Refreezing of cast-in-place piles under various engineering conditions

In the construction of the Qinghai-Tibet Power Transmission Line （QTPTL）, cast-in-place piles （CIPPs） are widely applied in areas with unfavorable geological conditions. The thermal regime around piles in permafrost regions greatly affects the stability of the towers as well as the operation of the QTPTL. The casting of piles will markedly affect the thermal regime of the surrounding permafrost because of the casting temperature and the hydration heat of cement. Based on the typical geological and engineering conditions along the QTPTL, thermal disturbance ofa CIPP to surrounding permafrost under different casting seasons, pile depths, and casting temperatures were simulated. The results show that the casting season （summer versus winter） can influence the refreezing process of CIPPs, within the first 6 m of pile depth. Sixty days after being cast, CIPPs greater than 6 m in depth can be frozen regardless of which season they were cast, and the foundation could be reffozen after a cold season. Comparing the refreezing characteristics of CIPPs cast in different seasons also showed that, without considering the ground surface conditions, warm seasons are more suitable for casting piles. With the increase of pile depth, the thermal effect of a CIPP on the surrounding soil mainly expands vertically, while the lateral heat disturbance changes little. Deeper, longer CIPPs have better stability. The casting temperature clearly affects the thermal disturbance, and the radius of the melting circle increases with rising casting temperature. The optimal casting temperature is between 2 ℃ and 9 ℃.

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, pileesoileraft 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.

Axial Load Capacity of Cast in Place Piles from SPT and CPTU Data

This paper aims to deal with the assessment of axial load capacity for cast in place pile foundations, which are made by the earth drill method, by using the data taken from Standard Penetration Tests (SPTs) and Piezocone Penetration Tests (CPTUs). These tests were carried out as part of the investigation program for P.N.G. Terminal-Power Plant, near Semani beach, in Hoxhara marsh, in the western part of Albania. The design of axial load capacity of piles is based on empirical formula using SPT and CPTU values. This study presents the results of axial load capacity analysis of cast in place piles by different analytical calculation methods, which are based on in situ tests results, and also referring to the Building Standard Law of Japan. In the end of our work, differences between calculations methods by using different in situ tests results are shown in tables and graphs.

Experimental Analysis of Influencing Factors on Heat Exchange Efficiency of Cast-in Place Energy Piles

The energy pile is a kind of building energy-saving technology using shallow geothermal energy,and its heat exchange performance is an important factor affecting its development.In this study,heat exchange tests were carried out on three full-size cast-in place energy piles,and the influence of various factors on the heat exchange amount and the heat exchange rate was analyzed.The following conclusions are drawn:(1)the heat exchange rate of the cast-in place energy pile is greatly affected by the inlet water temperature;(2)increasing the pile length can increase the heat exchange amount,but has little effect on improving the heat exchange rate;(3)the increase in the heat exchange amount by heat exchange pipes in series is not significant,and therefore the parallel-type heat exchange pipes should be considered in practical engineering;(4)the appropriate circulating water flow velocity gives the best heat exchange efficiency.

Experimental Research on Large Diameter Cast-in-Place Piles Embedded in Rock

This paper expatiated the field test of large diameter cast in place piles embedded in soft rock, including static loading test, high or low strain dynamic test, measurement of stresses and strains of pile body, and pressure measurements between pile tip and soft rock. The relative in situ test problems are discussed. Based on the limit equilibrium theory and the load transfer equation, a synthesis method of analyzing the ultimate carrying capacity of single large diameter pile is put forward. The research results show that the key to determining the ultimate carrying capacity of single pile with a large diameter is the analysis of the intensity of soft rock.

Field study of plastic tube cast-in-place concrete pile

The compositions, technical principles and construction equipments of a new piling method used for ground improvement plastic tube cast-in-place concrete pile were introduced. The results from static load tests on single piles with different forms of pile shoes and on their composite foundations were analyzed. The distribution patterns of axial force, shaft friction and toe resistance were studied based on the measurements taken from buried strain gauges. From the point of engineering application, the pile has merits in convenient quality control, high bearing capacity and reliable quality, showing higher reasonability, advancement and suitability than other ground improvement methods. The pile can be adopted properly to take place of ordinary ground improvement method, achieving greater economical and social benefits.

