抗滑桩水平地震作用修正系数分析——汶川地震典型抗滑桩破坏工点分析

桩板墙是一种重要的新型支挡结构,已被广泛应用于边坡加固。过去规范中针对桩板墙采用的设计方法主要是拟静力法,较少涉及动力设计方法。选用汶川地震中的典型抗滑桩板墙工点,利用有限元软件ABAQUS对其进行分析。计算中选取"5.12"汶川地震卧龙台站记录作为激励,分析得到抗滑桩的弯矩、剪力、土压力分布等,并把分析结果与规范结果进行比较。比较结果表明,当采用规范计算时,地震综合影响系数应取0.4才能使工程满足设计要求。在此基础上,将汶川波的PGA(峰值地震动加速度)折换到7度(0.1g)、8度(0.30g)、9度(0.40g),分别对抗滑桩进行分析,将分析结果与规范对比发现,当按照规范计算时,地震综合影响系数分别取0.30(7度)、0.35(8度)、0.40(≥9度)时,能保证工程设计要求。     

Reliability of design approaches for axially loaded offshore piles and its consequences with respect to the North Sea

In the near future, several offshore wind farms are planned to be built in the North Sea. Therefore, jacket and tripod constructions with mainly axially loaded piles are suitable as support structures. The current design of axial bearing resistance of these piles leads to deviant results regarding the pile resistance when different design methods are adopted. Hence, a strong deviation regarding the required pile length must be addressed. The reliability of a design method can be evaluated based on a model error which describes the quality of the considered design method by comparing measured and predicted pile bearing resistances. However, only few pile load tests are reported with regard to the boundary conditions in the North Sea. This paper presents 6 large-scale axial pile load tests which were incorporated within a new model error approach for the current design methods used for the axial bearing resistance, namely API Main Text method and cone penetration test (CPT)-based design methods, such as simplified ICP-05, offshore UWA-05, Fugro-05 and NGI-05 methods. Based on these new model errors, a reliability-based study towards the safety was conducted by performing a Monte-Carlo simulation. In addition, consequences regarding the deterministic pile design in terms of quality factors were evaluated. It is shown that the current global safety factor (GSF) prescribed and the partial safety factors are only valid for the API Main Text and the offshore UWA-05 design methods; whereas for the simplified ICP-05, Fugro-05 and NGI-05 design methods, an increase in the required embedded pile length and thus in the GSF up to 2.69, 2.95 and 3.27, respectively, should be considered to satisfy the desired safety level according to DIN EN 1990 of β = 3.8. Further, quality factors for each design method on the basis of all reliability-based design results were derived. Hence, evaluation of each design method regarding the reliability of the pile capacity prediction is possible.     

重庆市嵌岩桩单桩承载力可靠指标分析

对重庆市近10年102根嵌岩灌注桩试桩资料进行统计分析,建立单桩无量纲随机变量的极限状态方程,采用改进JC法和Monte-Carlo法计算得到重庆地区嵌岩桩基目标可靠度的范围,进而提出重庆地区嵌岩桩基目标可靠指标的建议值。最后引入可靠系数(代替荷载分项系数和抗力分项系数)并计算了嵌岩桩单桩承载力的可靠系数,提出采用可靠系数表达的极限状态设计实用表达式,该表达式体现了简单实用的原则,利于设计人员掌握和理解,便于工程应用。     

基于沉降标准的卵砾石层中后注浆桩承载力预测

 用桩顶位移标准来确定桩的承载力对后注浆桩的设计有重要意义。在分析后注浆减小桩基沉降机制的基础上,通过对桩端位于卵砾石层的后注浆桩承载力提高量的统计分析得出在不同桩顶位移条件下承载力提高系数的分布情况,并通过正态分布函数进行拟合,给出一种基于沉降控制标准的后注浆承载力预测方法,并通过一个工程实例说明该方法的合理性。另外,统计表明在现有极限承载力设计体系下,当桩顶荷载对应于设计注浆桩承载力特征值时桩顶沉降比范围为1～2;当桩顶荷载对应于设计注浆桩极限承载力标准值时,桩顶沉降比范围为3～4。     

