Thermal performance of cast-in-place piles with artificial ground freezing in permafrost regions

During the construction of cast-in-place piles in warm permafrost,the heat carried by concrete and the cement hydration reaction can cause strong thermal disturbance to the surrounding permafrost.Since the bearing capacity of the pile is quite small before the full freeze-back,the quick refreezing of the native soils surrounding the cast-in-place pile has become the focus of the infrastructure construction in permafrost.To solve this problem,this paper innovatively puts forward the application of the artificial ground freezing(AGF)method at the end of the curing period of cast-in-place piles in permafrost.A field test on the AGF was conducted at the Beiluhe Observation and Research Station of Frozen Soil Engineering and Environment(34°51.2'N,92°56.4'E)in the Qinghai Tibet Plateau(QTP),and then a 3-D numerical model was established to investigate the thermal performance of piles using AGF under different engineering conditions.Additionally,the long-term thermal performance of piles after the completion of AGF under different conditions was estimated.Field experiment results demonstrate that AGF is an effective method to reduce the refreezing time of the soil surrounding the piles constructed in permafrost terrain,with the ability to reduce the pile-soil interface temperatures to below the natural ground temperature within 3 days.Numerical results further prove that AGF still has a good cooling effect even under unfavorable engineering conditions such as high pouring temperature,large pile diameter,and large pile length.Consequently,the application of this method is meaningful to save the subsequent latency time and solve the problem of thermal disturbance in pile construction in permafrost.The research results are highly relevant for the spread of AGF technology and the rapid building of pile foundations in permafrost.

Field observation of the thermal disturbance and freezeback processes of cast-in-place pile foundations in warm permafrost regions

The bearing capacity of pile foundations is affected by the temperature of the frozen soil around pile foundations.The construction process and the hydration heat of cast-in-place(CIP)pile foundations affect the thermal stability of permafrost.In this paper,temperature data from inside multiple CIP piles,borehole observations of ground thermal status adjacent to the foundations and local weather stations were monitored in warm permafrost regions to study the thermal influence process of CIP pile foundations.The following conclusions are drawn from the field observation data.(1)The early temperature change process of different CIP piles is different,and the differences gradually diminish over time.(2)The initial concrete temperature is linearly related with the air temperature,net radiation and wind speed within 1 h before the completion of concrete pouring;the contributions of the air temperature,net radiation,and wind speed to the initial concrete temperature are 51.9%,20.3%and 27.9%,respectively.(3)The outer boundary of the thermal disturbance annulus is approximately 2 m away from the pile center.It took more than 224 days for the soil around the CIP piles to return to the natural permafrost temperature at the study site.

Quality Control Measures of Bored Pile in Bridge Construction

In order to improve the quality of bored piles in bridge construction and ensure the overall quality of the bridge,we analyzed a series of problems in the construction process of bored piles,then propose corresponding quality control measures,in hopes of improving quality control of bored piles in bridge construction in our country.

Experimental study and numerical analysis on bearing behaviors of super-long rock-socketed bored pile groups

A centrifuge modeling test and a three-dimensional finite element analysis（FEA）of super-long rock-socketed bored pile groups of the Tianxingzhou Bridge are proposed.Based on the similarity theory,different prototypical materials are simulated using different indicators in the centrifuge model.The silver sand,the shaft and the pile cap are simulated according to the natural density,the compressive stiffness and the bending stiffness,respectively.The finite element method（FEM）is implemented and analyzed in ANSYS,in which the stress field during the undisturbed soil stage,the boring stage,the concrete-casting stage and the curing stage are discussed in detail.Comparisons in terms of load-settlement,shaft axial force distribution and lateral friction between the numerical results and the test data are carried out to investigate the bearing behaviors of super-long rock-socketed bored pile groups under loading and unloading conditions.Results show that there is a good agreement between the centrifuge modeling tests and the FEM.In addition,the load distribution at the pile top is complicated,which is related to the stiffness of the cap,the corresponding assumptions and the analysis method.The shaft axial force first increases slightly with depth then decreases sharply,and the rate of decrease in rock is greater than that in sand and soil.

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 ℃.

