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.

山地光伏微孔灌注桩基础质量控制技术研究

为解决山地光伏支架微孔灌注桩施工难题,笔者分析山地光伏电站特点及微孔灌注桩基础施工难点,以甘洛320 MW光伏电站为例,研究微孔灌注桩基础质量控制技术与措施,以提高山地光伏电站建设水平,为微孔灌注桩施工提供参考。

高纬度季节性冻土区域光伏桩基础快速精准施工技术

在新疆极寒冻土区修建光伏电站,极易受到冻融和恶劣气候等不利因素的影响,在修建光伏支架微型钻孔灌注桩基时,不能简单复制其他地区的施工方式,而是要综合考虑当地水文地质、气候和环境等因素的影响。以浙能乌尔禾光伏电厂为实例,探索在极寒冻土区及戈壁滩上采用微型钻孔灌注桩的施工技术,形成一套简单、切实可行的方案,可为类似项目施工提供借鉴。

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.

山地光伏支架及其新型基础形式的应用分析

由于国家对能源的战略部署及其规划,山地光伏电站得到了迅速发展,为提高此类光伏电站的设计质量,增加其经济效益,以山地光伏电站的光伏支架(下文简称为“山地光伏支架”)结构及其基础形式为研究对象,对山地光伏支架设计采用固定式可调支架的必要性及其优势进行分析,并对设计、施工时的控制要点进行说明;对能与固定式可调光伏支架结合的微孔钢管灌注桩基础的受力性能及其优势进行介绍,并指出了其设计控制参数。以期可为固定式可调光伏支架及其微孔钢管灌注桩基础的设计应用及推广提供一个安全、经济的比选方案。

Application of a concrete thermal pile in cooling the warming permafrost under climate change

Permafrost degradation caused by climate warming is posing a serious threat to the stability of cast-in-place pile foundations in warm permafrost regions.Ambient cold energy can be effectively utilized by two-phase closed thermosyphons(TPCTs)to cool the permafrost.Therefore,we installed TPCTs in a cast-in-place pile foundation to create a unique structure called a thermal pile,which effectively utilizes the TPCTs to regulate ground temperature.And we conducted a case study and numerical simulation to exhibit the cooling performance,and optimize the structure of the thermal pile.The purpose of this study is to promote the application of thermal piles in unstable permafrost regions.Based on the findings,the thermal pile operated for approximately 53%of the entire year and effectively reduced the deep ground temperature at a rate of at least-0.1℃per year.Additionally,it successfully raised the permafrost table that is 0.35 m shallower than the natural ground level.These characteristics prove highly beneficial in mitigating the adverse effects of permafrost degradation and enhancing infrastructure safety.Expanding the length of the condenser section and the diameter of the TPCT in a suitable manner can effectively enhance the cooling capability of the thermal pile and ensure the long-term mechanical stability of the pile foundation even under climate warming.

冲击载荷对无烟煤微观孔隙的作用机理

为了研究冲击载荷对无烟煤微观孔隙的作用机理、提高煤层气的开发效果,以山西沁水煤田成庄矿无烟煤为研究对象,利用霍普金森冲击系统分别对垂直、平行、45°斜交层理方向煤样实施冲击载荷模拟实验,根据冲击载荷前后压汞、低温液氮和电镜扫描测试数据,分析了冲击载荷对无烟煤微观孔隙的作用规律与特点,进一步探究了冲击载荷对无烟煤微观孔隙的作用机理。研究结果表明:①冲击载荷能增大孔径和孔容,改善孔隙结构,提高孔隙连通性,增强瓦斯扩散、渗流与运移速度,并存在冲击载荷的最佳改善效果,不同方向最佳改善效果所对应的冲击载荷大小不同;②最佳改善效果以前的冲击载荷改善孔隙结构及渗透特征,最佳改善效果以后的冲击载荷增加微孔、堵塞中大孔、整体劣化孔隙孔渗特征;③冲击载荷对煤样的破坏模式可以分为两种——一种是始于微晶结构、基于位错塞积效应的破坏模式,另一种是始于宏观孔隙、基于煤基质破碎的破坏模式。结论认为,该研究成果对于进一步认识煤层气产气机理、产气特征及其控制因素具有重要的理论意义,对经济有效地开采煤层气具有现实意义及推广价值。

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.

山地光伏项目中微孔灌注桩施工管理研究

我国新能源产业在不断发展,对太阳能的利用方式也在逐步增加,山地光伏项目就是发展新能源的重要途径。在山地光伏项目中微孔灌注桩是常用的桩基,但是在微孔灌注桩施工中还存在着一些问题,文章根据存在的问题提出相对应的对策,旨在提高微孔灌注桩施工质量,推动山地光伏持续发展。

Capacity and failure mechanism of laterally loaded jet-grouting reinforced piles: Field and numerical investigation

This study presents the results of field and numerical investigations of lateral stiffness, capacity, and failure mechanisms for plain piles and reinforced concrete piles in soft clay. A plastic-damage model is used to simulate concrete piles and jet-grouting in the numerical analyses. The field study and numerical investigations show that by applying jet-grouting sur- rounding the upper 7.5D （D = pile diameter） of a pile, lateral stiffness and beating capacity of the pile are increased by about 110% and 100%, respectively. This is partially because the jet-grouting increases the apparent diameter of the pile, so as to en- large the extent of failure wedge and hence passive resistance in front of the reinforced pile. Moreover, the jet-grouting pro- vides a circumferential confinement to the concrete pile, which suppresses development of tensile stress in the pile. Corre- spondingly, tension-induced plastic damage in the concrete pile is reduced, causing less degradation of stiffness and strength of the pile than that of a plain pile. Effectiveness of the circumferential confinement provided by the jet-grouting, however, diminishes once the grouting cracks because of the significant vertical and circumferential tensile stress near its mid-depth. The lateral capacity of the jet-grouting reinforced pile is, therefore, governed by mobilized passive resistance of soil and plastic damage of jet-grouting.