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.

超近距箱涵顶进上跨既有地铁线路施工关键技术

以北京芙蓉路匝道箱涵顶进为工程背景,通过数值模拟、理论计算及现场监测等方法研究了箱涵顶进对既有地铁线路的影响。对工作坑支护、滑板施工、箱涵制作、箱涵顶进等施工要点进行把控,制订了科学、合理的防控措施。研究表明:箱涵顶进施工穿越既有地铁线对结构竖向和水平变形影响较大,顶推完成后结构变形呈椭圆形。采取“袖阀管注浆+非全嵌固桩”进行底板及基坑加固的方法可显著提高地基承载力,降低箱涵顶进对既有地铁线路的影响。

高层建筑桩基岩溶处理施工技术研究

针对高层建筑地基岩溶面广而深的特点,依托实际工程提出了先分区分块,后点-面相结合的岩溶处理方法。具体为先将整个场地划分为岩溶发育区、岩溶较发育区和岩溶弱发育区,对高密度发育区采用点-面结合的方法处理。其中面处理采用岩面注浆法加固岩面以上地层,以形成封闭。岩面以下则依据溶洞填充情况采用了袖阀管注浆或泵送水泥砂浆进行点处理,使洞内充填物与洞外介质固结成一个整体。后续地基处理及桩基检测结果均表明该岩溶处理施工技术能够满足高层建筑岩溶处理要求,值得进一步推广应用。

码头桩基钢套筒灌浆加固修复技术

针对服役多年码头桩基混凝土出现的破损裂缝问题,依托宁波某码头水工大修工程,对钢套筒、灌浆作业系统及超早强灌浆料等进行设计开发。结合施工工艺形成成套加固修复技术,开展现场施工并进行质量检测。结果表明:超早强灌浆料12 h抗压强度可达到20 MPa以上,其与钢套筒黏结强度可达6 MPa以上,其他性能参数均符合规范要求;使用该成套技术在6个月内成功加固修复232根桩基。

基于SACS和ABAQUS的裙桩套筒局部结构分析

裙桩套筒作为导管架平台附属部件,主要用于连接导管架平台和桩结构,起到支撑和提高稳定性的作用。但在导管架平台整体分析中,对其局部强度不能进行准确分析和校核。因此,需要采用SACS(海上结构设计分析软件)和ABAQUS(有限元工程模拟分析软件)有限元相结合的方法对裙桩套筒进行分析。以东海某导管架平台项目为例,基于SACS整体结构分析结果,得到较危险的工况,提取关键节点的位移、力和弯矩载荷,对目标裙桩套筒建立有限元模型,在三种极端载荷环境工况下进行局部强度分析和剪切板屈曲分析。结果显示,去除下斜撑与主腿连接处以及上斜撑、剪切板与主腿连接处等应力集中位置后,裙桩套筒局部整体强度基本满足强度要求;极端工况下剪切板屈曲校核后的最大计算使用系数满足DNV(挪威船级社)标准。研究结果证明,该分析方法可以提高分析平台桩腿局部结构强度和抗剪强度的准确度,对新建平台桩腿设计和在产平台后续改造适应性校核具有重要实际工程意义。

膨胀静力在灌注桩桩头破除工程中的应用

桥梁桩基破除桩头施工一直是施工技术难点,施工时经常会破坏桩头钢筋,从而影响桩基质量,并且桩头破除工程费时费力。以赣南大道快速路某桩基破除桩头施工为例,采用静力膨胀法破除桥梁灌注桩桩头,运用该工艺桩头能够有效实现截桩面部位出现微观水平裂缝,有利于桩头整体破除,桩头部位钢筋洁净无损伤,施工过程中无扬尘、无噪声,绿色环保,经济低碳,可以有效节约施工成本,提高施工效率。

袖阀管应用于灌注桩后注浆的现场试验

针对传统的灌注桩后注浆技术存在的不能多次压浆的问题,将袖阀管注浆技术与灌注桩后注浆技术结合,研究探讨将袖阀管应用于灌注桩后注浆施工。在雄安新区某工点通过开展现场试验,发现袖阀管注浆容易出现注浆管连接耗时长、注浆器与射浆孔匹配难度大、袖阀管堵塞等问题,采用相关技术措施后,袖阀管注浆施工顺利完成,验证袖阀管注浆应用于灌注桩侧、桩端加强的可行性。

污泥层置换砂桩套打水泥搅拌桩软基处理技术

水泥搅拌桩是软基处理的一种常用方式,针对水泥搅拌桩在污泥弱软层出现水泥固化效果差等问题,通过在污泥内预先施工小直径砂桩,用中粗砂置换污泥层中腐殖质,同时以“小桩套打大桩”的形式,将水泥搅拌桩套打在同桩位处的砂桩上,采用“四搅四喷”工艺,置换后砂土与水泥浆液充分均匀搅拌结合成桩,有效增强桩身强度,确保搅拌桩满足设计要求。在提高工效、确保桩体质量及降低综合施工成本等方面都突显出了独特的优越性,为解决腐殖质有机物污泥层中传统水泥搅拌桩施工效果差、成桩质量难以保证等问题提供了一种创新、实用的工艺技术。

