不同成桩工艺条件下冻结粉土中基桩承载性状试验研究

桩基础作为冻土工程中最适宜的基础形式,主要采用插入桩、静压桩和灌注桩三种桩型,因其成桩工艺不同对冻土地基及桩基自身造成的差异不一。为研究不同成桩工艺对冻土地基及基桩承载性状的影响,通过开展室内-1.5 ℃条件下单桩静载模型试验,分析在冻结粉土中不同成桩工艺对地温场、桩基极限承载力、桩身轴力以及桩侧摩阻（冻结力）的影响规律。试验结果表明：灌注桩对桩周地温场扰动剧烈,桩侧温度较高,地温变化幅度大。随着桩周土体的回冻,地温逐步降低,其中灌注桩桩侧降温速率最大;在承载力方面,2根灌注桩的极限承载力约为12.8 kN,静压桩为11.6 kN,插入桩极限承载力最小,仅为钻孔灌注桩的2/3。并对比4种不同成桩工艺的基桩在不同荷载条件下对应的沉降量,体现出成桩工艺对基桩沉降造成的差异性;随着桩顶荷载的逐级增加,桩侧摩阻（冻结力）和桩端阻力逐渐发挥作用,桩身上部1/3由于温度较低,致使桩侧摩阻（冻结力）较大,在10 cm深度附近温度最低,使桩侧摩阻（冻结力）达到最大值;并对比4种不同成桩工艺的基桩在荷载5.6 kN下轴力沿桩身的传递情况,发现静压桩更易把荷载传递到冻土区深层地基。     

Growth-rate-dependent prediction of pile set-up and its application in driven pile foundation construction

In this paper research was presented on the development of a growth-rate-dependent model for pile set-up prediction using the restrike and static/statnamic load testing data collected from different projects. The data included: a) restrike records from ninety-five production piles and restrike and load test results of nine instrumented piles driven in soft clays from the relocation project of Highway No. 1 in Louisiana (LA-1); and b) restrike and static load testing data of five fully instrumented square PPC piles driven at four different bridge sites in various soil layers from sands to clays in Florida. Research effort was focused on the prediction of the ultimate shaft resistances with pile set-up formulated using the pile resistance growth rate-dependent model. The timeframe of interest was studied for a practical set-up magnitude such as 90% of the ultimate shaft resistance (Q₉₀). As an application of the rate-dependent model, it was found that piles at the LA-1 relocation project, in general, reached about 95% of the ultimate shaft resistances at the time of 2 weeks after pile installation. The strategy of incorporation of pile set-up in adjusting pile driving criteria or/and design during pile construction, such as the experience-based plan of a two-week waiting period adopted by Louisiana DOTD, was investigated and justified.     

大直径超长钻孔灌注桩荷载分层传递特性试验分析

基于＂上海市虹桥综合交通枢纽交通中心工程西交通广场＂工程现场静载荷试验和桩身应力测试结果,分析竖向荷载作用下大直径超长钻孔灌注桩在成层土中的荷载传递特性。本工程试桩已加载至破坏,对此试验结果进行分析,能为深入研究大直径超长钻孔灌注桩的承载性状提供有价值的工程参考。本次试验结果表明：1）大直径超长钻孔灌注桩桩端承载力所占比例较低,荷载-沉降关系为陡降型,存在明显拐点;2）桩侧与桩端阻力非同步发挥且相互影响,而上下土层侧阻力系先后发挥至极限;3）根据试桩实测数据,土层埋深对桩周具有相似物理力学性质土层的侧摩阻力影响较大。     

薄壁筒桩在温州滩涂土地基处理中的应用研究

以温州浅滩一期半岛起步区首期1#地块为工程背景,对薄壁筒桩、钻孔灌注桩、预应力管桩进行现场静载荷试验,研究结果表明：在滩涂土地区,与钻孔灌注桩、预应力管桩相比,薄壁筒桩不会产生沉渣问题、桩身质量好、挤土效应弱、节约工程造价,承载力及桩顶沉降量均满足滩涂土地基加固要求,更适宜应用于滩涂土地基处理中,且在滩涂土地区,薄壁筒桩不会产生土芯闭塞效应,土芯的存在及桩顶封盖对于提高承载力具有一定的贡献,适当增加筒桩桩长可以提高承载力。     

Pile driving by numerical cavity expansion

A cavity expansion procedure for the simulation of pile driving is presented and assessed in this paper. The analysis uses a non-linear finite-element model and the penetration of the pile into the soil is simulated by a radial opening of the soil around the pile. The case of a pile advanced by expansion will be compared to a similar pile subjected to computational driving (referred to, respectively, as ‘expanded’ and ‘driven’ piles for convenience). The state of stress and deformation, and the evolution of pore-water pressure in the soil will be monitored for the expanded and driven piles. Further computational driving will be applied to both cases and the pile response and soil resistance will be compared. The computational cost of advancing the pile by expansion will finally be investigated. Copyright © John Wiley & Sons, Ltd.     

