Numerical investigation of dynamic behavior of composite foundation composed of soilbags and piles by 3D elastoplastic FEM

In this paper, a new composite foundation composed of piles and soilbags is proposed, and its mechanical behavior is investigated systematically with numerical analyses based on sophisticated models for both soils and structures. The foundation is characterized by the laying of soilbags between the pile heads and the footing on which the superstructure stands. The expected effects of the foundation are to cut off the fixed junction between the piles and the footing in order to reduce the bending moment of the piles and decrease the response acceleration of the structure. In this study, the seismic response of the proposed composite foundation is investigated numerically by a 3D elastoplastic FEM analysis, and the superiority of the proposed composite foundation over the conventional pile foundation is discussed in detail. From the analysis, it is found that both the bending moment and the response acceleration of the pier are suppressed because of the elongation of the natural period of the foundation and the increase in the hysteresis damping of the soilbags. In addition to the above-mentioned effect on the response acceleration, the bending moment of the piles is greatly reduced at the pile heads in the proposed composite foundation due to the effect of the stress dispersion of the soilbags.     

循环荷载作用下水泥土桩复合地基的临界循环应力比和永久变形分析

通过循环荷载作用下水泥土桩复合地基的室内模型试验,考虑复合地基的垫层、加固层以及下卧层的相互作用,研究了循环荷载作用下水泥土桩复合地基的竖向永久变形发展情况,考虑了复合地基的固结状态、循环应力比的大小及加荷频率对竖向永久变形的影响,得到了水泥土桩复合地基的临界循环应力比和竖向永久变形随加荷周数、加荷频率和固结状态而变化的规律,对于控制复合地基的工后沉降具有重要的参考价值。     

CFG单桩复合地基载荷试验的数值模拟研究

结合某实际工程,利用有限元分析软件ANSYS对CFG单桩复合地基载荷试验进行数值模拟并与载荷试验检测报告进行对比分析,得到了与试验结果相近的P—S曲线,从而证实了数值模拟的可行性,为进一步的研究分析提供了计算基础。     

CFG-PHC组合式长短桩复合地基试验研究

通过3组CFG-PHC组合式长短桩复合地基现场试验,研究复合地基承载力特征值、桩土应力比、应力分布、荷载分担比等性状。试验表明长桩的桩土应力比大于短桩;达到极限承载力后,短桩和长桩的应力比趋于稳定。在达到复合地基承载力特征值时,桩间土承载力发挥最多,长桩次之,短桩最少。建议用发挥度组合法计算组合式长短桩复合地基承载力,且长桩、短桩的承载力发挥度可分别取0.6～0.8、0.2～0.3,认为桩间土强度发挥度β可大于1,并建议取1.4～1.8。     

水平循环荷载下砂土中沉井加桩基础累积变形特性

 采用自主设计的水平循环加载装置进行模型试验,探讨单向、不完全对称及双向对称循环加载路径下沉井加桩复合基础及沉井基础的变形特征与位移预测模型。模型试验表明：在近一万次循环荷载作用下,基础的水平循环累积位移随循环次数的增大先增大后逐渐趋于稳定,呈两阶段变化特征,并且其大小受循环加载路径的影响较大,同时,与沉井基础相比,沉井加桩复合基础显著提高了基础抵抗水平循环变形的能力。最后,在对模型试验结果回归分析的基础上建立水平循环累积位移预测模型,并且其能较好地对不同循环加载路径下的基础变形进行预测。     

Evaluation of ultimate behavior of actual large-scale pile group foundation by in-situ lateral loading tests and numerical analysis

Structures resting on a liquefied natural gas (LNG) base must be completely stable during an earthquake in order to ensure a steady gas supply. Thus, the stability of the foundations on which these large-scale structures rest is one of the most significant factors for realizing the safety and ease of maintenance in the event of an earthquake. Problems can potentially occur when the actual bearing capacity of an in-service practical LNG tank foundation has not been verified, and when the operative design method for the pile-group effect is inadequate, especially for large-scale pile groups. To date, no prediction method capable of appropriately simulating the mechanical behavior of large-scale pile groups has been established. Therefore, in this study, in-situ lateral loading tests were conducted on a real-life practical tank foundation upon the demolition of an LNG tank after 40?years of service. A 3D elasto-plastic finite element method (FEM) analysis was also conducted. From the results, the bearing capacity and the failure level of the practical LNG tank foundation were investigated and its soundness was confirmed. In addition, the behavior of a large-scale pile group foundation for load share distribution was observed and compared with the operative design.     

波浪循环荷载作用下滨海软土水平向动力特性试验研究

为研究海上风电桩基在波浪荷载作用下,产生水平向循环荷载对桩基周围土体动力特性的影响,以唐山地区滨海软土为研究对象,通过室内动三轴试验,研究不同围压、动应力幅值和振动次数条件下对软土水平向动力特性的影响。结果表明:软土水平向动强度随围压增加而增加,随振动次数增加而减小;动应力幅值增大,破坏振次减小;水平向动应变εd随振动次数增加变大,且动应力幅值越大,增速越明显,变化规律遵循Monismith模型;动应力幅值改变时,软土水平向动模量变化明显,当围压减小,动弹性模量减小;曹妃甸软土水平向间具有明显的结构性,不同围压条件下,随动应力幅值增加动阻尼比均表现增大趋势。     

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.     

