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.

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 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.

FGC-15D large-diameter DTH air hammer drilling system and its application in offshore rock-socketed pile hole drilling

In the past two decades,numerous large-diameter rock-socketed piles were constructed in China to support foundations of skyscrapers,great bridges or to retain soil in potential geological hazard areas.However,drilling large-diameter rock-socketed pile holes with conventional drilling method such as rotary drilling or cable tool drilling is time-consuming and the cost is usually very high.In order to drill large-diameter rock-socketed pile holes faster at relatively low cost,the FGC15A large-diameter DTH air hammer drilling system was developed in 1987 and was given the second-class award by Ministry of Geology and Mineral Resources in 1991.Since it was innovated the drilling system has been used in more than twenty important and tough projects on land,and wonderful results were acquired.At the same time the large-diameter DTH air hammer drilling system was improved continuously.The FGC15D is the latest version of the technique.

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。

圆形抗滑桩-拱形挡土板支挡性能与参数分析

为规避当前施工方法的不足,发展安全、高效和可靠的施工方法。依托东北地区某高速铁路路堑边坡工程,采用适于机械钻孔的圆形抗滑桩代替现行矩形抗滑桩,预制拱形挡土板,本文提出一种新型拱形板-桩墙支挡体系;并构建有限元模型,分析桩长、桩径、桩间距以及拱形挡土板矢跨比对新型拱形板-桩墙支挡体系支护性能的影响;揭示不同参数下桩顶水平位移与桩身内力的变化规律。研究结果表明:桩长与桩身弯矩和剪力大小呈正相关关系,与桩顶水平位移呈负相关关系。桩长超过14 m时,其对桩顶水平位移、桩身弯矩和剪力的影响效应明显减弱;随着桩间距增大,桩顶水平位移增大但变化幅度逐渐减小。拱形挡土板矢跨比增大时,桩顶水平位移、桩身弯矩和剪力均减小。影响拱形板-桩墙体系支挡性能的因素依次为桩间距、桩长、桩径和矢跨比。研究结果为圆形抗滑桩-拱形挡土板支挡体系在实际工程中的应用提供新思路。

硬质岩滑坡深大抗滑桩一体化旋挖成孔技术及应用

硬质岩深大抗滑桩桩孔成孔效率是滑坡灾害防治的难点问题之一。选取川西南典型复杂山区硬质岩滑坡抗滑桩成孔为研究对象,系统分析了裂隙弱发育深部硬质玄武岩地层抗滑桩传统工艺成孔存在的问题,对此设计了9种组合旋挖成孔工艺方案进行现场试桩试验,从钻进效率、技术及经济可行性等方面进行了分析论证。现场试验及分析结果表明:采用“大扭矩截齿筒钻旋挖+扩孔直径差0.4 m”分级扩孔一体化成孔技术在硬质岩地层中钻进效率较高、钻具损耗较低、成本较低,验证了硬质岩抗滑桩高效旋挖分级扩孔技术的可行性和高效性,对比现有人工成孔、冲击钻成孔、旋挖一次成孔等工艺,从安全、效率、成本等方面均得到了极大提升。该技术成功应用于白鹤滩迁建集镇后缘滑坡桩基施工,每延米综合耗时1.43 h,较改进前旋挖一次成孔效率提升3倍,成本降低了60%。该技术为复杂地质条件下水敏型、震敏型硬质岩滑坡抗滑桩快速成孔施工提供了高效、可行的技术方案。

多维度深水浅层建井方式优选方法研究

深水钻井作业具有海洋环境多变、地质条件复杂、作业风险大和日费率高等特点,深水浅层建井的方式难以选择。针对海底土质强度、建井质量、作业时效、经济性、风险控制等因素,根据现场作业工程数据,分别对不同建井工艺的适应性进行了分析,建立了单维度的适应性分级机制,并借助雷达图进行定量可视化,形成了多维度深水浅层建井方式优选方法。研究结果表明,水深500~1500 m的南海北部海域,单井作业时,喷射法一般为浅层建井最优方法,作业效率相比钻入法可提高50%以上。该方法已在中国南海几十口深水井取得良好应用效果,可为深水复杂地层建井方式选择提供定量评价方法。