搅拌桩内套打钻孔灌注桩在暗浜深基坑围护工程中的应用

在暗浜地层内施工钻孔灌注桩时,易产生钻孔扩径、孔内塌方事故,产生“叉腿桩”及诱发搅拌桩次生“叉腿”,造成钻孔灌注桩与搅拌桩之间的产生大的缝隙.基坑开挖时,基坑外侧的地下水沿缝隙渗入,造成基坑外侧土体压缩变形,引起地表开裂、产生不均匀沉降.搅拌桩内套打钻孔灌注桩系先用低渗入量水泥将暗浜土搅拌成复合土,提高暗浜土的质量,然后在复合土内钻进成桩,其作用是避免钻孔灌注桩在暗浜土段的扩径、塌方超灌事故,确保了基坑桩基围护工程施工质量.     

粉土及粉质黏土对静压沉桩桩端阻力影响机制现场试验

为探讨在粉土及粉质黏土中桩端阻力随贯入深度的变化规律,通过在试验桩P1的桩端安装轮辐压力传感器,以及在试验桩P1、P2距桩端200 mm处安装光纤光栅（FBG）传感器,采用两种不同的测试技术全程监测了两根闭口预应力高强度混凝土（PHC）管桩现场贯入过程中的桩端阻力。试验结果表明：桩端阻力与土层的变化密切相关,土层越硬,桩端阻力越大,当桩端从粉质黏土层进入粉土层时,桩端阻力明显增大,粉土中的桩端阻力达到粉质黏土层的2倍;整个贯入过程中,同一土层不同位置的差异性对桩端阻力也存在较大影响,在距离地面1.50 m处,P1和P2桩FBG传感器桩端阻力的差值达到了89.29 kN,而在距离地面3.50~4.50 m处,两桩的桩端阻力则相差较小。     

抗液化排水刚性桩沉桩过程的土压力响应

抗液化排水刚性桩是一种将刚性桩与竖向排水体相结合的新桩型。基于某建筑桩基工程,开展了抗液化排水刚性桩和不含排水体的普通刚性桩的沉桩对比现场试验,采用了动态土压力传感器实时监测沉桩过程中桩周土体内产生的土压力响应,对比了排水桩与普通桩沉桩对桩周土体水平方向应力及有效应力影响的差异。试验结果表明：抗液化排水刚性桩能够有效减小沉桩过程对桩周深部可液化土体的扰动,在桩身近侧（距桩心0.6 m）深部埋深（-15 m）位置,排水桩的水平土压力响应峰值仅为普通桩的1/4;排水桩能够有效降低沉桩对可液化土层有效应力的影响,使桩周土体更加稳定;在单次沉桩过程中,对于浅部埋深（-5 m）,排水桩对桩周土压力峰值的影响作用较小,对于存在可液化土层的深部埋深（-10、-15 m）,排水桩对土压力峰值的有效影响半径可达4倍桩径。现场试验数据为抗液化排水刚性桩的桩间距选择提供了有力的设计参考依据。     

竖向高荷载作用下大直径超长钻孔灌注桩承载性能试验

通过机场—西华高速大直径超长钻孔灌注桩大吨位竖向单桩静载试验,分析了该地区大直径超长钻孔灌注桩的承载性状以及荷载传递机理。试验结果表明:试桩的Q-S曲线呈缓变型,桩端承载力占总荷载的比例均<10%,即均表现为摩擦桩特性;试桩的侧摩阻力自上而下逐步发挥,侧摩阻力和桩端阻力异步发挥且互相耦合;大直径超长钻孔灌注桩桩侧摩阻力的发挥与土层性质、土层埋深及桩顶荷载水平有关;在高荷载作用下桩侧上部土层摩阻力具有不同程度的软化现象,而中下部土层侧摩阻力具有不同程度的强化现象,甚至即使在最大加载情况下,桩身下部土层的侧摩阻力也并未完全发挥,因此在根据规范计算超长桩承载力时,不同深度土层的侧摩阻力应乘以相应不同的修正系数。     

沉桩引起的三维超静孔隙水压力计算及其应用

引入时间、深度参数分析在饱和软粘土中沉桩时引起的超静孔隙水压力,给出了考虑固结效应的超静孔隙水压力的三维解析解;分析了超静孔隙水压力的消散过程中桩周土发生曼德尔效应的时间和区域,提出了在群桩施工过程中土体中的超静孔隙水压力是消散与累加的综合过程,施工完毕后则变为单一的消散的计算模型,并给出计算公式。通过算例,分析了桩群不同桩距、不同入桩顺序对超静孔隙水压力的影响。     

