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.

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.

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.

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.

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.

督灸疗法联合易筋经“托天桩”功法在颈型颈椎病患者中的应用

目的:观察督灸疗法联合易筋经“托天桩”功法治疗颈型颈椎病的临床疗效。方法:运用简单随机化方法将82例颈型颈椎病患者,随机分为试验组和对照组,每组41例,以督灸疗法护理方案为对照组,以督灸疗法联合易筋经“托天桩”功法护理方案为试验组,治疗5次/周,连续治疗4周。结果:两组患者VAS评分和NDI评分均较治疗前明显改善,但试验组与对照组比较,两组间的改善程度存在差异,P<0.05,具有统计学意义。经TTM检测显示,两组患者的颈项部温度在接受治疗后,亦有明显变化且存在差异,P<0.05,具有统计学意义。在临床疗效方面,试验组总有效率为95.2%,高于对照组87.8%,P<0.05,两组临床疗效相比,具有统计学意义。结论:督灸疗法联合易筋经“托天桩”功法护理方案,在颈型颈椎病的治疗上,具有疗效显著,值得在临床上推广应用。

Exothermic process of cast-in-place pile foundation and its thermal agitation of the frozen ground under a long dry bridge on the Qinghai-Tibet Railway

A number of dry bridges have been built to substitute for the roadbed on the Qinghai-Tibet Railway,China.The aim of this study was to investigate the exothermic process of cast-in-place (CIP) pile foundation of a dry bridge and its harm to the stability of nearby frozen ground.We present 3D heat conduction functions of a concrete pile and of frozen ground with related boundaries.Our analysis is based on the theory of heat conduction and the exponent law describing the adiabatic temperature rise caused by hydration heat.Results under continuous and initial conditions were combined to establish a finite element model of a CIP pile-frozen ground system for a dry bridge under actual field conditions in cold regions.Numerical results indicated that the process could effectively simulate the exothermic process of CIP pile foundation.Thermal disturbance to frozen ground under a long dry bridge caused by the casting temperature and hydration heat of CIP piles was substantial and long-lasting.The simulated thermal analysis results agreed with field measurements and some significant rules relating to the problem were deduced and conclusions reached.

土体内混凝土桩基础钻孔雷达响应特征正演分析

采用FDTD方法,正演分析了单层及多层土体中无缺陷混凝土桩基,以及单层土体中设置有空洞、离析和缩径缺陷的混凝土桩基钻孔雷达响应特征。单层及多层土体中混凝土桩基桩底位置处的同相轴均出现扭曲等情况。圆形空洞反射波同相轴呈现双曲线形特征,方形空洞反射波同相轴呈现前部扁平曲线特征。离析缺陷雷达反射波同相轴特征与圆形空洞同相轴类似,但同相轴滞后。缩径处的反射波同相轴特征与方形空洞的同相轴特征相似,但缩径位置与天线位置的变化会导致同相轴滞后。

桩身质量检测的一种新方法——桩形估计法

本文提出了桩身质量检测的一种新方法——桩形估计法,或称“ESP”(Estimation of theShape of a Pile)法。该法基于一维杆应力波理论,将桩划分为N 个单元,根据波在各单元界面处反射与透射的传波原理,推导出在桩顶进行瞬态激振时桩顶的速度响应与界面反射系数以及桩的形状之间的关系。对几种典型缺陷桩进行了计算机模拟,其结果证明作者所提出的桩形估计法是可行的和有效的。现场试验更进一步证明:该法能够较准确地估计出缺陷桩或好桩的形状。

预制桩身复合载体夯扩桩在软土地区的研究与应用

复合载体夯扩桩是一种承载力高、成本低的桩基础,但在软土地区施工容易出现缩颈、断桩等缺陷,为此北京波森特岩土工程有限公司研究出了采用预制桩身的复合载体夯扩桩技术,该技术既具有传统复合载体夯扩桩承载力高的优点,同时又解决了软土地区混凝土桩施工中容易出现的缺陷问题,是一项值得大力推广的技术。详细介绍了预制桩身的复合载体夯扩桩的设计、施工等技术。

缩径基桩竖向承载性能的透明土试验研究

基于透明土试验,对1根完整基桩及8根缩径基桩进行了竖向加载试验,研究了缩径基桩的竖向承载性能.利用MatPIV软件对桩周土体颗粒的散斑场进行处理,得到桩周土体变形规律.在此基础上,通过对比完整桩,分析了缩径基桩竖向承载力的变化原因.研究结果表明:缩径对基桩竖向承载力有显著的影响.当缩径长度为桩长10%时,承载力损失可达45%;当缩径直径为桩径20%时,承载力损失27.3%.其原因为缩径尺寸较大时,缩径处的土体产生斜向下变形,降低了桩身与土体的相对位移,减小了桩身侧摩阻力,导致基桩承载力减小.对于某些工况,在缩径产生的阻力、桩端阻力及侧摩阻力的共同作用下基桩竖向承载力无变化,但其沉降较大,不宜承载.当缩径位于桩身浅部与中部时,承载力损失明显;当缩径位于深部时,缩径处土体与桩端土体会发生贯通现象,土体发生剪切破坏,削弱了桩的承载力.研究结果可为基桩的合理设计和加固提供一定的理论和技术参考.

