Finite element analysis of tunnel–soil–pile interaction using displacement controlled model

Significant additional loads could be induced in pile foundations adjacent to new tunnels. Accurate prediction of magnitude and shape of the ground displacements, which define curvature changes, is crucial for the computation of tunnelling induced bending and axial stresses in pile foundations. The finite element simulation of tunnelling by removing forces corresponding to initial stress-state, tend to predict incorrect shape of ground displacement profiles, hence incorrect forces in pile foundations adjacent to tunnels. To overcome this difficulty, this paper describes the development and application of a simple and useful displacement controlled model (DCM) to predict the effects of tunnel excavation on adjacent pile foundations. The DCM simulates tunnelling by applying displacements to the tunnel boundary. A method to determine magnitude and direction of tunnel boundary displacements, based on convergence patterns observed in field and centrifuge test results, is proposed. Back analyses of numerous greenfield tunnel case histories using the DCM indicate good agreement between computed displacement profiles and field/test data. The suitability of the DCM in modelling tunnel–soil–pile interaction problems is demonstrated through back analysis of a centrifuge test and a field case study.     

Effects of tunnelling on pile foundations

An efficient analysis method is presented for estimating the effects induced by tunnelling on existing pile foundations. The method is based on a two-stage procedure: (1) an estimate of the free-field ground movements caused by the tunnel excavation, and (2) an analysis of the pile group subjected to the computed free-field ground movements. The first step may be carried out using alternative approaches, ranging from empirical methods to 3D numerical analyses. The second step is performed by PGROUPN, a computer program for pile-group analysis based on a non-linear boundary element solution. The validity of the approach is assessed by comparing it with alternative numerical solutions and field measurements. The results indicate that the method is capable of generating reasonable predictions of pile response for many cases of practical interest, thus offering substantial cost savings over a complete 3D analysis of tunnel–soil–pile interaction.     

广州地铁西村站近接高架桥桩基影响分区及应用研究

 广州地铁西村站为暗挖法施工的隧道群,近接9组高架桥桩基,其中隧道与桩XJ33最小净距仅2.30 m。为保证高架桥的使用安全性,采用有限差分法和最小二乘法原理,对近接高架桥桩基进行近接影响分区研究,并根据分区结果对桩基进行分类,确定各类桩基沉降集中区,并对其提出可靠的加固方案;同时建立西村站近接桩基监控量测管理等级。研究结果表明：西村站隧道洞室开挖对近接高架桥桩基的影响可分为4个区,相应地把近接桩基分为4类;A类桩的桩基沉降集中区为掌子面前2.0D(D为隧道洞径)及掌子面后3.0D区段,B类桩为掌子面前1.0D及掌子面后3.0D区段,C类桩为掌子面前0.5D及掌子面后2.0D区段;西村站范围内的9组桩基均为A类桩,其中危险桩为XJ25,XJ32,XJ34。通过现场应用,有效地控制了西村站隧道群开挖对近接高架桥桩基的影响,保证了施工安全,积累了成功经验,可为相似工程施工提供参考。     

盾构隧道下穿桥梁引起桩基变位的数值分析

依托某地铁盾构隧道下穿既有桥梁桩基工程,考虑实际的工程地质水文地质条件、上部桥梁结构传递到承台顶的荷载、盾构设计及施工参数等因素的影响,建立FLAC3D数值计算模型,模拟盾构隧道顺桥向穿越桥梁桩基的全过程,对两种不同的桩基加固方案条件下地表沉降和桩身变形规律进行了分析。研究结果表明:隧道开挖引起桩的挠曲,桩身的水平位移随桩洞距离增大而减小;后开挖侧的桩身位移比先开挖侧大;桩和承台约束了地表的沉降。     

Setzungen und Beanspruchungen bei Gründungen auf Pfahlgruppen

Settlements and pile resistance of foundations by pile groups. Pile foundations are a commonly used type of deep foundations. It is well known that piles arranged in a group can affect each other and influence their resistance‐settlement behaviour. At present no satisfactory simplified approach that considers the group effect is available for pile groups. Therefore, the aim of this paper is to develop a simple design approach for pile groups which take into account the partial safety concept according to DIN 1054:2005‐01. The design approach is presented in form of design charts, which are developed based on extensive numerical parameter studies complemented by the results of extended analytical methods. With this design approach, the average settlement of a pile group as well as the distribution of the different pile resistances within the pile group can be predicted with due consideration of the group effect.     

