Nonlinear calculating method of pile settlement

To study calculating method of settlement on top of extra-long large-diameter pile, the relevant research results were summarized. The hyperbola model, a nonlinear load transfer function, was introduced to establish the basic differential equation with load transfer method. Assumed that the displacement of pile shaft was the high order power series of buried depth, through merging the same orthometric items and arranging the relevant coefficients, the solution which could take the nonlinear pile-soil interaction and stratum properties of soil into account was solved by power series. On the basis of the solution, by determining the load transfer depth with criterion of settlement on pile tip, the method by making boundary conditions compatible was advised to solve the load-settlement curve of pile. The relevant flow chart and mathematic expressions of boundary conditions were also listed. Lastly, the load transfer methods based on both two-broken-line model and hyperbola model were applied to analyzing a real project. The related coefficients of fitting curves by hyperbola were not less than 0.96, which shows that the hyperbola model is truthfulness, and is propitious to avoid personal error. The calculating value of load-settlement curve agrees well with the measured one, which indicates that it can be applied in engineering practice and making the theory that limits the design bearing capacity by settlement on pile top comes true.

Behavior of raft on settlement reducing piles:Experimental model study

An experimental program is conducted on model piled rafts in sand soil.The experimental program is aimed to investigate the behavior of raft on settlement reducing piles.The testing program includes tests on models of single pile,unpiled rafts and rafts on 1,4,9,or 16 piles.The model piles beneath the rafts are closed ended displacement piles installed by driving.Three lengths of piles are used in the experiments to represent slenderness ratio,L/D,of 20,30 and 50,respectively.The dimensions of the model rafts are 30 cm×30 cm with different thickness of 0.5 cm,1.0 cm or 1.5 cm.The raft-soil stiffness ratios of the model rafts ranging from 0.39 to 10.56 cover flexible to very stiff rafts.The improvement in the ultimate bearing capacity is represented by the load improvement ratio,LIR,and the reductions in average settlement and differential settlement are represented by the settlement ratio,SR,and the differential settlement ratio,DSR,respectively.The effects of the number of settlement reducing piles,raft relative stiffness,and the slenderness ratio of piles on the load improvement ratio,settlement ratio and differential settlement ratio are presented and discussed.The results of the tests show the effectiveness of using piles as settlement reduction measure with the rafts.As the number of settlement reducing piles increases,the load improvement ratio increases and the differential settlement ratio decreases.

Residual settlement calculation of geocell cushion over gravel piles

The calculation of residual settlement of bidirectional reinforced composite foundation, which is composed of geocell cushion over gravel piles, was studied. The geocell cushion was modeled as a thin flexible plate with large deflection. Based on the Kirchhoff hypothesis, the governing differential equations and boundary conditions of the deformation of geocell cushion under working load were founded using von Karman method and solved by Galerkin method. On theses bases, the gravel piles and inter-pile soils were assumed as Winkler ground with variable spring stiffness so as to execute the approximate calculation of the residual settlement of the bidirectional reinforced composite foundation. The calculation method was verified by two laboratory experiments concerning settlement of embankments. One experiment was with just geocell cushion installed to treat the soft clay under embankments; another one was with both geocell cushion and gravel piles installed. The results show that the calculated settlement curve and the maximum settlement are closed to the observed ones.

Settlement mechanism of piled-raft foundation due to cyclic train loads and its countermeasure

In this paper, numerical simulation with soil-water coupling finite element-finite difference（FE-FD） analysis is conducted to investigate the settlement and the excess pore water pressure（EPWP） of a piled-raft foundation due to cyclic high-speed（speed： 300km/h） train loading. To demonstrate the performance of this numerical simulation, the settlement and EPWP in the ground under the train loading within one month was calculated and confirmed by monitoring data, which shows that the change of the settlement and EPWP can be simulated well on the whole. In order to ensure the safety of train operation, countermeasure by the fracturing grouting is proposed. Two cases are analyzed, namely, grouting in No-4 softest layer and No-9 pile bearing layer respectively. It is found that fracturing grouting in the pile bearing layer（No-9 layer） has better effect on reducing the settlement.

