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.

Limit analysis of vertical anti-pulling screw pile group under inclined loading on 3D elastic-plastic finite element strength reduction method

Based on the functional theory, catastrophe theory, simultaneity principle and the idea of strength reduction method （SRM）, the bearing capacity functional anti SRM of pile group foundation were established, and the criteria of ultimate load and the concept of safety storage coefficient （Css） were advanced. The inclined ultimate loads by the static loading test, load increment method （LIM） and SRM are compared. Theoretically, the ultimate load of piles does not change with the loading levels when it is calculated by SRM. When the one strength reduction parameter is applied in the calculation boundary, there are calculating errors because the bearing capacity action of soils happened in the finite zone. The inclined 10adings are 108, 132 and 144 kN, and SSC are 1.07, 0.94 and 0.79, respectively, so the calculation values of ultimate loads are about 115.56, 124.08 and 113.76 kN, respectively. The error between calculations and observation values is less than 6%. But .the error between calculations of LIM and observations is 20%. Because of the effect of inclined loading, the push-rotation phenomenon of screw pile group appears. Under this testing, the ultimate bearing capacity of piles is mostly determined by the horizontal ultimate bearing capacity, and the effect of the vertical component of inclined load should also be considered.

杂填土地基中螺杆桩竖向承载特性研究

为了研究杂填土地基中螺杆桩竖向承载特性,基于极限平衡理论分析了桩-土咬合机理,结合桩-土之间的相互作用得到螺纹段等效侧阻力增大系数,在此基础上建立了单桩竖向承载力计算公式,并对杂填土地基中螺杆桩单桩进行缩尺模型试验。结果表明,螺杆桩在竖向荷载作用下,荷载-沉降曲线经历了弹性、弹塑性、塑性破坏3个阶段。根据模型试验所得桩身轴力和桩侧等效侧阻力沿深度的分布规律,总体上螺纹段等效侧阻力高于直杆段。当桩体达到极限承载力时,试验所得等效侧阻力扩大系数与理论分析结果基本一致。

砂土中抗拔螺杆桩承载力及变形特性研究

通过基于数字图像相关技术的模型试验和离散元数值模拟方法(PFC),研究了叶片设置、砂土相对密实度、受荷工况等因素对螺杆桩抗拔承载力和变形特性的影响,利用试验结果校准离散元程序的细观参数,模拟结果用于分析螺杆桩的荷载分担特性、螺纹叶片间距对承载力的影响以及桩周土体滑动面特征。结果表明:抗拔工况下桩基荷载位移曲线呈现软化特征,0.05 D破坏准则适用于抗拔桩的极限承载力判定,叶片的设置使得桩承载力提高37%,螺纹段荷载分担比接近70%,直杆段荷载分担比是抗压工况的2倍;上拔承载力随叶片间距增加呈降低趋势,H/D=1.00是此工况下合理参数;砂土相对密实度的提高使得螺杆桩和直杆桩抗拔承载力分别增长19.4%、26%,而对抗拔桩周围土体位移模式影响有限,螺杆桩桩周土体最大影响范围约为4 D,而直杆桩约为2 D。

不同推进比下螺旋桩在砂土中安装力的离散元研究

螺旋桩作为一种颇具潜力的海上风电基础形式,其安装问题是螺旋桩能否成功应用在海上风电工程中的关键。基于离散元方法,采用PFC^(3D)模拟分析了螺旋桩在不同推进比下安装时的桩身受力情况以及对周围土体的影响。宏观上,研究了螺旋桩在不同推进比下安装时螺旋桩连杆侧壁、连杆端部、螺旋叶片上下表面的受力变化规律;微观上,分析了螺旋桩安装过程中桩周土体孔隙率、配位数及应力变化规律。研究表明:随着推进比的减小,螺旋桩安装时受到的竖向力与扭矩也随之降低,对桩周土体的扰动也更小;螺旋桩在推进比为1时安装与在推进比小于1时安装存在两种不同的安装机制。研究结果对进一步确定螺旋桩在不同推进比下的安装条件具有一定意义。

成层土中考虑桩侧土竖向作用的螺纹桩动力响应

螺纹桩与桩侧土的作用相对复杂,为研究竖向荷载作用下螺纹桩的振动特性,从桩侧土的三维波动出发,考虑土体的成层特性和桩侧土的竖向作用,对滞回阻尼地基中螺纹桩纵向振动特性进行研究。基于三维波动理论建立桩侧土波动方程,采用Laplace变换和修正阻抗函数传递法得到桩土完全耦合条件下的螺纹桩振动响应解。将理论计算结果与现场实测曲线进行对比,证明螺纹桩-土相互作用模型的合理性。结果表明:相比桩侧土为均质体的假定,考虑土体的成层特性可以充分考虑应力波在分层界面的反射和折射现象,更符合实际情况;在有效桩长范围内,随着桩长的增加,桩侧土对螺纹桩的侧摩阻力越大,阻尼效应越明显;螺纹桩螺牙的存在对桩体具有一定的减振效应,且随着螺牙外径的增加,桩顶复刚度曲线的振幅水平和共振频率均显著减小。理论模型能较好地模拟螺纹桩与成层土的相互作用机理,为螺纹桩的应用提供理论支撑。