Seismic performance analysis and design suggestion for frame buildings with cast-in-place staircases

Many staircases in reinforced concrete （RC） frame structures suffered severe damage during the Wenchuan earthquake. Elastic analyses for 18 RC structure models with and without staircases are conducted and compared to study the influence of the staircase on the stiffness, displacements and internal forces of the structures. To capture the yielding development and damage mechanism of frame structures, elasto-plastic analysis is carried out for one of the 18 models. Based on the features observed in the analyses, a new type of staircase design i.e., isolating them from the master structure to eliminate the effect of K-type struts, is proposed and discussed. It is concluded that the proposed method of staircase isolation is effective and feasible for engineering design, and does not significantly increase the construction cost.

ON THE DRIVING BEHAVIOR，VARIABLE－DIAMETEREFFECT AND BEAKING CAPACITY OF DRIVENCAST－IN－SITU PILE WITHFLAT

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钻孔灌注桩在基坑维护施工中的质量控制措施

结合实际案例,按照钻孔灌注桩施工工艺流程顺序,从钻孔前、钻孔中、成孔、钢筋笼吊装及混凝土浇筑等环节,对钻孔灌注桩在深基坑维护施工中的质量控制措施进行简要分析,如钻孔前应建立合理的质量控制流程等;钻孔过程中要做好护筒埋设、设备就位与安装、成孔等环节的质量控制;钢筋笼吊装要做好钢筋笼起吊点计算等工作;混凝土浇筑应避免堵孔、断桩等质量缺陷。

大直径嵌岩灌注桩承载特性试验与有限元模拟

基于青岛某工程的4根大直径嵌岩灌注桩,在现场开展了单桩竖向抗压静载试验和桩身完整性测试,以明确大直径嵌岩灌注桩在竖向荷载下的承载特征,拟揭示不同荷载水平下嵌岩灌注桩的承载力发挥机制。结合不同嵌岩深度的单桩承载性能有限元模拟,重点分析了单桩在不同嵌岩深度下的竖向抗压承载特性和桩土位移分布特征。研究表明:在笔者试验条件下,大直径嵌岩灌注桩单桩竖向抗压承载力具有一定离散性,桩头部位的损坏会极大影响单桩承载力的有效发挥;加载完成后,数值模拟得到的桩顶沉降量为31.56 mm,约为现场实测值的1.1倍,模拟结果与实测结果较为吻合;通过不同嵌岩比的模拟结果对比发现,嵌岩深度越大桩端阻力占桩顶荷载的比例越小,且桩端阻力占比的变化速率随着嵌岩比的增加逐渐减缓;嵌岩深度取2.5倍桩径为宜。

高水位流沙层地质下旋挖灌注桩施工技术研究

随着桩基技术的发展与新技术的推广应用,桩基工程对施工的质量、经济性、安全性、高效性等也提出了多方面的要求。旋挖灌注桩因具有工作效率高、环保性强、泥浆排放量少、现场施工文明化水平高等特性,能够适应桩基工程的需求,受到了建设单位及施工单位的一致青睐。而随着施工场景的复杂化,常规的旋挖灌注桩工艺已无法满足工程需求,必须结合工程现状,制订契合工程需求的工艺体系。本研究从施工工艺创新角度出发,提出了解决以上复杂环境的方法,具有一定的推广价值。

基于混凝土水化热的灌注桩成桩质量评价方法及应用

针对如何基于灌注桩水化热温度场准确评价桩身质量的问题,利用与桩身横截面中心径向距离大于0.5倍桩半径范围内,混凝土水化热温度随径向距离的变化近似线性的特征,建立根据钢筋笼上对称两测点的水化热温度计算钢筋笼偏位的方法。结合缺陷灌注桩的水化热仿真分析,系统分析缺陷几何特征和热工参数变化对灌注桩水化热温度场的影响规律,提出根据沿钢筋笼主筋布置水化热温度测点获得的温度-深度曲线特征,判断缺陷存在并分析缺陷空间分布位置的方法。结合现场试验及与低应变、跨孔超声波检测结果的对比,验证了该方法的可行性。