全直桩与斜桩高桩码头结构地震动力损伤对比研究

为探究斜桩与全直桩高桩码头结构受强地震作用的损伤特点,以一典型钢管高桩码头为研究对象,并以同一建设场地为背景,在相同服役条件及地质条件下,分别建立斜桩与全直桩高桩码头结构的有限元解析模型,进行结构体系的数值模拟仿真。通过桩体截面弯矩-轴力关系曲线,分析了全直桩码头、斜桩码头两种不同结构形式受不同水平地震作用下的动力响应特性及损伤特点,为高桩码头结构的设计与选型提供合理的依据和参照。研究结果表明,在斜桩结构的内力中,斜桩的轴力起主要支配作用,可有效分担地震的作用力,在输入地震动为350 gal时,考虑到结构的水平位移满足设计要求等因素,斜桩结构在抗震性能方面要稍优于直桩结构,在输入地震动为1 000 gal时,两种结构均发生了塑性破坏,通过塑性损伤开展以及对于地震能量的吸收与耗散的对比分析,得到全直桩结构具有优于斜桩结构的抗震性能的结论。     

不平衡堆载作用下邻近结构桩的侧向受力机制

为了分析结构桩在邻近堆载作用下的受力特性,以条状梯形分布堆载为代表,基于布西奈斯克弹性理论解得出堆载作用下地基附加侧压力表达式,并据此通过对局部塑性变形理论模型的改进,推导得出桩前土体侧移作用下桩身实际附加荷载,同时考虑土体可能发生绕桩流动的极限状态,结合绕桩极限模型给出桩身被动荷载的合理确定方法;在此基础上,考虑地基土体塑性屈服和桩顶结构荷载的影响,基于三参数Winkler弹性地基梁模型,建立桩身自由段、被动受荷段和主动作用段的微分方程,并通过桩身离散与矩阵传递法给出结构桩被动受力特性的半解析解。实测结果、文献理论解与该解答的对比分析,验证了本文计算方法的可靠性。     

成层软土地区高层建筑PHC管桩抗震性能研究#br#

以天津成层软土地质条件下采用桩筏基础的高层建筑为背景,利用有限元方法建立上部结构-桩-土相互作用模型,对PHC管桩在地震荷载下的内力进行研究。研究表明,桩筏基础在地震荷载作用下,角桩将产生较大的内力,减少承台对管桩转动自由度的约束可有效改善其抗震性能。有地下室的结构在地震荷载下的桩顶内力将小于无地下室的结构,但其影响范围在桩顶以下10倍桩径范围内,此外,桩顶周围软弱土层的存在也会对桩顶内力产生较大影响,因此在进行设计时,应充分考虑地下室和桩顶周围软弱土层的综合作用,仅考虑地下室的影响可能会使管桩在地震荷载下处于不利的受力状态,改良桩顶周围土体的性质可明显降低地震荷载作用下的管桩顶部内力。     

桥梁逆作承台–群桩复合基础应用与现场实测研究

为进一步研究逆作复合基础在桥梁工程中的适用性,开展了桥梁逆作承台–群桩复合基础的实桥应用研究,针对有水和无水的施工环境,分别提出了桩与承台间预留孔采用带剪力连接键的钢护筒和收口网模板的构造设计与连接施工工艺,并通过12组接头承载性能试验验证了工艺方案的可靠性。通过现场实测,揭示了桥梁逆作承台–群桩复合基础受力机理,测试结果表明:采用逆作方案,先施工桩基和预留桩孔的承台,待盖梁施工完成后再封桩,使桩与承台形成整体共同参与受力,能够使封桩前承台底土承担上部荷载,有效发挥浅层地基土的承载能力;封桩后,基础沉降增量和承台底土压力增量均明显减缓,继续增加的上部荷载主要由桩基承担,能够有效控制封桩后桥梁沉降。     