A simplified analysis method for the influence of tunneling on grouped piles

One of key issues of tunneling in urban areas is to assess the likely impact on adjacent piled buildings of tunnel construction. Simple and reliable predictions of tunneling-induced bending and axial stresses in pile foundations are important to the safety of tunneling. In this paper, a simple two-stage analysis method for determining the response of pile groups caused by tunneling was presented. At the first stage, an analytical solution proposed by Loganathan and Poulos [Loganathan, N., Poulos, H.G., 1998. Analytical prediction for tunneling-induced ground movement in clays. J. Geotech. Geoenviron. Eng., ASCE 124 (9), 846–856] is used to estimate the free-field vertical and lateral soil movements induced by tunneling. At the second stage, assuming no slippage at the soil-pile interface, the Winkler model is first adopted for simulating the pile-soil interaction, combined with finite difference method in the case of multi-layered soils. Then, shielding effect is considered for the interaction between two passive piles using a logarithmic attenuation function suggested by Randolph and Wroth [Randolph, M.F., Wroth, C.P., 1979. Analysis of the vertical deformation of pile groups. Géotechnique 29 (4), 423–439] for vertical response and Mindlin’s solution for lateral response. Finally, the response of a passive pile group due to tunneling is obtained by the superposition principle. Solutions obtained by the proposed approach for the analysis of single piles and piled groups subjected to ground movements induced by tunneling are compared with those using the boundary element program GEPAN. Comparisons are also made between the observed behavior of centrifuge model tests as well as field measurements and those computed by the proposed method. It is demonstrated that the present method can in general give a satisfactory prediction of the response of passive piles subjected to tunneling.     

Lateral response of pile foundations in liquefiable soils

Liquefaction has been a main cause of damage to civil engineering structures in seismically active areas.The effects of damage of liquefaction on deep foundations are very destructive. Seismic behavior of pile foundations is widely discussed by many researchers for safer and more economic design purposes. This paper presents a pseudo-static method for analysis of piles in liquefiable soil under seismic loads. A freefield site response analysis using three-dimensional(3D) numerical modeling was performed to determine kinematic loads from lateral ground displacements and inertial loads from vibration of the superstructure. The effects of various parameters, such as soil layering, kinematic and inertial forces,boundary condition of pile head and ground slope, on pile response were studied. By comparing the numerical results with the centrifuge test results, it can be concluded that the use of the p-y curves with various degradation factors in liquefiable sand gives reasonable results.     

Group effect on ultimate lateral resistance of piles against uniform ground movement

In earthquake engineering, pile foundations are designed to withstand the lateral loading that results from large displacements due to ground movement caused by strong earthquakes. The distress and failure of superstructures occurs when the lateral load exceeds the ultimate lateral resistance of the piles. The aim of this study is to estimate the ultimate lateral resistance of piles especially in terms of the group effect induced by the pile arrangement. Several experimental and numerical analyses have been conducted on pile groups to investigate the group effect when the groups are subjected to uniform large horizontal ground movement. However, the ultimate lateral resistance of the pile groups in these studies was calculated by applying load to the piles. The present study directly assesses the ultimate lateral resistance of pile groups against ground movement by systematically varying the direction of the ground movement. Although the load bearing ratio of each pile in a pile group, defined as the ratio of the ultimate lateral resistance of each pile in a pile group to that of a single pile, is an important design criterion, it was difficult to assess in past works. This study focuses on the load bearing ratio of each pile against ground movement in various directions. The use of the finite element method (FEM) provides options for simulating the pile-soil system with complex pile arrangements by taking the complicated geometry of the problem into account. The ultimate lateral resistance is examined here for pile groups consisting of a 2?×?2 arrangement of four piles, as well as two piles, three piles, four piles, and an infinite number of piles arranged in a row through case studies in which the pile spacing is changed by applying the two-dimensional rigid plastic finite element method (RPFEM). The RPFEM was extended in this work to calculate not only the total ultimate lateral resistance of pile groups, but also the load bearing ratio of the piles in the group. The obtained results indicate that the load bearing ratio generally increases with an increase in pile spacing and converges to almost unity at a pile spacing ratio of 3.0 with respect to the pile diameter. Moreover, the group effect was further investigated by considering the failure mode of the ground around the piles.     