考虑桩底沉渣的随钻跟管桩竖向承载特性模型试验研究

随钻跟管桩施工不能完全清除桩底岩土沉渣,从而影响桩基端承力。为揭示桩底沉渣对随钻跟管桩承载力的影响机制,开展了考虑桩底沉渣影响的随钻跟管桩竖向承载特性模型试验研究。试验结果表明:在密砂地层中,具有桩端水泥土扩大头的随钻跟管桩,其桩顶荷载-沉降曲线为缓降型,而模拟试验的其他管桩均为陡降型;桩底沉渣降低随钻跟管桩的极限承载力在22%以内,且其桩顶荷载主要由桩侧摩阻力承担,承担占比超过90%;与存在一定厚度沉渣的钻孔灌注桩相比,随钻跟管桩的桩底沉渣对降低承载力的影响相对较小;靠近桩端的轴力随着沉渣厚度的增加而减小,沉渣越厚,减少的幅度越明显;桩端水泥土扩大头施工可提高随钻跟管桩约37%的承载力,且桩端阻比均小于15%。现场原位测试(桩长为15.5 m,长径比为15.50)和室内模型试验(桩长为1 m,长径比为15.87)结果均表明:存在桩底沉渣时,随钻跟管桩是以发挥侧摩阻力为主的端承摩擦型桩。研究成果有助于进一步加深对随钻跟管桩承载性状的认识。     

砂土中开口管桩沉桩过程的颗粒流模拟研究

基于颗粒流理论,采用PFC2D程序,模拟再现不同型号开口管桩在沉桩过程中土塞的形成演化规律、土颗粒细观结构变化以及桩周土应力场分布情况,并通过分析土体细观变化模式揭示沉桩过程中宏观力学响应的内在机制。计算结果表明,管桩直径对土塞效应影响很大,外径为30 mm的开口管桩,沉桩过程中土塞增量填充率（IFR）值较小,土塞效应明显,土塞高度小,类似闭口桩;随着管桩直径的增大,土塞效应迅速减小,大直径管桩在砂土中沉桩全部闭塞的可能性很小。细观因素（孔隙率和滑动比例）与土体宏观位移表现之间存在着明显的相互对应关系,并依此将桩周土划分3个区域。桩周土体水平应力、竖向应力和剪应力都在桩底附近形成“应力核”,不同型号管桩桩周土应力场分布相近。     

基于Gudehus-Bauer模型的砂土中灌注桩护壁套管高频振动贯入速率

随着全套管护壁振动取土灌注桩施工工艺的发展,灌注桩在工程中的应用越来越广泛,但是关于灌注桩护壁套管高频振动贯入速率的研究较少。通过假定套管为刚体,将套管周围土体划分为同心轴的圆环柱体,利用Gudehus-Bauer亚塑性本构模型计算套管外侧各土体单元接触面间的剪应力和套管端部土体竖向应力,并考虑套管振动贯入过程中土塞效应,建立灌注桩套管高频振动贯入砂土中贯入速率的计算模型。将所提出的贯入速率计算结果和物理模型试验结果与有限元结果进行对比分析,验证了计算模型的合理性。通过参数分析,得到地基土体孔隙比、振动频率和套管直径对贯入速率的影响规律,为实际工程中快速、准确地预测套管振动贯入速率提供了可靠方法。     

桩端后注浆超长灌注桩桩侧极限摩阻力计算方法

目前超长灌注桩普遍应用于大型桥梁、超高层建筑工程中,且常采用桩端后注浆技术,但其桩侧极限摩阻力的确定尚未有切合其实际特点的方法。大量现场实测数据反分析表明,采用 法计算时,存在桩侧土压力大于土层静止土压力的现象。在探讨桩侧土压力增大机制的基础上,对 法进行改进,建立了桩侧土压力确定方法,并给出了桩侧土压力系数的取值公式。工程实例计算结果与试桩实测结果比较分析表明,所建立的桩端后注浆超长灌注桩桩侧极限摩阻力计算方法具有合理性与可行性。     

Beam–solid contact formulation for finite element analysis of pile–soil interaction with arbitrary discretization

This paper presents an embedded beam formulation for discretization independent finite element (FE) analyses of interactions between pile foundations or rock anchors and the surrounding soil in geotechnical and tunneling engineering. Piles are represented by means of finite beam elements embedded within FEs for the soil represented by 3D solid elements. The proposed formulation allows consideration of piles and pile groups with arbitrary orientation independently from the FE discretization of the surrounding soil. The interface behavior between piles and the surrounding soil is represented numerically by means of a contact formulation considering skin friction as well as pile tip resistance. The pile–soil interaction along the pile skin is considered by means of a 3D frictional point‐to‐point contact formulation using the integration points of the beam elements and reference points arbitrarily located within the solid elements as control points. The ability of the proposed embedded pile model to represent groups of piles objected to combined axial and shear loading and their interactions with the surrounding soil is demonstrated by selected benchmark examples. The pile model is applied to the numerical simulation of shield driven tunnel construction in the vicinity of an existing building resting upon pile foundation to demonstrate the performance of the proposed model in complex simulation environments. Copyright © 2014 John Wiley & Sons, Ltd.     