自平衡法和高应变检测在码头灌注桩中的应用

海南某高桩码头工程地质条件特殊,表面覆盖硬岩层,下部为黏土层,码头基础采用灌注桩结构,以硬黏土层为持力层。施工后分别采用自平衡法静载试验和高应变法检测桩基承载力,将检测结果与规范理论计算结果进行比较。结果表明,通过自平衡检测的荷载-位移关系曲线判断,桩端依旧处于弹性阶段,并根据在加载过程中桩身轴力沿桩身轴向的分布情况,可分别判断各桩侧摩阻力、端承载力的发挥值,从而判断桩阻力并没有全部发挥,桩基的实际极限承载力大于试验值。高应变检测法检测桩侧摩阻力结果大于理论计算结果,桩端承载力小于理论计算结果,总桩基承载力二者比较接近,因此可以作为静载试桩的辅助验证。在港口码头工程中采用自平衡静载测试法,并用高应变检测法进行验证,可以解决水域条件下大质量桩基承载力的检测问题。

黄土地基不均匀沉降对现浇管廊影响的试验研究

湿陷性黄土地基浸水后的不均匀沉降会使现浇管廊结构产生附加内力,造成管廊结构开裂甚至破坏,严重威胁其运营安全.为深入研究黄土地层湿陷变形对现浇管廊的影响,开展了黄土地层局部浸水湿陷的模型试验.在试验中,设定局部浸水范围,以地基浸水深度为变量,对管廊结构底部位移、土体接触压力、表面应变和地表沉降进行了监测,并分析了其发展趋势.试验结果表明:当局部基底浸水深度达到20 cm,管廊结构在浸水区出现明显的弯曲变形,结构在湿陷区内的最大下沉达到0.57 mm;结构与土体接触压力变化较大区域为湿陷区边缘的管廊底板处;在管廊底部局部浸水发生不均匀沉降时,结构受力形式表现为底板受拉,顶板受压,且越远离管廊中部,所产生的应变越小,拉压应变在湿陷区边缘发生转变;地表也会发生沉降现象,最大沉降值为2.24 mm.试验结果可为湿陷黄土地区现浇管廊的设计和相关病害的防治提供参考.

大直径嵌岩灌注桩竖向承载特性试验研究

为了探讨泥岩地基大直径灌注桩竖向承载性能,依托青岛某机场桩基工程,对嵌入泥岩的9根大直径灌注桩进行了竖向抗压静载试验和桩身应力监测,分析大直径嵌岩灌注桩的竖向抗压性能,明确大直径灌注桩的荷载传递规律。结果表明,试桩桩身完整性较好,桩身质量可靠;试桩荷载沉降(Q-s)曲线均为缓变型,加载初期存在明显的压密段,桩顶沉降量为10.31 mm,最低回弹率大于50%;最大荷载作用下,各试桩的竖向抗压极限承载力特征值均不小于6480 kN,试桩竖向抗压承载能力仍有较大潜力;试桩均表现出端承摩擦桩的特性,轴力自桩顶到桩端逐渐衰减,桩侧摩阻力峰值发生在中风化泥岩段,桩端阻力占桩顶荷载的10.55%~34.48%。