Regressive approach for predicting bearing capacity of bored piles from cone penetration test data

In this study, th e least sq u are su p p o rt v ecto r m achine （LSSVM） alg o rith m w as applied to predicting th ebearing capacity o f b ored piles e m b ed d ed in sand an d m ixed soils. Pile g eo m etry an d cone p e n e tra tio nte s t （CPT） resu lts w ere used as in p u t variables for pred ictio n o f pile bearin g capacity. The d ata u se d w erecollected from th e existing litera tu re an d consisted o f 50 case records. The application o f LSSVM w ascarried o u t by dividing th e d ata into th re e se ts： a train in g se t for learning th e pro b lem an d obtain in g arelationship b e tw e e n in p u t variables an d pile bearin g capacity, and testin g an d validation sets forevaluation o f th e predictive an d g en eralization ability o f th e o b tain ed relationship. The predictions o f pilebearing capacity by LSSVM w ere evaluated by com paring w ith ex p erim en tal d ata an d w ith th o se bytrad itio n al CPT-based m eth o d s and th e gene ex pression pro g ram m in g （GEP） m odel. It w as found th a t th eLSSVM perform s w ell w ith coefficient o f d eterm in atio n , m ean, an d sta n d ard dev iatio n equivalent to 0.99,1.03, an d 0.08, respectively, for th e testin g set, an d 1, 1.04, an d 0.11, respectively, for th e v alidation set. Thelow values o f th e calculated m ean squared e rro r an d m ean ab so lu te e rro r indicated th a t th e LSSVM w asaccurate in p redicting th e pile bearing capacity. The results o f com parison also show ed th a t th e p roposedalg o rith m p red icted th e pile bearin g capacity m ore accurately th a n th e trad itio n al m eth o d s including th eGEP m odel.

A case study on behaviors of composite soil nailed wall with bored piles in a deep excavation

A complete case of a deep excavation was explored. According to the practical working conditions, a 3D non-linear finite element procedure is used to simulate a deep excavation supported by the composite soil nailed wall with bored piles in soft soil. The modified cam clay model is employed as the constitutive relationship of the soil in the numerical simulation. Results from the numerical analysis are fitted well with the field data, which indicate that the research approach used is reliable. Based on the field data and numerical results of the deep excavation supported by four different patterns of the composite soil nailed wall, the significant corner effect is founded in the 3D deep excavation. If bored piles or soil anchors are considered in the composite soil nailed wall, they are beneficial to decreasing deformations and internal forces of bored piles, cement mixing piles, soil anchors, soil nailings and soil around the deep excavation. Besides, the effects due to bored piles are more significant than those deduced from soil anchors. All mentioned above prove that the composite soil nailed wall with bored piles is feasible in the deep excavation.

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.

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.

Dynamic estimation and reliability control of quality assurance for bored piles

A systematic method was proposed to estimate the occurrence probability of defective piles(OPDP) from a site according to quality assurance inspection. The OPDP was firstly suggested as the criterion to weight the performance of a pile foundation. Its prior distribution and updating distribution were deduced to follow Beta distributions. To calibrate the OPDP, a dynamic estimation model was established according to the relationships between prior mean and variance and updating mean and variance. Finally, a reliability-control method dealing with uncertainties arising from quality assurance inspection was formalized to judge whether all the bored piles from a site can be accepted. It is exemplified that the OPDP can be substantially improved when more definite prior information and sampling formation become available. For the example studied herein, the Bayesian estimator of updating variance for OPDP is reduced from 0.0037 to 0.0014 for the first inspection, from 0.0014 to 0.0009 for the second inspection, and with less uncertainty by incorporating experience information.

Comparative Examinations of Single Bored Piles Using International Codes

This paper presents some aspects of the load-settlement behavior for large diameter bored piles using four different international codes, namely: ECP 202 [1], DIN 4014 [2], AASHTO [3] and French Code [4]. Ultimate capacities for 38 pile load tests founded in realistic multi-layered soils in Delta and Port Saidareas atEgyptare evaluated using modified Chin (1970) method and compared to ultimate load predictions obtained by the aforementioned codes. Many statistical analyses were conducted on the total pile loads and individual contributions of tip and skin resistances. Based on code predictions of ultimate pile loads, an empirical modified load-settlement model is proposed. This model will simplify to a great extent the analysis of piled-raft systems as it can effortlessly predict pile settlement due to the load on pile itself. Comparisons showed that the pile load test is an irreplaceable process for determining the ultimate capacity of piles.