深水钻井井口吸力桩稳定性计算和校核方法

深水松软土质下,传统喷射法安装的表层导管在钻井阶段存在下沉和倾斜的风险。为了提升深水钻井水下井口的稳定性,提出了井口吸力桩表层建井的方法,开展了吸力桩下入深度和稳定性分析。基于桩基承载理论和广义Winkler地基梁模型,建立了井口吸力桩稳定性理论模型,并通过有限元方法和有限差分法研究了井口吸力桩稳定性特征及影响因素。以南中国海某井工程参数为依据,对井口吸力桩稳定性开展理论计算,与有限元计算结果进行对比,竖向极限承载力误差为4.23%,最大水平位移误差为6.0%,最大弯矩误差为2.96%,印证了计算结果的准确性。在钻井工况极限载荷作用下,相比于传统表层导管,井口吸力桩竖向承载力提升了43.91%,水平位移减少了63.21%,抗弯安全系数是前者的24.5倍。分别对井口吸力桩施加不同水平位移和弯矩,结果表明水平载荷对横向稳定性影响较为显著。井口吸力桩具有很高的竖向承载力和横向承载力,显著提高了水下井口的稳定性,研究结果为其在深水松软土质中表层建井提供了理论依据。

南宁盆地特殊地层静压随钻跟管桩适应性及单桩承载力特性研究

为了研究静压随钻跟管桩在南宁盆地特殊地层的适应性及单桩承载特性,对现场4根直径800mm的静压随钻跟管桩进行静载及低应变反射波法试验。研究结果表明:各地层中的试验桩承载力均能满足设计要求且均属于Ⅰ类桩,桩身回弹率较高,说明各试验桩仍具有较大的承载潜力;未进行破坏性试验时各地层中单桩承载力综合增强系数均大于1.00,以经验公式确定静压随钻跟管桩的单桩竖向承载力时应乘以相应的增强系数。基于以上研究,将静压随钻跟管桩工艺应用于实际工程,经验收检测表明,各项目受检基桩的桩身质量完好,承载力均满足设计要求,且仍具有一定的承载力富余量,良好的实际工程应用效果表明该工艺具有良好的适用性以及较高的推广使用价值。

CFG桩在高铁软土路基建设中的应用

CFG桩是一种针对软土路基施工建设的有效施工工艺,对软土路基处置具有良好效果。该文以某高铁工程为例,对CFG桩加固软土层的基本原理进行叙述,从长螺旋钻孔施工工艺、桩帽施工等关键环节对CFG桩施工工艺流程进行探讨,并针对性地提出CFG桩施工的关键要点和注意事项。该文旨在为类似工程施工的优化设计提供参考。

埋入式后压浆管桩竖向抗拔破坏模式研究

埋入式后压浆管桩作为一种新型桩基础,集钻孔灌注桩和预应力管桩的优点于一体,可以解决灌注桩桩身质量不可靠和预应力管桩沉桩困难等问题。对埋入式后压浆管桩的竖向抗拔试验进行CT扫描,得到不同位移状态下模型桩的CT扫描图像,直观地揭示破坏面的发展规律,并建立破坏面方程。对其他学者提出的抗拔桩破坏面方程进行优化,通过静力平衡方程,获得优化后破坏面方程对应的埋入式后压浆管桩抗拔极限承载力函数,并采用遗传算法确定优化后破坏面方程的参数。将多种理论假定破坏面与试验获得的破坏面进行对比分析,验证所得破坏面方程的准确性。研究发现:埋入式后压浆管桩在上拔位移到达0.45倍桩径时发生破坏,此时灌浆层与土体紧密结合,两者之间未出现明显位移,破坏仅发生在灌浆层之外的桩周土体中;破坏面从桩底向桩顶延伸,越靠近桩顶,破坏面距离桩身越远,破坏面在桩底与混凝土侧壁相切,在地面与水平面的夹角接近42°,呈喇叭形破坏;本文的理论假定破坏面与试验获得的破坏面的相关指数R^(2)=0.97,前人的理论假定破坏面与试验获得的破坏面的相关指数R^(2)=0.91,本文的破坏面方程更加贴近试验获得的破坏面。研究成果对埋入式后压浆管桩的设计计算具有重要的参考价值。