刚性桩复合地基设计与实测研究

 桩土应力比与沉降是复合地基设计的2个重要参数,针对复合地基设计中难以考虑位移协调的情况,通过对地基基础共同作用方程进行简化,建立考虑桩土位移协调的设计分析方法,并结合广东地区3个不同类型刚性桩复合地基上建筑物在施工期间的沉降与桩土应力监测实例,得到桩土荷载分担以及沉降的变化规律。实测结果表明,所采用的地基承载力和沉降计算方法较好地反映了工程实际状况,比规范方法更加接近实际情况,研究成果可供今后类似工程设计参考。     

深圳地铁隧道邻接桩基施工力学行为研究

针对深圳地铁新建隧道邻接既有桩基的地铁工程 ,进行了三维有限元数值模拟的施工力学行为研究。研究结果表明 :与无桩情况相比较 ,邻接桩基施工将引起新建隧道自身结构 ,特别是与既有桩基邻接一侧边墙不利的受力状况和变形特征 ;并且 ,邻接施工还将引起既有桩基产生偏向隧道水平方向的“拉伸”变形情况。因此 ,需要采取加固措施 ,以确保隧道自身结构以及邻接地下“建筑物”的安全。此外 ,考察不同刚度折减系数时 ,发现桩基水平侧向变形几乎没有变化 ,这表明改变盾构管片的拼装方式 ,在整体上对邻接地下“建筑物”几乎无影响     

盾构法施工对近距离侧穿桥梁桩基的影响分析

盾构法施工地铁隧道近距离侧穿高速公路桥梁桩基时,引起地层移动和应力调整,导致桩基位移和内力发生变化,给上部结构带来安全隐患。以杭州地铁3号线工大站—留和站盾构区间双线施工为依托,运用三维有限元软件模拟盾构开挖施工的全过程,研究开挖过程对地层沉降及邻近桥梁桩基影响规律。结果表明,先行隧道开挖导致地表形成沉降槽,后行隧道开挖沉降曲线向后行线扩展;桩基竖向呈现刚体位移,单线开挖时在横向(Y方向)上嵌入土体桩基上半部分向隧道内倾移,下半部分背离隧道方向倾移,在纵向(X方向)上桩基呈现拱形弯曲,双线开挖时桩基横向位移发生反向叠加效应,导致最终横向位移基本接近初始状态,纵向上弯曲位移发生正向叠加效应;双线隧道先后开挖使桩基产生附加摩阻力和附加轴力,在隧道顶面分界线以上桩基总侧摩阻力较初始状态不断减小,分界线以下增加,位于-2.5 m以上桩基轴力较初始状态减小,以下增加;单线开挖时桩基弯矩变化明显,双线开挖弯矩出现反向叠加效果,基本保持初始状态。     