Numerical Analysis of the Settlement of Composite Foundation Reinforced with Crushed Stone Grouting Pile and Rigid Bearing Plate

This paper deals with a new type of crushed stone grouting pile with a rigid bearing plate. The load transfer characteristics were analyzed, and a settlement model of the composite foundation reinforced with crushed stone grouting pile and rigid bearing plate was built by FEM program. The effects of replacement ratio of capping plate, replacement ratio of pile, replacement ratio of grout diffusion zone, pile-soil modulus ratio, and serous-soil modulus ratio, on the composite foundation settlement were discussed. It is concluded that the proposed crushed stone grouting pile with a rigid bearing plate is effective in decreasing the settlement of composite foundation.

Settlement of composite foundation with discrete material pile considering modulus change

Based on deeply discussing the deformation mechanism of composite foundation with discrete material pile, firstly, the settlement of composite foundation in rigid foundation conditions was assumed to consist of two parts, an expanding part and an un-expanding part. Then, in view of the differences of deformation properties between the expanding part and the un-expanding part, the relationships between the pile modulus and the applied load in these two parts were respectively developed. Thirdly, by introducing the above relationships into settlement analysis, a new method to calculate displacement of composite foundation with discrete material pile was proposed by using the multi-stage loading theory and the layer-wise summation approach. This method is effective not only for accounting for the effect of variations of pores on deformation modulus of the pile body in different depths, but also for describing the characteristics of different deformation mechanisms of the pile body with varying depth. Finally, the proposed method was used to a practical composite foundation problem, whose theoretical results were presented and compared to those of other methods. The rationality and feasibility of this method are identified through comparative analysis.

Settlement Characteristics of Cement Fly-Ash GravelPile-Plate Composite Foundation of High-Speed Railway

Field tests on settlement characteristics were carried out on the cement fly-ash gravel （CFG） pile-plate composite foundation on Beijing-Xuzhou section of Beijing-Shanghai high-speed railway. The settlements of the piles and the soil between pries were measured and analyzed. The results show that the settlement-time dependency experienced three phases： rapid development phase, stable development phase and stable phase. Therefore, surcharge preloading was necessary to reduce the settlement after construction. The finite element software Plaxis was used to calculate the deformations of the pile top and the soil between piles at the embankment center, as well as the settlements of CFG pile reinforcement area and the underlying stratum under surcharge preloading. The calculation results and the field test results were compared and analyzed. Both the results show that the settlement of the composite foundation mainly occured in underlying stratum. The settlement characteristics of pile-plate composite foundation under high embankment are also concluded.

Effects of topography on dynamic responses of single piles under vertical cyclic loading

This paper describes model tests of single piles subjected to vertical cyclic compressive loading for three kinds of topography: sloping ground, level ground, and inclined bedrock. Comprehensive dynamic responses involving cyclic effects and vibration behaviours are studied under various load combinations of dynamic amplitude, mean load,frequency and number of cycles. Test results show that permanent settlement can generally be predicted with a quadratic function or power function of cycles.Sloping ground topography produces more pronounced settlement than level ground under the same load condition. For vibration behaviour,displacement amplitude is weakly affected by the number of cycles, while load amplitude significantly influences dynamic responses. Test results also reveal that increasing load amplitude intensifies nonlinearity and topography effects. The strain distribution in a pile and soil stress at the pile tip are displayed to investigate the vibration mechanism accounting for sloping ground effects. Furthermore, the dynamic characteristics among three kinds of topography in the elastic stage are studied using a three-dimensional finite method. Numerical results are validated by comparing with experimental results for base inclination topography. An inclined soil profile boundary causes non-axisymmetric resultant deformation, though a small difference in vertical displacement is observed.