Interaction between inclined pile groups subjected to harmonic vibrations

Vertical and inclined piles are used in seismic areas where they could be subjected to oblique harmonic vibration loads. The effect of closely spaced battered piles on the pile–soil–pile interaction has not yet been fully recognized. A simple analytical method, based on the elasto-dynamic theory by Novak and his associates, is used in the present study to characterize vertical and inclined isolated cylindrical piles subjected to inclined harmonic vibrations. The free field movement of the ground in the vicinity of the piles is determined using an approximate approach based on the interference of the cylindrical wave field originating along each pile shaft and spreading radially outward. In calculating the interaction factor between two battered piles, an analysis has been carried out to demonstrate the effect of the presence of a neighboring pile (receiver) while the first pile (source) is loaded. In this situation, it has been found that the movement of the source pile head is decreased when a receiver pile is present. Also, the effects of the pile–pile distance, the group geometry, the length of the piles, and the inclined angle for each or all of the piles have been studied and the corresponding results will be presented.     

基于概率极限状态设计的复合桩基分项系数研究

本文在可靠性分析的基础上,对复合桩基承载力极限状态概率统计方法进行了研究,计算了与目标可靠性指标相对应的各抗力和荷载效应的分项系数,使得复合桩基的可靠度设计能方便地应用到工程实践。     

单桩静载试验的桩顶沉降稳定收敛特征研究

采用单桩静载试验,系统地对36根工程桩的324个荷载级进行沉降稳定分析和荷载稳定时间分析,并通过中等直径灌注桩和超长大直径嵌岩桩两种桩型的15根单桩进行了对比分析。研究表明:不论是嵌岩桩还是非嵌岩桩,各级荷载总体都在1小时内稳定且占比高达96.3%;桩顶的沉降速率、加速度均表现出随时间衰减的特征,证明了桩顶位移具有基于时间、位移的收敛的性特征,且试验耗时降低40%以上。采用P-s曲线法、LogP-s曲线法、E-Δs/ΔQ法判定了单桩的极限荷载,验证了桩顶沉降值在合理的范围之内,同时也表明对桩顶沉降收敛特征的研究可以有效的缩短桩基础检测的时间并提高检测效率。     

嵌岩桩嵌岩段的岩石极限侧阻力系数

嵌岩桩在岩土工程中已得到广泛应用,但如何准确计算嵌岩段桩的极限侧阻力仍是工程设计人员面临的重要课题。收集整理了不同时期、不同地区、不同岩石强度和不同嵌岩条件下开展的145个嵌岩桩竖向下压承载力试验成果,主要包括嵌岩段岩石类型及其单轴抗压强度、嵌岩桩的直径与嵌岩深度、嵌岩段桩的极限侧阻力等。定义嵌岩段桩的极限侧阻力和岩石单轴抗压强度的比值为嵌岩桩嵌岩段岩石极限侧阻力系数,分析了桩径、嵌岩深度、嵌岩深径比和岩石强度对嵌岩段极限侧阻力和岩石极限侧阻力系数的影响规律,建立了嵌岩段岩石极限侧阻力系数与岩石单轴抗压强度之间的拟合关系式,给出了不同可靠度水平下岩石侧极限阻力系数取值。     

洞庭湖软土地区大直径超长灌注桩竖向承载力试验研究

以洞庭湖软土地区某特大拱桥主墩桩基础为工程依托 ,进行了一试桩竖向静载荷试验 ,并基于所获得的现场实测数据探讨了软土地区大直径超长灌注桩的荷载传递机理和竖向承载特性。结果表明 ,软土地区大直径超长桩为典型的摩擦桩 ,其竖向承载力主要由桩侧摩阻力提供 ,而桩端阻力往往难以发挥甚至趋于零 ,设计计算时尚应重视桩身压曲稳定分析和桩顶变形验算。同时 ,建议根据上部结构对基础沉降的要求按桩顶容许沉降量来控制超长桩的竖向承载力 ,并基于线弹性 -全塑性桩侧荷载传递模型给出了相应的计算公式 ,以此获得的桩顶荷载位移计算曲线与实测值吻合良好 ,证明了该思路与分析方法的可行性。     