Ground Behavior Due to Tunnel Excavation with Existing Foundation

Two-dimensional (2D) and three-dimensional (3D) model tests of tunnel excavation with nearby existing foundation are carried out to investigate the influence of the existing foundation due to the interaction between ground and the existing structures. Three types of foundations: flat foundation, group-pile foundation and piled raft are considered. 2D and 3D finite element analyses using subloading t_ij model are also conducted. The deformation mechanism and distribution of earth pressure during tunnel excavation in the ground with nearby foundation are found to be different from those of green field condition. Surface settlement trough due to tunnel excavation in the ground with existing foundation does not follow the usual pattern of a Gaussian distributive curve, which can be observed in the case of green field. Especially, in the case of pile foundation, D_p, the distance between pile tip and tunnel is an important factor for the ground deformation and surface settlement. For a short distance D_p, although the length of pile is long, the ground deformation is concentrated at a place near the front pile and the rotation of foundation becomes larger. The maximum surface settlement in the case of existing foundation is also larger than those in the case of green field. Due to the existing foundation, unsymmetrical distributions of earth pressure occurred at the bottom of the ground due to tunnel excavation, both in model tests and numerical analyses. The earth pressure at the crown of tunnel in the case of existing foundation is almost the same as those in the case of green field. The arching at the shoulder of tunnel in the case of existing foundation, however, is much larger than those in the case of green field due to the dead load exerted on the foundation. The numerical results agree well with the results of the model tests.     

静压桩的静压力施加技术研究与应用

对承重墙下条形基础加固工程中的静压桩技术进行了讨论,通过工程实例指出了在砌体危房加固工程中,当由于软弱地基上墙下条形基础沉降过大时,可优先采用本文改进的静压桩静压力技术加固承重墙下基础.     

One-dimensional wave equation analyses for pile responses subjected to seismic horizontal ground motions

This paper presents a numerical one-dimensional wave equation analysis technique for piles and pile groups subjected to seismic horizontal ground motions in liquefiable zones. The so-called Earthquake Wave Equation Analysis for Piles (EQWEAP) procedure is introduced for piles subjected to horizontal earthquake excitations. Disregarding the effects of kinematic soil–pile interaction, the seismic responses of piles can be obtained by approximating the free-field ground response analysis, the ultimate earth pressure model, and the ground displacement profiles. The nonlinearities of the concrete piles were modeled using the approximate tri-linear moment–curvature relationships. A case study and application concerns were presented. Although the analysis is in one dimension, it is found to be effective and able to provide a rapid estimation in foundation design when seismic pile behaviors are of interest. The advantages of this analysis are the time efficiency of the seismic design of pile foundations and the relative simplicity of the analysis. In addition, it suggests alternative modeling for the dynamic analysis adopting the commonly known static models and/or methods.     

Mechanical Behavior of Pile Foundation Constructed in Composite Ground and its Evaluation

This paper describes a foundation design method in which the ground is improved around the heads of pile foundations in soft ground or loose sandy ground and its practical effectiveness. The shear strength increased due to ground improvement is reflected in the horizontal resistance of piles. In this design method, the influence range of the horizontal resistance of piles and the necessary range of ground improvement are determined by taking account of three-dimensional domain formed with the gradient of the surface of passive failure. The horizontal subgrade reaction of piles is evaluated by converting the shear strength of improved ground to the modulus of deformation. In this study, the validity of design method for the pile foundation with ground improvement was confirmed through an in-situ horizontal loading test. The dynamic behavior of pile foundation constructed in improved ground was also investigated through a series of centrifuge model tests and numerical analyses. The influence of the difference in strength between the original and improved grounds on piles during an earthquake was also confirmed based on the numerical analyses. The cost performance of the proposed method was discussed by comparing with the case without ground improvement.     

