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.

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.

自升式钻井平台插拔桩技术发展现状及趋势

针对自升式钻井平台在插桩和拔桩过程中面临的土工问题,系统地梳理插桩和拔桩涉及的关键技术。分析插桩过程中的土工试验技术、土体破坏机理、上硬下软地层承载力和桩脚滑移问题;阐述拔桩过程中的拔桩阻力计算方法和减小拔桩阻力方法的研究进展情况。在此基础上,根据目前面临的新挑战,提出今后自升式钻井平台插拔桩技术研究的重点,并预测其发展趋势。

褶皱逆深地带机械冲孔桩施工技术

在传统桥梁工程桩基础施工中,大直径深孔桩常采用机械成孔桩法。云南红河州逆深大断裂地带为褶皱与断层交织地质,该地质形成年代晚,地质结构复杂、地理环境高差达440m。在以上条件限制下,常规深孔桩冲孔施工方法难以实现。以元江至蔓耗高速公路土建项目桩基工程为背景,总结褶皱逆深地段25~40m深孔机械成孔桩施工技术,主要从技术特点、工艺原理、施工工艺流程及操作要点、材料设备及质量控制等方面进行详述。

边坡加固的冲孔灌注桩基质量多法融合检测技术研究

地灾区域的边坡塌陷问题会直接影响该地区的长期安全性和稳定性,对该地区的灾后重建工作形成较大负面影响。针对地灾区域边坡塌陷加固作业工程中质量检测方面存在的问题,研究以某地灾区域边坡塌陷加固作业工作为例,对该类型工程中开展冲孔灌注桩施工的桩基质量进行检测分析。结果表明,相比于单一的检测方法,将采用低应变法、超声波透射法和钻芯法等相互结合的检测方法可以更为精准地发现桩基结构完整性描述,超声波-低应变联合法具有检测效率适中、破坏性小、检测精度理想等优势。

单桩水平静载试验在万州体育中心工程中的应用

对万州体育中心工程的3根机械冲孔灌注桩进行单桩水平静载试验,在桩身强度和水平位移两种控制工况下,确定其单桩水平承载力特征值、水平极限承载力,并推定地基土抗力系数的比例系数m值。试验结果验证了特殊地质条件下的桩基施工工艺,给出了桩基水平承载力与位移的计算参数,并为类似桩基工程的设计与施工提供了基础试验数据。

提高岩溶桥梁桩基质量的应用研究

广西区域喀斯特地貌的特殊地质,其中溶洞、地下河、暗湖、石钟乳、石笋对于桥梁桩基的影响尤为严重,溶洞良好的处理及桩基的完整性是整个线性工程中桥梁质量把控中关键的一环;桥梁桩基的质量决定了桥梁整体的稳定性及安全性。利用合理的施工工艺及溶洞处理方式方法来提高岩溶桥梁桩基的成桩质量合格率及施工效率,同时达到降本增效的目的。

复杂地质条件下钢板桩引孔施工技术研究与应用

本文以某项目工程为例,通过了解该项目地质特性,确定施工方案,重点从旋挖引孔、引孔灌砂、插打钢板桩、承台施工及钢板桩拆除等具体施工技术进行论述,最后对钢板桩围堰稳定性进行验算及监测,结果显示各杆件稳定性均符合要求,基坑水平位移监测数据达标,验证了该工艺的可行性。

喀斯特地区冲孔灌注桩施工特点及质量控制要点

冲孔灌注桩是一种常用的基坑支护和地基处理技术,被广泛应用于建筑、桥梁、隧道、码头等各类土木工程。冲孔灌注桩通过冲入基槽内的橡胶管道灌注钢筋混凝土,形成一个坚固的桩身,起到支撑和加固土体的作用。冲孔灌注桩作为一种有效的地基处理和基坑支护技术,施工过程需要严格按照设计要求和施工规范进行,以确保桩身的质量和整体工程的安全性。文章结合实际案例,主要探讨了喀斯特地区冲孔灌注桩施工特点及质量控制要点,旨在为同类工程项目的开展提供参考和借鉴。

防洪工程冲孔灌注桩常见施工问题分析

为了探究防洪工程冲孔灌注桩施工问题及处理措施,以福建省三明市沙溪流域防洪工程为例进行分析,并对该工程部分冲孔灌注桩进行超声波透射检测。结果显示,HH0+518.39号桩和HH0+619.29号桩的声速分别为5.2和5.5km/s,波幅分别为1 200和1 180pV,且二者的功率谱密度均未发生变化,表明HH0+518.39号桩和HH0+619.29号桩均不存在缺陷,为Ⅰ类桩。而其他桩的声速、波幅及功率谱密度均在相应位置发生突变,表明其他桩均存在一定缺陷。

