果进行对比,分析表明利用该方法分析堆载对邻近桩基的水平及竖向影响是合理可靠的。     

Static analysis of vertically loaded pile group in multilayered transversely isotropic soils

In this paper, a coupling approach is presented to study the static responses of vertically loaded pile group embedded in multilayered transversely isotropic soils. The individual pile in pile group is modeled by the finite element method, while the analytical layer-element method is applied to represent the soil's behavior. Then, the interaction equation of piles and soils is obtained by considering the force equilibrium and displacement compatibility conditions and solved by a FORTRAN program. The results computed by the proposed approach compare favorably with those from some existing solutions and field test. Some typical parametric analysis cases are investigated to study the effect of soil anisotropy, pile stiffness ratio, and pile spacing on the behavior of vertically loaded pile group.     

Analysis of kinematic seismic response of tapered piles

In the present work, a two-phase analysis is used to assess the lateral movement of a tapered pile due to kinematic seismic loading resulted from earthquakes. First, the free-field horizontal displacement of the ground due to earthquake is estimated using available theory of wave propagation. Second, these estimated soil movements are imposed on the taper pile in a closed-form solution to compute the pile response. The governing differential equation for an arbitrary pile segment is obtained, which includes the free-field horizontal movement estimated from the first phase. The equation is solved explicitly to obtain the horizontal deflection. Parametric studies show that tapered piles tend to be more flexible than uniform piles of the same volume and length under earthquake loading, which is soundly interesting.     

Vertical response of pile raft foundations subjected to tunneling‐induced ground movements in layered soil

A simplified analysis method has been developed to estimate the vertical movement and load distribution of pile raft foundations subjected to ground movements induced by tunneling based on a two‐stage method. In this method, the Loganathan–Polous analytical solution is used to estimate the free soil movement induced by tunneling in the first stage. In the second stage, composing the soil movement to the pile, the governing equilibrium equations of piles are solved by the finite difference method. The interactions between structural members (such as pile–soil, pile–raft, raft–soil, and pile–pile) are modeled based on the elastic theory method of a layered half‐space. The validity of the proposed method is verified through comparisons with some published solutions for single piles, pile groups, and pile rafts subjected to ground movements induced by tunneling. Good agreements between these solutions are demonstrated. The method is also used for a parametric study to develop a better understanding of the behavior of pile rafts influenced by tunneling operation in layered soil foundations. Copyright © 2011 John Wiley & Sons, Ltd.     

Analysis of pile‐raft foundations under complex loads in layered soils

Considering there is hardly any concerted effort to analyze the pile‐raft foundations under complex loads (combined with vertical loads, horizontal loads and moments), an analysis method is proposed in this paper to estimate the responses of pile‐raft foundations which are subjected to vertical loads, horizontal loads and moments in layered soils based on solutions for stresses and displacements in layered elastic half space. Pile to pile, pile to soil surface, soil surface to pile and soil surface to soil surface interactions are key ingredients for calculating the responses of pile‐raft foundations accurately. Those interactions are fully taken into account to estimate the responses of pile‐raft foundations subject to vertical loads, horizontal loads and moments in layered soils. The constraints of the raft on vertical movements, horizontal movements and rotations of the piles as well as the constraints of the raft on vertical movements and horizontal movements of the soils are considered to reflect the coupled effect on the raft. The method is verified through comparisons with the published methods and FEM. Then, the method is adopted to investigate the influence of soil stratigraphy on pile responses. The study shows that it is necessary to consider the soil non‐homogeneity when estimating the responses of pile‐raft foundations in layered soils, especially when estimating the horizontal responses of pile‐raft foundations. The horizontal loads and the moments have a significant impact on vertical responses of piles in pile‐raft foundations, while vertical loads have little influence on horizontal responses of piles in pile‐raft foundations in the cases of small deformations. The proposed method can provide a simple and useful tool for engineering design. Copyright © 2013 John Wiley & Sons, Ltd.     