群桩地基中应力及变形规律数值分析

采用基于有限元法的“数值分析试验” ,研究了桩基的荷载传递规律 ,分析了单桩和群桩基础中地基的应力场和变形场的分布 ,为深入了解群桩的荷载传递特性及变形特性提供依据     

双隧道不同开挖顺序对临近群桩的影响研究

双隧道不同开挖顺序对临近群桩承载性能的影响鲜有报道。针对此问题,采用基于地层损失比的位移控制有限单元法(DCM),对软土地基中不同埋深双隧道不同开挖顺序对处于工作荷载下群桩工作性能的影响进行研究,并与相关离心模型试验结果进行比较。得到以下结论:双隧道不同开挖顺序对群桩桩顶附加沉降和群桩承载能力损失影响差别显著,当先开挖上覆隧道时,群桩桩顶的附加沉降量为先开挖下置隧道时的1.25倍,且群桩承载能力的损失率约为后者的1.2倍;两工况中第二个隧道的开挖使得群桩中前桩附加弯矩和后桩附加弯矩均有明显的增大,此与分居群桩两侧但埋深相同的双隧道开挖对群桩弯矩的影响规律迥异;两工况下均产生较大的附加弯矩和附加轴力,且最大附加弯矩和最大附加轴力均位于上覆隧道轴线附近。     

深基坑施工对高架基础的变形影响及控制研究

以上海某下穿高架并紧邻高架基础的地铁车站深基坑为背景,采用简化分析、三维数值模拟及原位监测等手段,对复杂环境下深基坑施工对邻近高架基础的变形影响及变形控制措施进行研究。采用基坑开挖环境影响简化分区图并根据保护对象与基坑的相对位置关系,确定高架桩基的影响类型。根据混凝土结构裂缝控制要求,采用等代荷载法普通梁模型建立由桩身应力控制的桩基及基坑围护结构变形控制指标。通过建立三维整体计算模型,动态预测基坑围护结构及高架桩基的变形性态。主要监测结果与上海地区其他工程的对比分析表明,本工程采取的基坑围护结构及支撑体系调整、坑内地基加固及结构局部逆作等一系列变形控制措施,不但有效控制基坑自身的变形,也有效保护邻近高架基础的安全。     

Physical modeling of behaviors of cast-in-place concrete piled raft compared to free-standing pile group in sand

Similar to free-standing pile groups, piled raft foundations are conventionally designed in which the piles carry the total load of structure and the raft bearing capacity is not taken into account. Numerous studies indicated that this method is too conservative. Only when the pile cap is elevated from the ground level, the raft bearing contribution can be neglected. In a piled raft foundation, pile–soil–raft interaction is complicated. Although several numerical studies have been carried out to analyze the behaviors of piled raft foundations, very few experimental studies are reported in the literature. The available laboratory studies mainly focused on steel piles. The present study aims to compare the behaviors of piled raft foundations with free-standing pile groups in sand, using laboratory physical models. Cast-in-place concrete piles and concrete raft are used for the tests. The tests are conducted on single pile, single pile in pile group, unpiled raft, free-standing pile group and piled raft foundation. We examine the effects of the number of piles, the pile installation method and the interaction between different components of foundation. The results indicate that the ultimate bearing capacity of the piled raft foundation is considerably higher than that of the free-standing pile group with the same number of piles. With installation of the single pile in the group, the pile bearing capacity and stiffness increase. Installation of the piles beneath the raft decreases the bearing capacity of the raft. When the raft bearing capacity is not included in the design process, the allowable bearing capacity of the piled raft is underestimated by more than 200%. This deviation intensifies with increasing spacing of the piles.     

Development of tunneling influence zones for adjacent pile foundations by numerical analyses

In this study, three-dimensional elasto-plastic numerical analyses are conducted to investigate the influences of tunnel excavation on existing loaded piles. The effects of parameters on pile responses to be used as criteria for suggesting the influence zones can be captured from the analysis method. A large number of analyses are then carried out to generate the artificial data of pile responses from tunneling for establishing the influence zones. Deep and shallow tunneling are separately considered with their influence zones are suggested from the normalized pile settlement and the induced pile bending moment, respectively. The two zones are then combined. Based on the ground condition, geometries of the pile and the tunnel, tunnel construction method, and evaluated criteria considered in this study, the unique significant zones of influence for various conditions can be identified.