高压旋喷技术在某办公楼基础工程中的应用

结合某办公楼工程概况,对高压旋喷注浆法的优点进行了阐述,并对其施工工艺和注意事项进行了介绍,最后在质量检测方面进行了论述,以推广高压旋喷技术在软弱基础工程中的应用。

基于GTS的黏土中缩颈桩检测研究

利用GTS有限元分析软件,对黏土中的缩颈缺陷桩进行数值模拟,得到不同阻尼系数及在不同缺陷位置下的反射波曲线。对结果对比分析,总结出振幅比随缩颈桩缺陷变化的规律,以及缩颈桩的缺陷程度与波振幅比之间的关系,对于工程桩的检测具有重要的指导意义。

沉管扩底灌注桩在工程中的应用

根据沉管扩底灌注桩的理论计算，结合工程实例进行静载试验、沉降观测等试验研究，对沉管扩底灌注桩的技术特点和经济指标做了对比分析，认为沉管扩底灌注桩优于其它桩型，为沉管扩底灌注桩的进一步推广应用提供了依据。

黄泛区长螺旋压灌法CFG桩施工力学问题分析

在地下水较高的粉土、粉质黏土交互土层中采用长螺旋钻孔压灌法施工时,常出现缩颈、桩头下沉、断桩等问题.基于土力学基本理论和施工力学方法,对黄泛区高水位条件下CFG桩病害机理进行分析,表明桩周粉土、粉砂土剪切液化处形成扩径与桩底空孔过长等原因会引起流态混凝土下沉,对上部混凝土产生的拖曳作用可导致桩顶下沉、桩身缩颈.超固结土、灵敏度高的黏土受施工扰动抗剪强度降低,在长螺旋钻杆移出桩孔后孔内流态混凝土产生的侧压力小于相应位置处主动土压力,加剧了这一进程,严重时可形成断桩.提出针对性的设计施工措施,可供研究与工程参考.

穴位埋线结合易筋经托天桩治疗颈型颈椎病临床研究

目的:观察穴位埋线结合易筋经托天桩治疗颈型颈椎病的临床疗效。方法:将颈型颈椎病患者60例随机分为对照组和治疗组,对照组采用单纯穴位埋线治疗,治疗组在对照组的基础上联合易筋经托天桩练习。两组均干预2个月,干预前后采用颈椎多功能评估训练系统(Multi-Cervical Unit,MCU)评测,以颈椎活动度、最大肌力、疼痛评分为观察指标。结果:治疗组脱落3例,对照组脱落2例。治疗组颈椎活动度、最大肌力、疼痛评分的改善程度明显优于对照组,差异有统计学意义(P<0.05)。结论:穴位埋线结合易筋经托天桩治疗优于单纯穴位埋线治疗,具有一定临床推广价值。

缩径承台基桩竖向承载力及桩周土体变形

为了探究含缩径缺陷的承台基桩竖向承载性能和桩周土体变形规律,以缩径位于桩体深部为例,对1根完整承台基桩和6根缩径承台基桩进行了室内透明土模型加载试验,得到了桩荷载-沉降曲线。通过数据处理,得到承台沉降4 mm(极限承载力)时的桩周土体位移场。研究了缩径轴向尺寸和径向尺寸对承载力和桩周土体变形的影响规律,给出了承载力的变化原因。研究结果表明:缩径会严重影响基桩承载力,当轴向尺寸为20 mm、径向尺寸为8 mm时,极限承载力损失可达17.59%。承台和缩径周围的土体产生了较大的变形,减少了桩与土之间的相对位移。随着缩径轴向尺寸和径向尺寸增大,土体变形增大。缩径周围和桩端之间的土体发生贯通现象,桩与土体产生了同步位移,且随着缩径尺寸越大,贯通现象越明显。承台和缩径周围的土体与桩之间的相对位移,以及缩径周围与桩端之间的土体产生的贯通现象导致桩的侧摩阻力降低,从而损失了桩的承载力。

Complex variable solution for boundary value problem with X-shaped cavity in plane elasticity and its application*

A new type of displacement pile, the X-section cast-in-place concrete （XCC） pile, has recently been developed in China. Extensive field tests and laboratory experi- ments are undertaken to evaluate its performance and quantify the non-uniform deforma- tion effect （NUDE） of the X-shaped cross section during installation. This paper develops a simplified theoretical model that attempts to capture the NUDE. Based on the theory of complex variable plane elasticity, closed-form solutions of the stress and displacement for the X-shaped cavity boundary value problem are given. Subsequently, the analytical solution is used to evaluate the NUDE, the concrete filling index （CFI）, and the perimeter reduction coefficient of the XCC pile cross section. The computed results are compared with field test results, showing reasonable agreement. The present simplified theoretical model reveals the deformation mechanism of the X-shaped cavity and facilitates applica- tion of the newly developed XCC pile technique in geotechnical engineering.

青藏铁路多年冻土区钻孔桩基础个别问题探讨及设计对策(英文)

Part of soil around cast-in-place pile will thaw because the heat of hydration produced by concrete during construction. In this paper soil upfreezing action to pile during refreezing process is analyzed, and the measures to the action are put forward. Furthermore, soil frictional forces to pile due to the thawing of part of soil around pile and the slowness of refreezing after construction is discussed and a rational method is suggested.