基于MFO-BPNN的螺旋钻机钻速预测研究

针对利用现有经验公式所建立的螺旋钻机钻速预测模型存在准确度不足的问题,提出了一种基于飞蛾扑火算法(MFO)的反向传播神经网络(BPNN)钻速预测模型。首先,对MFO算法的基本原理进行了研究,构建了MFO算法优化BPNN的具体流程;接着,采集了江苏无锡某施工现场钻探数据,并分析了钻速影响因素,运用小波阈值降噪、归一化和灰色关联度分析等系列方法对采集数据进行了预处理,得到了训练和测试集;然后,将MFO算法运用于神经网络的权值和阈值训练,以代替原有梯度下降法,建立了MFO-BPNN钻速预测模型;最后,对上述预测模型与BPNN模型、遗传算法优化反向传播神经网络(GA-BPNN)模型以及粒子群优化算法优化反向传播神经网络(PSO-BPNN)模型的预测结果和评价指标进行了详细的对比分析。研究结果表明:运用MFO-BPNN建立的钻速预测模型,其可靠性达到了91.65%,其决定系数(R 2)优于其他3种预测模型,3项误差指标也是其中最低的,说明该模型的预测精度良好,适合于桩基础工程的实际应用,可为复杂因素影响下的钻速预测提供一种新思路。

钦州港首座大型高桩梁板码头建设关键技术与对策

钦州港目前已有大型码头主要为重力式结构,依托金鼓江作业区19号泊位工程,探讨本地区首座大型高桩梁板码头实施的关键技术问题与对策,并统计分析其实施功效,为当地的港口发展提供一种新的结构选型,有效丰富了技术储备。本文论证了高桩结构的适应性,突出建设中克服的主要难点,包括采用引孔植桩的新工艺,解决了深水裸岩环境下的临时辅助钢管桩与灌注桩钢护筒的施打难题;首次研究了炸礁与灌注桩浇筑的交叉施工机制,填补了行业实践空白;提出了新结构形式下的相应安全管理要求,补充了当地的安全文化建设。

湿陷性黄土地区百米超长桩施工关键技术研究

依托陕西省铜川市玉皇阁二号桥及引线工程百米超长桩项目,比选了各类成孔方法和钻机,选定旋挖成孔法施工工艺和徐工XR800E旋挖钻机,完成该百米超长桩施工。介绍了百米超长桩的关键技术要点和质量控制措施,并从钻孔前的预处理、钻孔过程中的综合处理、钢筋笼施工控制措施等方面保证该超长桩施工的可靠性。通过试桩试验,确定了百米超长桩在湿陷性黄土场地的承载力和沉降特性。施工过程中,严格把控土层钻进参数、泥浆循环系统、沉渣厚度、钢筋笼安全稳定性验算、水下混凝土灌注等关键环节,为类似湿陷性黄土地区百米超长桩施工提供解决方案。

既有挡土墙加固关键技术研究

本文依托某挡土墙加固工程,结合实施环境、项目实施工期、安全、经济、技术等因素对加固方案进行了综合比选,选用扰动小的拼接螺旋钢桩进行地基加固处理,选择轻型悬臂式挡土墙结构方案,采用自钻锚杆将既有浆砌片石挡土墙与悬臂式挡土墙进行连接,打通泄水孔,加强现场监测,最终确定合理的路基结构形式。本研究为类似挡墙加固工程提供了借鉴案例。

特大枢纽型机场桩基工程的质量信息化管理研究

对群桩施工过程中桩基工程质量管理进行了研究,旨在通过信息化手段实时记录桩基工程过程信息。重点梳理了钻孔灌注桩涉及的关键数据字段和工作流程,同时接入了钢筋笼加工管理系统、混凝土材料供应商系统,开发了一套基于桩基工程质量信息化管理的模块,实现了现场移动端进行关键数据填报并拍照记录生成无纸化表单并形成一桩一档的资料,进一步对过程数据进行分析,及时掌握桩基质量情况,以期为类似工程项目实施提供借鉴。