桩基础岩溶(土)洞处理施工技术

桩基础工程建设项目,由于岩溶(土)洞的发育,容易导致工程质量事故,为保证桩基础设施安全,质量、进度、成本目标可控,在实际的工作中,选用较为成熟的袖阀管注浆处理技术,通过地勘资料的分析,明确溶(土)洞的发育区域,合理布置注浆孔与排气孔的位置与深度,选取合适的注浆参数及工艺,能有效解决岩溶引发的危害。该方法可用于喀斯特地貌地区一定规模的溶(土)洞地基处理。

徐工XR580HD、XR400E旋挖钻机在硬质泥岩超长桩基中的应用

G244线庆阳过境段公路工程火巷沟特大桥16号主墩桩基工程的墩位出露地层为第四系风积黄土夹古土壤,下伏白垩系泥岩,桩身上部黄土地质中粉质黏土层极易塌孔,加之下部泥岩层抗压强度大,具有桩基数量多、桩长深、孔径大、地质复杂,成孔难度大、施工工期紧等难点。文章根据技术分析,选用了徐工XR580HD、XR400E旋挖钻机进行成孔,取得了良好效果。在此基础上,总结了前述不利条件下的大直径超长桩基主要施工技术,旨在为今后黄土地区超长桩基底部泥岩层旋挖成孔施工提供参考。

抗拔管桩预埋机械套筒连接件轴拉承载性能试验及数值分析

【目的】验证用于抗拔管桩的预埋机械套筒连接件的轴拉承载性能。【方法】完成了6组抗拔管桩张拉机械套筒连接件的轴拉试验,得到了最大破坏荷载和破坏形态,结合有限元计算结果,分析试件应力分布规律,找出应力集中点或薄弱部位。【结果】结果表明:预埋机械套筒连接件整体满足抗拉承载力要求,试件极限破坏时,表现出的主要破坏现象为套筒卡槽和钢棒墩头破坏。墩头和卡槽接触面倾斜角度减小对改善试件应力集中现象有明显效果,建议倾斜角度控制在15°以内。【结论】桩身混凝土会限制套筒连接件的变形,提高试件抗拔过程中的安全可靠性。

地铁高架桥桩基施工对邻近构筑物的影响及控制

以长沙地铁1号线北延线上跨铁路框架桥和芙蓉北路空心板桥桥台为工程背景,构建桩基邻近既有构筑物施工三维有限元模型,分析三种钻孔顺序、地层未加固和加固情况下,桩基施工对地层、既有框架桥和桥台的受力及变形的影响。结果表明:地层沉降大致呈U形分布,沉降大小与距桩基距离成反比;排式钻孔、连续钻孔、跳桩钻孔对土体的扰动依次减弱;框架桥破坏受拉应力控制,最大拉应力出现在框架桥顶面;在采取跳桩钻孔和袖阀管地面注浆、桩基反循环钻机成孔和实时监测三种变形控制措施后,框架桥和桥台的最大沉降与未采取控制措施时相比分别减少46.7%和48.5%,其中部和两端的差异沉降分别减少31.9%和44.4%,拉应力减少64.4%。这些措施显著提高了框架桥和桥台的安全性,可保证既有线正常运营。

堆载条件下软土地区桥梁桩基础负摩阻力受力机理及特性研究

某主跨为97.6 m的下承式系杆拱桥桥墩设置在黄河大堤外侧的淤背区,由于其内堆载土的逐年抬升使得桩基础受到负摩阻力作用。为准确把握负摩阻力的受力机理以及对桩身承载能力的影响,分别采用公路和铁路桩基础设计规范中的计算方法,并结合有限元方法,计算了有无套筒条件和不同堆载土高度条件下桩基的受力特性,研究了负摩阻力的受力机理和分布特性,获得了桩身在负摩阻力影响下的轴力。计算结果表明:填土前,桩身最大轴力在桩顶以下15 m深度处;施加填土荷载后,桩身轴力增加60.2%,最大轴力位置在桩顶以下50 m深度处。钢套筒虽然能减小其范围内桩基最大负摩阻力的75%~78%,但由于桩基中性点下移和钢套筒范围外负摩阻力的增大,桩基的最大轴力值仅减小5%,整个桩基的沉降量相差最大为2%。由以上结果可知,钢套筒无法有效提升负摩阻力影响下的桩基承载力,因此提高桩身材料强度是提升其承载力的可行方案。

深基坑工程地下连续墙与咬合桩接头处施工关键技术

针对同时采用地下连续墙和咬合桩2种支护形式时,结合处简单地采用高压旋喷桩封闭存在较大质量和安全隐患的问题,对接头工艺进行了改进。提出了采用套铣工艺对地下连续墙和咬合桩结合处进行施工的方法,使结合处通过混凝土连接形成整体,总结出了深基坑工程地下连续墙和咬合桩结合处施工关键技术。新技术使得地下连续墙和咬合桩结合处过渡连接更为可靠、方便、经济,具有较好的推广应用价值。

袖阀管注浆技术在管桩引孔加固中的应用分析

文中阐述了预应力高强混凝土管桩采用引孔法施工出现的问题—管桩入土深度小于引孔深度,分析了引孔后管桩入土未及孔底的原因,制定并实施对于引孔后预制管桩进行袖阀管注浆的加固处理方案。经检测结果显示,袖阀管注浆技术可有效解决引孔后管桩施工出现的此类问题,对桩底沉渣及桩周松弱土体具有显著的加固作用,可规避桩身承载力无法满足设计要求的风险。