Influence of the penetration of adjacent X-section cast-in-place concrete(XCC)pile on the existing XCC pile in sand

A series of small-scale 1g X-section cast-in-place concrete(XCC)pile-penetration model tests were conducted to study the effects of soil density and pile geometry on the lateral responses of an existing pile and the variations in surrounding soil stress.The results showed that the bending patterns of existing XCC piles varied with penetration depth.The lateral response of the existing pile was sensitive to the change in relative density and pile geometry.For example,the bending moment of the existing pile increased along with these parameters.The development of the radial stressσ′r/σ′v0 of the soil around an existing pile showed different trends at various depths during the penetration of the adjacent pile.Moreover,the change in radial stress during the penetration of the XCC pile did not exhibit the“h/R effect”that was observed in the free-field soil,due to the shielding effect of the existing piles.The peak value of radial stressσ′r_max/σ′v0 decreased exponentially as the radial distance r/R increased.The attenuation ofσ′r_max/σ′v0 with r/R in the loose sand was faster than in the medium-dense or dense sands.Theσ′r_max/σ′v0 at the same soil location increased with the cross-section geometry parameter.

黏性土中桩侧后注浆单桩抗压承载性能室内模型试验研究

后注浆技术的应用能有效减少桩周土体扰动和泥皮等造成的不利影响,从而提高钻孔灌注桩的承载力。目前有关后注浆技术的研究多集中于桩端后注浆,少有涉及桩侧后注浆,影响了桩侧后注浆技术进一步推广应用。根据桩侧后注浆工艺,开展单桩抗压室内试验,以研究桩侧后注浆量对单桩抗压承载性能的影响。结果表明:在荷载水平较大的工况下,相较于桩侧后注浆量为1 L的单桩,注浆量为2、3 L时单桩极限抗压承载力分别提高18.2%和66.0%,提高幅度与注浆量呈正相关,并且可以在很大程度上减小桩顶沉降量;桩侧后注浆桩桩身轴力在侧注浆点位附近减小较快,在整个加载过程中桩端附近处桩身轴力值均比较低,单桩受压表现为摩擦桩;水泥浆液同时存在上返和下劈扩散,在室内试验条件下,两者扩散高度均约为14倍桩径,在浆液扩散段桩侧摩阻力得到显著提高,且越靠近注浆点位提高程度越明显。

G3铜陵长江公铁大桥桥塔基础设计

G3铜陵长江公铁大桥主桥为主跨988 m的斜拉-悬索协作体系桥,桥塔采用钢筋混凝土门形结构。3号桥塔基础比选了钻孔桩基础与沉井基础方案,4号桥塔基础比选了钻孔桩基础与地下连续墙基础方案,综合考虑基础受力性能、经济性、施工风险等,均推荐采用钻孔桩基础方案。3号、4号桥塔塔座高4.5 m,承台分别为厚7 m的圆形-哑铃形和矩形-哑铃形结构,基础分别采用68根和64根φ3.0 m钻孔桩,桩长分别为95 m和79 m,按摩擦桩设计。承台系梁采用ANSYS软件进行实体有限元分析,系梁横向受力主筋、竖向抗剪箍筋采用优化分区配置,满足受力要求。

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。

软土地区轨道交通桥梁下方灌注桩小净距施工影响

[目的]我国沿海软土地区灌注桩与运营轨道交通桥梁、桥墩间小净距的施工影响正成为制约城市轨道交通规划与建设的又一瓶颈因素,因此有必要研究小净距钻孔灌注桩施工对周围地层和桥墩的变形影响。[方法]以浙江某城市道路桩板结构形式下穿轨道交通桥梁在建工程为例,通过设计土体侧向变形监测试验和搭建既有桥梁、桥墩的位移智能监测系统,分析了深厚软土区地层与测斜孔的协同变形特性,以及钻孔灌注桩施工对周围地层及桥墩变形的影响规律。[结果及结论]为使测斜孔与地层具有较好的协同变形,软土区测斜孔施工后应稳定停留6 d;淤泥层对施工加载、卸载过程的敏感性较高,由施工引发的软土地层变形量最大;在施工条件下,桥墩3个方向的变形幅值约为未施工的2~3倍;成孔开挖和混凝土浇筑工序对桥墩深度方向的扰动影响较显著,因此已运营轨道交通桥梁下部施工的监测重点应为竖向位移。