高速公路软基路堤改扩建若干问题与建议

基于大量高速公路改扩建工程实践,对软基路堤改扩建的若干问题给出建议。水塘路段不能在塘底进行地基处理施工时宜采用搅拌桩,采用刚性桩时宜按桩承堤设计。既有边坡下软基处理工作面宜采用削陡边坡或钢板桩支护等方式形成。拼宽路堤沿其与既有路堤的界面滑动稳定性满足要求时,可不设置台阶。普通土拼宽路堤台阶壁宜倾斜、平台宜水平或轻微外倾。竖壁台阶高度大于44cm时会在台阶壁附近形成虚土区,需利用液压夯实或冲击碾压等对虚土进行压实,台阶高度大于2m时可利用注浆将虚土胶结。轻质土拼宽路堤不应用作挡土墙,应设置易于施工的排水设施,以避免轻质土拼宽路堤后面积水。轻质土拼宽路堤滑动稳定分析时,应只考虑斜坡范围内的交通荷载。除长螺旋桩机施工的钻孔桩外,其他桩帽之间不必设置连梁。

Experimental study on load bearing behavior of large-scaled caps with pile groups

The objective of this investigation was to study the behavior of deep pile caps and the ultimate load-carrying capacity. Four 1/10 scaled models of nine-pile caps were cast and tested on vertical loads to failure. The destruction shapes of pile caps,the correlation between load and displacement,and the internal stresses were analyzed systematically. The results demonstrated that the failures of all the four models are resulted from punching shear; the internal flow of the forces in nine-pile caps can be approximated by "strut-and-tie" model. Furthermore,the failure loads of these specimens were predicted by some of the present design methods and the calculated results were compared with the experimental loads. The comparative results also indicated that the "strut-and-tie" model is a more reasonable design method for deep pile caps design.

埕岛油田反复插拔桩区域就位风险及预防措施

胜利埕岛油田位于现代黄河水下三角洲,海底地层复杂,沉积物粉土、粉质黏土互层变化大.近年来,动平台作业在同一井口或相近井口反复插拔桩作业次数越来越多.海上动平台反复插拔桩区域存在滑桩、穿刺、液化下沉风险,在该类型区域插桩存在较大隐患.针对该风险进行预防措施分析,降低动平台插桩风险.

冲孔灌注桩技术在建筑工程施工中的应用分析

冲孔灌注桩技术依托自身所具有的灵活性强、负面影响小和单桩承载力大等优势,得以在建筑工程施工实践中广泛应用。在冲孔灌注桩技术应用阶段,受到多重因素影响,容易诱发一系列问题,影响建筑工程施工效果。基于此,该文对冲孔灌注桩技术于建筑工程施工中的应用予以研究,在简单介绍冲孔灌注桩技术内涵、优势和应用流程基础上,引进具体的工程案例,结合案例探讨冲孔灌注桩技术应用,并针对性构建质量控制措施,期望以此提升冲孔灌注桩技术应用水平,从整体上强化建筑工程施工质量,在一定程度上也可为后续阶段建筑工程施工中冲孔灌注桩技术的应用提供参考。

冲孔灌注桩施工常见问题分析及解决对策

冲孔灌注桩是建筑工程中普遍应用的一种桩基施工工艺,具有施工操作方便灵活、场地环境适应性强、承载力大、桩端穿透力强等优点。文中结合实际工程案例分析了冲孔灌注桩施工工艺,提出施工存在的常见问题及解决办法,仅供参考。

冲孔灌注桩施工技术在深厚砂层基础工程中的应用

冲孔灌注桩在水利、桥梁、建筑等工程中应用广泛,但在深厚砂层复杂地质工程中,其在砂层地质中的护壁效果较差,易在冲孔施工中出现护壁塌陷埋钻、砂层扰动造成桩身缩颈、桩身夹泥等问题。对此,文章结合广州北站综合交通枢纽工程农新路高架桥项目,深入研究了深厚砂层基础工程中应用冲孔灌注桩施工技术存在的问题及处理方法,详细阐述了冲孔灌注桩施工技术要点,以期为同类工程施工提供有益参考。