黄淮冲积平原地区高速铁路螺杆桩复合地基应用研究

依托郑徐客运专线兰考南站路基工程螺杆桩复合地基项目,通过试桩和桩身质量检测,总结黄淮冲积平原地区螺杆桩关键施工工艺和质量控制措施,同时对螺杆桩承载力特性及复合地基进行工后沉降分析。结果表明:黄淮冲积平原地区粉质黏土、粉砂、细砂、圆砾土土层中可以形成螺杆桩螺纹段桩身,成桩质量良好;螺杆桩单桩承载力和复合地基承载力满足设计要求;该地区同等条件下,螺杆桩极限承载力较普通灌注桩提高约24%;复合地基沉降主要发生在填筑期;双曲线法预测得出三个观测断面工后沉降量均小于15mm,满足无砟轨道铺设要求。

螺杆灌注桩在某高层项目中的应用

建筑桩基大多采用预应力管桩和灌注桩,前者出现的质量问题较多,后者造价较高,寻找新的桩基类型成为具有实际意义的一个工程问题。本文对一种较为新型的桩型——螺杆灌注桩的受力原理、设计、施工重点及在某高层项目中应用的情况做了详细介绍,供工程设计人员参考。

钢螺杆锚桩压重联合静载试验钢梁设计与验证

为保证大吨位桩基静载试验安全实施,加载反力结构的合理设计至关重要。建立了加载反力平台钢梁的简化计算模型,分析不同受荷工况下钢梁的内力与变形发展规律,并对主、次钢梁截面尺寸进行优化设计。最后,针对实际静载试验项目对钢梁竖向位移、倾斜及锚桩抗拔力进行监测,验证钢梁计算模型的合理性。分析结果表明,钢梁在整个静载试验过程中的变形实测值均小于理论计算值;当施加的静载试验总反力超过堆载块和钢梁自重后,钢螺杆锚桩参与受荷,随着加载荷载的增大,其抗拔承载力的贡献逐渐增大;对于采用钢螺杆锚桩联合压重反力装置的静载试验,监测的各根钢梁竖向位移和倾斜总体一致,未出现明显偏心受力情况,表明该静载试验方法具有较高的安全性。

不同工况下螺锁式预应力混凝土实心方桩的桩身完整性分析

螺锁式预应力混凝土方桩的桩身完整性检测方法主要采用低应变法,实测曲线中接桩位置经常出现明显同向反射,对桩身完整性的合理判定产生影响。在没有桩周土和施工工艺影响的工况下,采用低应变法对 3 组桩的桩身完整性进行对比验证,结果表明同时采用螺锁式和密封材料连接工艺的桩的桩身完整性可满足要求。在实际项目的桩基承载力问题处理过程中,低应变法无法准确判定接桩位置的缺陷程度,而高应变法可以作为基桩施工质量综合判定的有效手段。

螺杆桩自动定位成桩工艺在软基处理中的应用

研究了螺杆桩自动定位成桩工艺在软基处理中的应用。软基是土木工程中常见的地基类型,其稳定性对工程的安全性和长期使用性具有重要影响。提出了一种基于螺杆桩的自动定位成桩工艺,通过引入现代化的测量和控制技术,实现了在软基处理过程中的高精度成桩定位。实验结果表明,螺杆桩自动定位成桩工艺可以显著提高软基处理的效率和准确性,降低工程成本,并改善工程的稳定性和使用寿命。

螺杆桩竖向抗拔承载性能原位试验研究

本文研究了螺杆桩的竖向抗拔承载性能,分别对螺杆桩与直杆桩进行了现场原位静载与内力测试试验,记录了不同荷载下桩身轴力分布情况,对比分析了螺杆桩与直杆桩竖向抗拔承载性能差异及原因。研究结果表明:在本文地层条件下螺杆桩的竖向抗拔承载力较直杆桩可提升5%~15%;随着上拔荷载增大,螺杆桩螺纹段侧阻优势开始发挥,桩轴力在螺纹段下降更快,轴力曲线呈折线;相同上拔荷载作用下,螺杆桩产生位移更小,其抗拔力主要由螺杆段产生,其上部直杆段侧摩阻力发挥水平不如直杆桩侧摩阻力。

长螺旋钻孔压灌桩施工中的孔壁稳定性控制研究

长螺旋钻孔压灌桩是一种被广泛应用于各种工程项目中的地基处理技术,其施工过程中孔壁稳定性的控制是保证施工安全和质量的关键环节之一。本文探讨了孔壁稳定性控制的重要性、土壤类型、钻孔直径和深度、孔壁支护方式和材料选择、钻进液的选择和使用、土壤处理措施等内容,并对不同工程项目的孔壁稳定性控制方法进行了比较分析,以期为相关领域的工程技术人员提供参考和借鉴,提高工程质量和效率。

螺杆灌注桩抗拔承载机理的试验研究

通过室内模型试验和现场试验对螺杆灌注桩抗拔承载机理进行了研究,研究表明螺杆灌注桩作为抗拔桩优势较为明显。通过室内模型试验,采用图像处理技术直观地得到上拔荷载作用下螺杆桩和直杆桩的桩周土体位移场和上拔破坏模式,进而对其抗拔承载机理进行探讨,并采用现场试验进行验证。模型试验和现场试验表明,螺杆桩的单桩抗拔承载力高于同直径的等截面桩,螺纹段的抗拔侧阻力与等截面桩的侧阻力相比明显提高。对于螺纹段,极限抗拔侧阻力表现为周边土体的抗剪强度,破坏面为连续的拱形;对于直杆段,极限抗拔侧阻力表现为桩体与桩周土体的摩擦阻力,破坏面为桩周圆柱面。根据试验结果从工程实用的角度,引入抗拔折减系数给出螺杆灌注桩单桩抗拔极限承载力的表达式,并给出相关参数的取值建议。