秦望广场塔楼沉降后浇带提前封闭技术研究

研究目的:带超高层物业开发的大型地下综合体,为了不影响裙房和地下室提前投入使用,需要研究超高层建筑沉降后浇带提前封闭可行性,通过杭州富阳秦望广场通道工程实际项目,采用有限元整体建模分析,提出了残余沉降差计算方法,利用试桩静载试验P-S曲线确定了桩基线刚度参数,推导出沉降后浇带提前封闭楼层,通过实际监测数据,对理论计算结果进行对比验证。研究结论:(1)试桩线刚度与单桩承载力特征值比值在76~119之间,与传统经验值100倍单桩承载力特征值相比,试桩结果离散性较大;(2)塔楼总沉降量与楼层数关系不大,与桩基线刚度取值关系较大;(3)塔楼桩基采用相同线刚度,楼层数与总沉降量成正比;(4)塔楼沉降呈线性发展趋势,理论计算与监测数据基本一致,因成桩工艺控制不稳定,导致工程桩线刚度有离散性,理论计算沉降速率与监测相比均偏小,导致后浇带提前封闭楼层判断有误差;(5)本研究成果可为超高层建筑提前封闭沉降后浇带设计提供参考。

Study on Optimization of Design Parameters for Precast Pipe Pile Treatment of Soft Soil Subgrade on the Jianghan Plain

The soft soil in the Jianghan Plain is characterized by a high water content, liquid limit, sensitivity, porosity ratio, and organic matter content and low strength and is commonly known as “five highs and one low” soft soil. Thus, the construction of expressways in this area is likely to cause subgrade settlement. The manuscript investigated the influence of the design parameters of precast pipe piles on the soft ground treatment in the Jianghan Plain based on the case of the soft soil subgrade project of the Xiaogan south section of the Wuhan city ring expressway, China. Midas GTS NX 2019 finite element software was used to analyse the settlement pattern of the subgrade under the variations in pile length, pile spacing and pile diameter. The results show that precast pipe piles are effective in reducing the settlement of soft soil subgrades with a high compressibility and water content;the soft foundation settlement decreases with increasing pile length and pile diameter and decreasing pile spacing. As the pile tip is embedded in the bearing stratum, the settlement of the soft foundation is greatly affected by the variations in pile length and pile spacing while slightly influenced by the variation in pile diameter;in combination with the curve fitting obtained from the real-time monitoring data, the analysis concludes that the soft foundation treatment plan with a pile length of 15 m, pile spacing of 1.2 m to 1.5 m and pile diameter of 0.6 m can better contain the soft soil subgrade settlement in this section.

赛车道软土路基PHC桩负摩阻力中性点的时间效应

武汉赛车场跑道下伏的深厚软土层给桩基的设计与施工带来不小阻碍,桩基的负摩阻力及其中性点的确定对工程的稳定性具有非常重要的影响。为检验武汉赛车场跑道软土路基的预应力高强度混凝土管桩(PHC)设计的合理性以及为后续桩基改进方案的提供技术支持,选择典型地层开展了堆载作用下桩与桩间土的沉降监测,根据桩与土的相对沉降变化规律研究负摩阻力中性点的时间效应。结果表明,该场地的PHC桩的桩周土沉降约是桩沉降的6倍,具有明显的负摩阻力特性,桩周软弱土的下限深度为13.0 m;随着时间增加,中性点向上移动;中性点深度随时间变化具有三阶段特征,采用指数函数的中性点时间效应预测模型能较好反映中性点初始值、中性点变化规律与速率以及最终稳定值,与规范法比较,预测模型计算的中性点深度具有更高的准确性,由此得到了两个试验场地的中性点稳定值分别为4.43 m与5.72 m。研究还表明三向土工格栅能有效降低桩间土的沉降,也能降低桩-土的差异沉降,使中性点上移,减小负摩阻力,提高桩的承载力。本研究对了解负摩阻力对桩基性能发挥有一定的参考价值。

考虑桩底沉渣时沉降后浇带提前封闭时间计算方法:以秦望广场综合体工程为例

为了合理确定沉降后浇带提前封闭时间,开展了考虑桩底沉渣效应时沉降后浇带封闭时机研究工作。首先基于模型试验结果和秦望广场综合体工程的原位试桩试验结果,提出了考虑桩底沉渣效应的单桩沉降模型。基于该单桩沉降模型,根据施工过程中的常见工况组合,考虑了浮荷载对单桩刚度的改变,计算得到后浇带最早可封闭时间,并分析了沉渣厚度对后浇带可封闭时间的影响。模拟结果表明:当沉渣厚度符合设计要求时,恒载、活载、浮载组合工况引起的后浇带两侧差异沉降最大,但随着沉渣厚度的增大,恒载、活载组合工况逐渐转变为控制工况;当沉渣厚度较小时,所有工况下的差异沉降均呈线性发展,此时可以通过前期沉降观测值对后期变形进行预测;当沉渣厚度较大时,后浇带可封闭时间显著延后,甚至无法提前封闭。