软黏土中PHC管桩打入过程中土塞效应研究

当开口管桩打入土层中,土体进入桩内形成土塞,土塞效应对桩的打入特性和承载能力具有重要影响。基于上海典型软土地基中长PHC桩的现场试验,统计分析3个不同场地共44根桩打桩过程中的土塞数据,探讨软土地基中PHC桩打桩过程中土塞长度与内径之比、土塞增量与桩打入深度增量之比(IFR)随打入桩长与内径之比变化的规律,并线性拟合出土塞增量与桩打入深度增量之比与土塞高度和桩打入深度之比(PLR)的经验公式。研究表明,大部分PHC桩在打桩过程中,土塞部分闭塞,桩从浅部较硬土层打入较软土层,IFR值减小,土塞闭塞作用大;桩从较软土层打入深部较硬土层,IFR值逐渐增大,土塞闭塞作用小,且土塞长度增量与桩打入深度增量之比与土塞长度与桩打入深度之比基本呈线性关系。     

中钢天津响螺湾工程后压浆钻孔灌注桩施工技术

中钢天津响螺湾工程基础桩采用后压浆钻孔灌注桩,最大桩长达76 m,设计单桩承载力特征值达15000 kN。阐述该工程钻孔桩的施工工艺、施工中出现的问题和解决方法、孔口钢筋笼直螺纹对接技术、后压浆施工工艺。通过现场试验,全部桩为Ⅰ类桩,单桩承载力达到设计要求,极限承载力实测值最大为35000 kN,为工程桩的设计提供了可靠的技术参数。     

软土中已打入桩对土体位移的遮帘作用研究

采用侧向移动土中桩的理论分析结果以及有限元数值模拟,较为系统地分析了沉桩工程中已打入桩对土体侧向位移及地表隆起的遮帘作用,结果表明已打入桩对土体侧向位移的总体遮帘作用并不显著。采用不同切向劲度系数的接触面单元以模拟已打入桩间隔时间长短对地表隆起的遮帘效应,结果表明：打入桩间隔期较短时,已打入桩对地表隆起的遮拦作用主要体现在对土体位移的“水平阻隔”上,土体沿桩土界面发生流动向上隆起;而间隔期较长时,桩土间存在有较大的摩阻力而限制了土体的位移,此时已打入桩对地表隆起的遮拦作用体现在竖向摩阻力对土体隆起的“抑制”上。     

高速铁路超长桥桩承载特性试验研究

超长桩广泛已应用于土木工程各个领域,但黄土地区超长桩的承载性状和变形特性尚不十分清楚,需要进一步研究。通过对郑西铁路客运专线某特大桥的4根超长桩现场测试项目的资料分析,得出了超长桩桩身轴力及侧阻力的变化规律,对超长桩的承载特性有了更为清晰的了解。试验结果表明,在桩顶竖向荷载作用下,桩身轴力随荷载的增加发生了局部调整,砂性土层的桩侧摩阻力具有增强效应,黏性土层的桩侧摩阻力具有退化效应;单桩竖向刚度随桩顶荷载的增加而减小,单桩竖向刚度降低40 %～70 %。     

Comparison of the load response of closed-ended and open-ended pipe piles driven in gravelly sand

Slow-maintained static load tests were performed on closed-ended and open-ended steel pipe piles driven side by side in a gravelly sand soil profile. The site investigation consisted of multiple cone penetration tests (CPTs) and standard penetration tests (SPTs), as well as laboratory tests on soil samples collected at various depths from the test site to determine basic soil properties. The test piles were densely instrumented with a combination of electrical-resistance and vibrating-wire strain gauges. The open-ended test pile was a specially fabricated double-wall, fully-instrumented pile, allowing for separation of the measurements of the inner and outer shaft resistances. Detailed comparison of the load test results, in terms of driving resistance, load response and profiles of unit shaft and base resistances for the two test piles, is presented and discussed. The applicability of three CPT-based pile design methods is assessed through a layer-by-layer comparison of the estimated resistances with those measured in the static load tests.