静钻根植桩竖向承载性能现场试验研究

通过一组现场试验对同一场地内静钻根植桩的抗压和抗拔承载性能进行研究,试桩均加载至极限状态。通过抗压和抗拔静载试验对静钻根植桩的抗压和抗拔承载性能进行对比分析。试验结果表明:静钻根植桩的抗压承载性能相比传统钻孔灌注桩有显著提高,本次试验场地各土层中静钻根植桩的桩侧摩阻力是规范建议的钻孔灌注桩极限侧摩阻力的1.49~3.21倍;静钻根植桩抗拔承载性能也明显优于钻孔灌注桩,抗拔试桩的极限承载力是根据规范计算的钻孔灌注桩极限抗拔承载力的1.52~1.55倍;静钻根植桩抗拔状态下桩侧摩阻力明显小于抗压状态下的桩侧摩擦力,即静钻根植桩也存在侧阻抗拔系数。

钻孔灌注桩数字化施工技术及应用

随着钻孔灌注桩向超孔深、超大直径方向发展,其施工控制难度也显著增加。传统施工工艺高度依赖人工对施工参数进行采集和记录,难以满足精细化施工管控的要求。为了保证超长大直径桩施工质量,以双柳长江大桥北主塔桩基施工为依托工程,对钻孔灌注成孔、成桩关键施工参数及采集方式进行分析,并针对成孔、成桩过程的关键参数进行自动化监测装置的开发与应用。研究结果表明采用数字化技术手段,能有效减少施工过程中人工采集工作量,提升钻孔灌注桩施工精细化管控水平,可供类似工程借鉴。

静钻根植桩施工环境效应现场试验研究

静钻根植桩采用先钻孔注浆后植桩的施工技术,可以解决传统预制桩施工过程引起的挤土效应问题。然而,静钻根植桩施工过程中钻孔和注浆过程会对周围土体产生一定扰动,目前没有相关研究。文章通过一组现场试验对静钻根植桩施工过程中的钻孔、注浆搅拌和植桩阶段对周围土体的扰动规律进行研究。静钻根植桩施工前在钻孔周围土体中埋设土压力传感器,孔隙水压力传感器和测斜管,分别测试静钻根植桩施工过程中周围土体中水平土压力,超静孔隙水压力和深层土体水平位移的变化规律。研究结果表明:静钻根植桩施工过程会对周围土体造成一定扰动,而施工完成后周围土体中的水平土压力、超静孔隙水压力和深层土体水平位移迅速恢复;当水平距离达到4D(D为钻孔直径)时,静钻根植桩施工过程基本不会对土体产生影响;试验结果可以为静钻根植桩在地铁周围等施工位移控制敏感区域的应用提供理论依据。

岩溶强发育及超厚回填土地层钻孔灌注桩施工技术

针对复杂地层中钻孔灌注桩常见的施工问题,以某工程项目为例,基于岩溶强发育及超厚回填土地层钻孔灌注桩施工技术,对超厚回填土地层、岩溶强发育地层以及上覆超厚回填土与下伏岩溶强发育叠加地层3种不同工况进行了应用分析,结果表明:该技术有效解决了复杂地层中成孔难、易扩缩径、灌注难等诸多工程难题,相较于传统工艺,可大大节省施工材料、降低施工成本,为项目缩短工期15天,累计节约旋挖钻机30台班,将灌注桩充盈系数从1.5降至1.05~1.1,为项目桩基施工节约综合成本35%以上。根据低应变检测结果可知,灌注桩均为Ⅰ,Ⅱ类桩,其中Ⅰ类桩占所测桩数的96.5%,Ⅱ类桩仅占所测桩数的3.5%,充分证明了该技术的可靠性。