基于桩-土界面剪切特性的单桩沉降和承载问题研究

近年来,西北地区出现了许多高填方场地,为减小建筑物基础的不均匀沉降,基础类型广泛使用桩基础。与一般场地不同,黄土填方场地中的单桩桩周土受力后仍会产生较大的变形,该类场地单桩沉降机制复杂。桩顶总沉降计算是桩基设计的重要依据,为此,建立了高填方黄土场地单桩桩顶总沉降计算模型。基于传统的荷载传递法和剪切位移法,分别考虑桩-土界面的桩-土相互作用和桩-土界面外剪切带土体的剪切变形。依据桩端边界,将单桩类型分为摩擦桩和端承摩擦桩,分别建立桩周土弹性阶段和塑性阶段的桩身位移控制微分方程,结合边界条件进行求解,得到桩身位移、轴力、侧摩阻力,并通过弹塑性理论求解了桩周土剪切带土体剪切变形,进而通过叠加原理求得桩顶总沉降。用桩长与桩周土塑性发展深度的比值,定义了桩基承载力安全系数K。通过算例分析与现场试验数据对比分析,研究结果表明:使用新的模型计算得到的桩顶总沉降与现场试验结果相近;当桩顶荷载较小、桩周土处于弹性阶段时,桩端边界对桩身轴力、位移和侧摩阻力影响很小,但桩周土进入塑性滑移阶段后,桩端边界的影响开始变大,考虑桩端土的承载能力会极大提高单桩极限承载力;建立了将荷载传递法和剪切位移法综合起来的计算模型,不仅可以考虑桩-土界面的相对滑移,还可以计算桩-土界面外土体剪切带的剪切变形,使得桩顶总沉降计算更加精确,可为类似场地中单桩沉降的分析与控制提供一定的参考。

砾砂地层盾构切削大直径群桩的刀具研究

刀盘刀具是盾构机实现切削功能并保证其顺利推进的主要部件,合适的刀具选型与设计在保证盾构机成功切削大直径钢筋混凝土群桩的过程中至关重要。依托沈阳地铁4号线盾构切桩工程,结合盾构穿越砾砂地层的工程特点以及切削大直径钢筋混凝土群桩的工程需求,对刀盘刀具进行选型,采用LS-DYNA软件模拟不同刃宽、刃角及前角的贝壳刀切削钢筋的效果。将分析结果应用于该工程,通过对比切桩前后既有隧道沉降变化规律论证刀具选型设计的合理性,并根据切桩后的贝壳刀磨损规律及盾构机排出钢筋情况,评估刀盘刀具切削效果,从而优化刀盘的刀具布置。研究结果表明:盾构刀盘采用面板+辐条复合式直角刀盘,切桩刀具采用前角-45°零后角、5 mm刃宽、90°刃角的双面刃贝壳刀,可提升盾构切桩效果;盾构切桩过程中既有隧道的变形波动控制在1.1 mm内,切桩完成后既有隧道的最大沉降仅为1.96 mm,未对既有隧道造成明显影响;刀盘布置贝壳刀时,应增加高度为210 mm的贝壳刀数量,尤其需要避免单个辐条上仅有1把高度为210 mm贝壳刀的情况,可有效提高刀盘刀具切削钢筋的能力并降低崩刃风险。刀具选型设计结果成功应用于沈阳地铁4号线切桩工程,可为今后类似工程提供参考。

高层建筑迫降法纠倾方案研究

建筑物因勘察、设计、施工或环境等不良因素影响,导致不均匀沉降,从而产生超过规范允许范围的倾斜。而高层建筑纠倾工程技术难度大、风险高。结合工程实例,通过对现有技术资料分析得出倾斜的原因,并根据工程实际情况,提出桩侧射水扰动迫降纠倾方案,对迫降法的原理、方案设计和施工工艺流程进行了介绍,可为类似工程提供参考。