海洋高桩基础水平单调及循环加载现场试验

开展了海洋软黏土中 2 根大直径高桩基础的现场水平单调和循环加载试验,实测获得了桩顶荷载 – 位移关系、桩身变形和桩身弯矩及桩侧土压力和孔隙水压力,揭示了水平单调和循环荷载作用下桩土相互作用规律及桩基水平位移和桩身弯矩发展规律。利用实测桩身水平位移推算了桩周土反力,在此基础上提出了相应的双曲线型 p – y 曲线,通过引进 Poulos 循环弱化模型建立了水平循环荷载作用下的桩基双曲线型 p – y 曲线分析模型,水平单调及循环荷载作用下桩顶荷载 – 位移关系、桩身变形和桩身弯矩及桩侧土压力等计算结果与实测值均吻合良好。通过现场试验发现规范 p – y 曲线法计算结果偏保守的主要原因是所采用的 p – y 曲线的刚度偏小;不同时段的循环荷载对桩基循环累积变形有叠加效应。 建议设计中应考虑桩基全寿命服役期内所承受的所有循环荷载的影响, 对于重要工程应开展相应的现场水平加载试验,实测桩身水平位移或桩身弯矩,进而利用所推算的桩周土反力来分析桩基受力变形及承载力。     

复合地基桩土应力测试方法研究

传统复合地基试验测量方法是利用压力传感器对复合地基直接进行测量,存在试验费用和精度等问题。提出了一种新的复合地基试验方法,放弃了传统的直接测量桩体和土体的应力,转而测量载荷板在复合地基反力作用下的应变,然后运用一种基于板壳理论的半解析半数值的数学方法,有效地将应变值转换为所要测量的复合地基反力。该方法能够弥补传统方法的不足,可以较直观得出在荷载作用下桩和桩间土上的应力分布情况,为分析复合地基的力学性能提供了有力支持。     

碎石垫层沉箱基础水平循环加载模型试验与分析

碎石垫层沉箱基础是一种适于强震区深厚软弱土地基的深水基础形式,具有抗震性能良好、承载力高、便于施工、造价合理等优点。采用自主设计的大周数水平循环加载装置对碎石垫层沉箱基础开展了室内1g模型试验研究,探讨了基础在水平静力加载、循环加载以及循环后静力加载下的承载变形特性。试验研究表明,基础在长期水平循环荷载作用下具有安全可靠的工作性能。静载下基础的荷载–水平位移关系可视为刚塑性、滑动破坏前为双曲线关系。循环荷载下基础的累积位移发展呈现收敛的趋势,并随着荷载幅值的增大而增大;基础在单向循环荷载下的累积位移大于同等大小荷载静载下的位移。循环加载后基础沿各方向的水平承载力和刚度均有所提高。然后,对基于边界面理论的修正G-M弹塑性接触面本构模型进行改进,利用ABAQUS接触面子程序UINTER将其嵌入ABAQUS中,对碎石垫层沉箱基础的模型试验进行了有限元数值分析,模拟得到的规律与试验结果相近。     

水平旋喷拱棚新工艺与载荷试验研究

基于国内水平旋喷技术存在成桩质量差、加固体周围岩土体变形大等问题,以中国京冶工程技术有限公司开发的全方位高压喷射注浆新工法及相应设备为依托,进行了水平旋喷拱棚现场新工艺和载荷试验。新工艺现场施工过程中,孔口止浆器止浆效果良好;养护28 d后隧洞开挖显示,旋喷桩搭接紧密,开挖长度内无搭接裂缝,施工效果良好;清除上覆土层后,可见旋喷桩质地均匀,纵横向搭接良好,桩–桩咬合尺寸约0.2 m;返浆试块的抗压强度约为取芯试块的60%。拱棚结构载荷试验表明,Q–s曲线相对平稳;拱棚结构的破坏主要是由于旋喷桩体搭接处产生裂缝并不断扩展,最终贯通引起的剪切破坏。通过数值模拟与实测值的对比,拱棚结构的竖向位移和水平位移均随荷载的增大而增加,最大位移值均出现在拱顶位置处。     

薄钢板剪力墙结构水平反复荷载试验研究

给出了薄剪力墙板拉力带对周边框架梁、柱产生的作用力计算公式,以此设计了试件的原型结构。薄墙板采用建筑结构胶和高强度摩擦型螺栓固定在焊于框架梁柱的角钢连接板上。对三层薄钢板剪力墙结构进行水平反复荷载作用下的试验研究,结果表明:墙板先屈曲,后屈服,屈曲后能继续承受荷载,并形成多条非常明显的斜拉力带;二层墙板最先出现屈服,底层墙板屈服时间最晚;试件破坏时,钢框架柱脚及梁柱连接部位形成了塑性铰,柱脚翼缘严重扭曲,腹板向外鼓曲。墙板发生很大的平面外凹凸变形,并有多个裂口,且在靠近角部梁端处出现撕裂。     