静钻根植竹节桩桩端承载性能数值模拟研究

静钻根植竹节桩是一种新型组合桩基,桩端处存在的水泥土扩大头使其桩端承载性能优于传统桩基。通过模型试验以及ABAQUS三维建模计算对静钻根植竹节桩的桩端承载性能进行研究时发现,当桩端土体为砂土或黏性土时,预制竹节桩桩端处于水泥土扩大头中间位置附近时桩端极限承载力最大;当桩端持力层为岩石层时,竹节桩桩端与岩石层之间水泥土层厚度越小,桩端承载性能越好。由于现阶段静钻根植竹节桩主要用于东南沿海软土地区,实际工程中竹节桩桩端处于水泥土扩大头中间位置附近时桩端承载性能最好;桩端水泥土的内摩擦角、黏聚力以及弹性模量对静钻根植竹节桩桩端承载性能影响不大;增加水泥土扩大头的直径能够提高静钻根植竹节桩的桩端承载力。     

深层搅拌桩复合地基沉降计算理论研究

为了寻找深层搅拌桩复合地基沉降计算的简便方法，根据已有的研究成果，进行了桩与桩周土间摩擦力分布规律的假设和简化，运用单位元法推导了深层搅拌桩复合地基沉降计算公式和设计用曲线，同时采用轴对称有限元法，对一实际工程的实测成果进行了分析。计算结果表明：文中理论计算的中级荷载下的沉降量值介于分层总和法与有限元法计算值之间，但文中方法比有限元计算法简单，比分层总和法更合理。     

Nonlinear three-dimensional analysis of reinforced concrete piles subjected to horizontal loading

A three-dimensional finite element approach is proposed to predict the response of reinforced concrete piles to horizontal loading. This approach allows the nonlinear effects arising from the soil–pile interaction to be properly accounted for. In particular, the occurrence of plastic strains in the soil, concrete cracking and steel yielding in the pile as well as the occurrence of slip and gap at the soil–pile interface are reliably simulated using appropriate constitutive models. Another advantage of the present method is that it requires few material parameters as input data. In addition, these parameters can be readily obtained from conventional geotechnical and structural tests. The proposed approach is used to analyse the results from some loading tests documented in the literature concerning a large-diameter pile and a large-section rectangular pile (barrette) embedded in sandy soils. The theoretical results from these analyses are found to be in fairly good agreement with the experimental measurements available from the loading tests. Remarks of practical interest on the response of the structures considered to horizontal loading are also made.     

非均质地基中群桩竖向荷载沉降关系分析

运用剪切位移法计算了桩轴向荷载传递因子。对于桩端采用线性的荷载传递函数,推导了基于弹塑性模型的单桩竖向荷载沉降的解析解。分析过程中考虑了土体强度沿深度线性变化的特性和桩土间的滑移现象,因此更符合大部分土体的实际性状。在此基础上,建立了考虑桩土滑移的桩-桩相互作用系数的计算公式,并将上述方法应用于群桩的分析,获得了群桩的荷载沉降特性。该分析方法克服了目前应用较多的弹性理论方法夸大桩土相互作用的缺点,单桩和群桩的荷载沉降曲线的分析结果和实测数据吻合,证明了该方法的合理性。     

桩-土-承台共同作用的简化分析方法

从桩与承台下地基土共同分担荷载的基本条件出发,提出了一种从单桩荷载沉降曲线入手的桩-土-承台共同作用简化分析方法。在给定单桩荷载水平下,假定承台下桩间土的变形量等于此时单桩沉降量,求解相应的板底应力,从而可以得到任意桩荷载水平下的承台荷载分担量,而不必将桩的受荷水平限制在极限荷载。将基础沉降分为桩间土变形量及桩端下卧土层的整体压缩量两部分进行分析。桩间土变形量等于单桩在给定荷载水平下的沉降量,而桩端下卧土层的整体压缩量则由承台与桩两部分荷载引起,可以相对准确地预测基础沉降量。工程实例分析与实测结果的比较表明该方法是可行的。     