某高层剪力墙结构不均匀沉降分析及处理

无新增荷载情况下的持续不均匀沉降常常发生在地基有缺陷的高层建筑中,该类沉降产生的破坏是缓慢持续的。地基不均匀沉降会导致建筑物开裂、倾斜,进一步发生结构安全事故。依据某高层剪力墙结构以及不均匀沉降监测数据,采用ABAQUS建立该高层剪力墙结构三维有限元计算模型,分析不均匀沉降工况下上部结构的内力分布及整体变形发展规律。结果显示,有限元计算得到的模型内力增大位置与实际外墙主要开裂位置相吻合,整体沉降变形发展规律与现场实测数据基本一致,证明上部结构三维有限元模型的可行性。进一步建立剪力墙结构-桩筏基础-地基耦合计算模型,开展不同筏板厚度对整体结构不均匀沉降的影响分析。同时结合实际案例制定“增设筏板+补桩”处理方法,加固完成后监测数据显示房屋处于稳定状态,变形与裂缝均无发展。

盾构隧道下穿对高速铁路CFG桩复合路基影响规律研究

针对城市地铁隧道下穿高速铁路的施工过程中,CFG桩复合路基对地层沉降的响应机理不明确、路基的沉降预测与控制理论缺乏的问题,依托郑州轨道交通8号线盾构区间下穿郑州东站咽喉区工程,提出CFG桩复合路基沉降预测方法 .基于地基加固理论,分析CFG桩对路基的加固效果,并通过物理模型试验,研究盾构隧道下穿对高速铁路CFG桩复合路基的影响规律.研究结果表明:CFG桩可以大幅提升路基压缩模量,盾构隧道下穿不会影响CFG桩复合路基的整体性,复合路基的变形规律符合Peck沉降曲线;CFG桩复合路基能够将地层的等效内摩擦角提高约50%,降低约23%的地层沉降量;盾构隧道下穿引起CFG桩复合路基的沉降量最大值为1.65 mm,在盾构隧道掌子面距离路基中线10 m时地层开始发生显著沉降变形,其中83.6%沉降量发生在盾构穿越阶段;盾构远离阶段,路基沉降量仍会继续增大,建议沉降监测持续到沉降稳定.研究结果可为盾构隧道下穿高速铁路CFG桩复合路基的沉降预测和加固处理提供参考.

考虑桩土间黏滑机制的桩基沉降动态特性研究

基于桩基运动控制方程,对荷载作用下的桩基沉降动态特性进行了研究。首先,考虑桩土间黏滞与滑动两种接触状态,建立了一种考虑侧阻软化及侧阻恢复的桩土相互作用模型;然后采用该模型及修正三折线模型分别模拟桩与桩侧土以及桩端土间的相互作用,结合有限差分法建立了考虑桩土间黏滑机制的桩基沉降分析方法;最后,基于建立的桩基受荷沉降分析方法,研究了桩基沉降的黏滑动态特性以及桩身的力学特性,并讨论了桩土相互作用模型中的参数对桩基沉降动态特性的影响。结果表明:当桩顶荷载接近桩基单级加载极限值,桩基沉降表现出黏滞位移与快速滑动位移交替的动态特性;桩身力学响应表现出加载路径相关性,与单级加载相比,分级加载时桩侧摩阻力能得到更好发挥,桩基极限承载力更高;提出的分析方法能够较好地模拟桩基静载试验沉降过程。

某高速公路深厚软基路桥过渡段跳车病害处治研究

结合某在役高速公路路堤深厚软基桥头跳车实测累计沉降量40.60 cm病害处治工程,开展室内土工试验、理论计算及GEO5模型计算分析。结果表明:15年内的累计剩余沉降量均超规范中桥头路基容许工后剩余沉降量不大于10.00 cm要求;提出“CFG桩+路面重铺”等处治方案,进行检算校核,计算得剩余总沉降量9.76cm,表明处治设计方案较为可靠。结合工点需保通及基底存在前期已施工CFG桩特点,制定通车条件下的施工顺序及工艺,确保方案可行。