半刚性连接梁柱组合节点低周反复荷载试验研究

为研究半刚性连接梁柱组合节点的抗震性能,开展了1个纯钢框架梁柱中节点、2个组合框架梁柱中节点的低周反复荷载试验。梁柱采用平齐式高强螺栓端板连接,试验采用柱顶加载模式。对试件的试验现象、变形、应变和耗能能力等进行了分析和比较。试验结果表明,组合节点相对于纯钢节点而言,由于负弯矩作用下钢筋的抗拉作用和正弯矩作用下混凝土板的抗压作用,其受弯承载力和转动刚度均有较大程度地提高;连接弯矩-转角关系的滞回曲线饱满、稳定,具有良好的耗能能力;平端板连接梁柱组合节点在反复荷载作用下具有较高的承载力和良好的延性,抗震性能良好。     

Numerical evaluation of group-pile foundation subjected to cyclic horizontal load

In this paper, three-dimensional (3D) finite element analyses of a real-scale group-pile foundation subjected to horizontal cyclic loading are conducted using a program named DBLEAVES. In the simulations, nonlinear behaviors of ground and piles are described by subloading t_ij model and the axial-force dependent model (AFD model) which considered the axial-force dependency in the nonlinear moment-curvature relations. In order to consider the influence of an effective stress path on the prediction of the group-pile foundation, the analyses are conducted within the framework of the soil-water coupling method with finite-difference and finite-element regime. The material parameters of soils are determined based on conventional triaxial drained compression tests on undisturbed and remolded specimens. The applicability of the proposed numerical method is encouraging, and therefore, it is quite confident to say that the method can be used to predict the mechanical behaviors of group-pile foundation to a satisfactory accuracy, particularly with the effective stress analysis.     

小型桩基竖向循环加载模型试验系统研制与应用

介绍了一种小型桩基竖向循环加载模型试验系统,包括加载系统、压力室、起吊装置、模型桩、数据采集系统。该系统通过伺服电机驱动联轴器带动滚柱丝杠转动,进而带动加载板对模型桩施加竖向位移荷载;通过水压加载法对土样进行围压加载,模拟不同深度土层的应力状态及固结情况。可针对不同固结状态的地基土和多种桩基型式,开展不同荷载组合下桩基竖向循环加载模型试验研究,适用于饱和软土、一般黏性土、粉土、砂土等均质或非均质地基。应用该设备进行了单桩竖向循环加载模型试验,并与数值模拟进行对比。研究结果表明,动荷载幅值、地基土固结压力对桩基承载力特性影响很大;动荷载幅值较小时,桩基承载力几乎没有弱化;地基土固结压力升高时,桩基承载力提高显著,桩基承载力弱化速度减慢;随着振次及振幅的增加,桩顶轴力逐渐弱化至残余值;位移循环荷载作用下,桩周土的弱化导致桩基产生了负摩阻力,进一步降低了桩基承载力。模型试验结果与数值模拟结果吻合较好。通过初步应用,证明了该试验系统弥补了常规1g小比例尺模型试验中低围压的不足,可以较好地反应出桩顶荷载和位移的非线性关系和承载力循环弱化现象。同时该系统输出荷载波形精确,量测系统灵敏度高、稳定性好...     

PHC管桩水平承载特性现场试验研究

针对PHC管桩水平承载特性研究的不足,以泗许高速亳州段工程为依托,对管桩水平载荷进行了试验,研究了PHC管桩的水平承载特性,得到了PHC管桩的水平承载力大小以及管桩在水平荷载作用下的变形规律。     

高承载素混凝土桩复合地基现场试验研究

某高层建筑采用素混凝土桩复合地基。基坑开挖后,进行了多项原位载荷试验以及桩、土应力测试;在建造过程中,进行了沉降观测,并对复合地基应力变化做了跟踪测试。基于试验数据,对复合地基的工作机理、荷载分担特性、桩身应力分布特征、基础沉降特点等进行了分析,并探讨了桩底后注浆工艺对复合地基工作性状的影响。研究表明：素混凝土桩复合地基具有高承载力和低变形的性能;非挤土成桩工艺增强体,对桩间土也具有加固效果;采用桩底后注浆工艺,可提高复合地基承载力、减小变形,加固效果在高荷载水平下较好;基础底板及上部结构刚度对桩的不均匀受力具有一定的调整能力。提出了根据荷载和地基反力分布状况,采取针对性的变刚度调平措施的建议。     

某铁路CFG桩复合地基试桩静荷载试验与分析

通过对龙厦铁路某里程段CFG桩复合地基试桩静荷载试验,确定了该里程段的复合地基承载力特征值,对试验过程和结果进行了分析,试验结果显示,该CFG桩单桩复合地基承载力满足设计要求,为施工过程提供了科学依据和数据参考。