刚性桩复合地基沉降计算的简化方法

既简便又具有较好精度的刚性桩复合地基沉降计算方法仍是工程实践中迫切需要而又有待于解决的问题,文中提出了一个解决的简化方法。在刚性桩复合地基中,由于桩间土荷载水平不高,将桩间土荷载-沉降曲线近似为线性。当桩的荷载水平不高时,对桩的沉降可按线性考虑,线性关系可按线弹性方程计算得到。若桩可能进入非线性甚至塑性,则假设桩的荷载-沉降曲线满足双曲线规律,可较好地考虑桩的非线性沉降过程。通过计算单桩承载力特征值下的沉降,然后通过双曲线方程特点得到桩的非线性沉降方程。对于有单桩静载试验的情况,提出直接利用单桩试验曲线建立单桩的双曲线方程。最后依据共同作用时桩和桩间土的变形协调条件和静力平衡方程,即可计算其实际基础下复合地基的沉降,从而得到一个刚性桩复合地基沉降计算的简化方法。工程实例表明,简化是方法既简便又具有较好的精度,可为工程实践提供一个有效实用的计算方法。     

Wave absorbing-boundary method in seismic centrifuge simulation of vertical free-field ground motion

In this paper, an evaluation of the method of experimental seismic simulation of vertical free-field ground motion by wave-absorbing boundary and centrifuge modeling is presented. By means of a large soil container with Ductseal lining and an in-box shake-table system, a series of seismic tests on a sand stratum model of uniform density and a large width-to-depth ratio was conducted at multiple g-levels. With a focus on the vertical motion produced, the time-domain data was processed using the transfer function approach. By examining the measured resonant regimes for the vertical mode as a function of the g-level or length scale over the frequency spectrum, the capability of the Ductseal boundary approach in simulating vertical free-field motions in one-dimensional inhomogeneous site response theory is highlighted. In seeking a comprehensive basis of synthesis for the modeling methodology, the benefits of using the three-dimensional elastodynamic model in the interpretation of the vertical free-field measurements are demonstrated.     

基于影像源法的基坑开挖对邻近单桩影响简化分析

邻近建筑物进行基坑开挖会使桩基产生附加变形和内力,降低其承载能力,如果桩基变形过大会威胁到上部结构的安全。针对该领域目前存在的三维有限元数值模拟法建模复杂且计算耗时的现状,同时为了充分利用基坑围护位移较易通过现场监测技术获取的优势,提出了基于影像源法的基坑开挖引起邻近单桩变形影响的两阶段简化分析方法。同时,引入了Kerr地基模型,并针对Winkler地基模型进行改进,弥补了Winkler地基不能考虑土体连续性的缺陷。在第1阶段基于影像源法,由基坑围护变形计算基坑开挖引起的土体水平自由场位移;第2阶段分别基于Winkler和Kerr地基模型,将土体自由场位移施加于桩基,建立桩基在被动位移扰动下的微分控制方程,得到基坑开挖对邻近桩基影响的简化解析解,包括基坑开挖引起桩基的水平位移、弯矩和剪力等。将计算结果与既有理论结果、监测数据以及三维有限元数值模拟结果进行对比,取得了较好的一致性,其中基于Kerr地基模型的简化解比基于Winkler地基模型的简化解更为精确。该简化方法可为有效分析基坑开挖对邻近桩基的变形影响提供一定理论依据。     

深厚软土中抗滑桩的修正悬臂桩计算方法

大面积堆填或开挖时深厚软土地层内部常产生较大滑移,这为该类地层中考虑桩土作用的抗滑桩分析带来较大困难。考虑深厚软土的滑移性状,针对既有悬臂桩法计算存在的问题进行修正,滑动面上部桩身受荷段的桩身荷载采用等腰三角形分布且极值点为极限侧土压力,设滑动面下部桩身锚固段上侧桩周软土为理想弹塑性以考虑软土大位移条件,下侧为弹性状态,并通过位移叠加原理对传统方法求解产生滑动面不连续的缺陷进行修正。通过现场桩侧堆载试验验证,修正悬臂桩法的弯矩和位移计算结果较好,桩顶位移误差小于3%,桩身最大弯矩误差小于10%。所提方法有助于深厚软土地层抗滑桩的设计和计算。