水泥土加固桩水平循环承载性能足尺试验研究

选取宿迁典型饱和粉土场地,采用水泥土搅拌桩加固钻孔灌注桩桩周土,开展灌注桩水平循环加载足尺试验。探究水泥土加固试验桩(TPI)和未加固试验桩(TPU)在循环水平荷载作用下的滞回特性、弯矩、刚度退化、位移延性、水平承载力与残余变形能力。试验结果表明:TPI比TPU桩的弯矩减小47%,屈服位移减小41%,水平承载力提升150%,有效刚度提升240%,等效黏滞阻尼比提升233%,桩的开裂荷载提升150%,屈服荷载阶段桩体的残余变形能力减小17%。水泥土搅拌法不仅可以限制桩侧粉土液化,增加桩基础的耗能及位移延性,而且可以显著的降低桩头水平位移和弯矩的发展。从抗震性能的角度分析,水泥土搅拌桩加固既有桩基,有效抑制了加固范围内地基土刚度衰减,增强了桩-土结构的总耗能、等效黏滞阻尼系数与变形恢复能力,提高桩基础在大变形阶段的抗震性能。

燃气爆炸荷载作用下钢筋混凝土板抗爆性能数值模拟

为了研究不同燃气爆炸荷载下混凝土板的动态响应特点,本文通过已有文献中的钢筋混凝土板抗爆试验,利用LS-DYNA有限元软件建立数值模型并验证其可靠性。然后,在混凝土板面施加不同峰值的简化三角燃气荷载,研究混凝土板在燃气荷载作用下的位移、速度、加速度等。结果表明:试验用钢筋混凝土板的最大位移和振动都出现在板跨中位置,且随着荷载峰值的增大而增大;当荷载峰值超过20 kPa时,随着作用在混凝土板上荷载量值的增加,结构的刚度受到削弱,在受迫振动阶段结构振动加速度的特性会发生变化,出现高频振荡;高频振荡开始时间随着燃气爆炸荷载峰值的提高而提前;当荷载结束后,混凝土板进入自由振动阶段,板的振动周期受残余刚度的影响,随着荷载峰值的提高而增大。

超宽鱼腹式现浇箱梁混凝土裂缝控制

近年来,国内站城综合体TOD开发成为一种新型建筑模式,地铁、市政道路、商业综合体等多种大体量建筑形态交叉同步施工成为工程实践难题。基于此,结合已经实施的成都博览城综合交通枢纽工程,解析福州路桥鱼腹式多幅四箱室闭合型超宽箱梁,采取优化混凝土配合比、U形冷却管布置、后浇带布置、支架沉降控制、箱梁浇筑顺序等5种措施,解决大体积C60梁体混凝土防开裂难题,并取得明显实践成果,获得国家优质工程奖和土木工程詹天佑奖。

XCC桩群桩沉桩挤土效应透明土模型试验研究

现浇X形混凝土桩(X-sectional cast-in-place concrete pile,简称XCC桩)作为一种非圆截面异形桩,利用等截面异形周边扩大原理,将圆弧正拱变成反拱,达到扩大截面周长、提高承载力的目的。XCC桩安装过程其实是一个挤土过程,群桩沉桩挤土效应与单桩相比较为复杂,关于XCC桩群桩挤土效应方面的研究较少,而XCC桩群桩沉桩挤土效应是研究XCC桩的重要课题。基于透明土试验,开展了XCC桩与圆形桩群桩沉桩贯入试验,通过粒子图像处理技术获取沉桩过程的位移场变化规律,研究不同桩型和不同沉桩顺序对群桩沉桩挤土效应的影响。试验结果表明:对于最后贯入桩背桩面土的位移,XCC桩贯入引起的位移会比圆形桩率先达到峰值,XCC桩和圆形桩在群桩沉桩贯入过程中都会产生明显的累积效应和遮拦效应,且XCC桩的遮拦效应要强于圆形桩。最后,给出了XCC桩和圆形桩在不同沉桩顺序条件下群桩遮拦效应的经验公式。

有限空间作业条件下中小桥梁加固安全施工及风险管理

以某现浇箱梁的维修加固工程为依托,分析了有限空间条件桥梁维修加固工程的施工安全风险以及应对措施。工程具有工作面高度极其有限、工序复杂、风险因素多等不利因素。利用风险源评估方法LEC评价法对风险因素进行评估,对重点的风险因素进行监控管理,并总结了恶劣施工条件下的安全措施要点,工程期间未发生安全事故,为后续公路工程箱梁内部维修加固工程提供一定的参考价值。