群桩基础桥墩静力PUSHOVER分析

静力Pushover分析是应用能力谱法进行抗震分析的重要一步,也是研究结构非线性受力特征的重要方法之一。本文采用Pushover分析方法, 同时考虑地基土与桩构件的非线性,研究了群桩基础桥墩的侧向加载模式与非线性受力特征。研究结果表明:(1)侧向加载模式不同,获得的能力曲线不同。均匀分布和墩顶集中力加载模式分别反映了结构能力曲线的上、下限,一阶振型和SRSS分布介于两者之间。建议群桩基础的Pushover分析应采用SRSS加载模式。(2)墩高小于20 m时,高速铁路简支箱梁桥的水平惯性力主要集中在墩顶及承台处。(3)受拉侧单桩初始屈服后,群桩基础的承载能力还可继续增加,但横桥向多排桩(4排桩)群桩基础增加更明显。横桥向多排桩群桩基础的位移延性系数高于纵向两排桩的情况。     

基于BP算法的CFG桩复合地基承载力的神经网络预测

将BP神经网络模型应用于CFG桩复合地基承载力的设计计算中,通过对影响CFG桩复合地基承载力因素间的非线性关系分析,指出影响CFG桩复合地基承载力的主要因素有:桩的参数、置换率、土的物理力学特性、褥垫层厚度和施工工艺。为成功预测CFG桩复合地基承载力,收集了大量工程实测资料(包括地质资料、工程设计资料、静载荷试验资料),并按照神经网络的训练要求对工程资料进行了分析整理,建立了基于人工神经网络的CFG桩复合地基承载力预测模型。通过对网络的学习训练,得到的复合地基承载力预测值达到了预期精度。     

Improved methods of testing piles in frozen soils

Conclusions 1. The approach adopted in this article for analysis of pile test data based on the above relations made it possible to work out three accelerated testing methods — stepped with equal-duration steps; dynamometric; and stepped-dynamometric.2. The results of the performed tests are characterized by an increased information content, which makes it possible to design the piles for two groups of limit states and to take into account the pile load variation.3. The reliability of the proposed testing and design methods is confirmed by comparison with experimental data.Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 4, pp. 14–18, July–August, 1990.     

可控刚度桩筏基础桩土共同作用的工程实践

端承型桩等支承刚度较大的桩基础,在正常工作荷载作用下,桩顶较小的竖向变形无法保证桩土共同作用的实现,这可能导致良好的天然地基承载力得不到充分利用,造成极大的浪费。介绍的典型工程成功解决了支承刚度较大的桩基础难以实现桩土共同作用的问题,同时还实现了桩筏基础的变刚度调平,通过对工程设计及数值分析过程的详细论述与描写,并将现场测试数据与分析结果进行对比分析,验证可控刚度桩筏基础应用于该项目的合理性与可靠性。数值分析及现场实测结果显示,本工程采用可控刚度桩筏基础后,建筑物总沉降和差异沉降均满足设计要求,地基土分担了上部结构63%的的荷载,桩基数量大幅度降低,减少了高能耗建材的使用和桩基施工对周边环境的影响,取得了显著的经济效益和社会效益。本工程可控刚度桩筏基础实施所取得的相关经验与成果可供类似工程借鉴与参考。     

竖向动荷载下桩土参数对基桩动力稳定性的影响

探讨了桩在竖向动力荷载作用下桩侧负摩阻力、桩土阻尼、桩的尺寸效应、地面荷载、桩土质量等参数对基桩动力稳定性的影响问题 ,并提出了桩截面尺寸选择的控制条件 ,为桩基础动力设计与分析提供参考依据     

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.