能源桩桩身加筋混凝土热-力学性质试验研究

能源桩是将地埋管换热器置于建筑桩基础中来实现地下换热的一种新型的地源热泵技术。然而,不同季节运行条件下,冷热变化导致的能源桩桩身混凝土的膨胀和收缩会影响能源桩的持续使用甚至危及建筑的安全。因此,寻找到一种热力学性能较好的桩身混凝土对能源桩技术安全使用和推广至关重要。探讨了桩身素混凝土和掺入不同含量的钢纤维,聚丙烯纤维桩身加筋混凝土热力学特性。导热系数测试表明,钢纤维的掺入能提高能源桩桩身混凝土的导热系数,聚丙烯纤维的掺入降低了能源桩桩身混凝土的导热系数。钢纤维掺入量为1.3%时,导热系数最大,为2.44 W/m·K;热力学梯级加温试验表明,能源桩桩身混凝土掺入钢纤维,聚丙烯纤维均能有效减小应变,钢纤维最大应变减少量为62.43%,聚丙烯纤维最大应变减少量为61.11%;热力学全过程试验表明,钢纤维能有效减少制冷收缩应变,全过程中应变最小。综合对比3种能源桩桩身混凝土热物性参数及热-力学特性可知钢纤维加筋混凝土更适合作为能源桩桩身材料。     

混凝土水化作用对群桩热力学特性影响现场试验

混凝土早期水化作用不仅释放大量热量、而且会引起桩基先期变形与约束应力,影响桩基承载性能。目前针对地层温度对早期混凝土水化作用引起的桩基热力学特性(尤其是群桩效应)影响的研究仍相对较少。开展3×3群桩在早期混凝土水化作用下的桩基热力学特性现场试验,实测了桩身温度、应变等变化规律,着重分析了地层温度对桩基水化热消散、桩身约束应力的影响规律;并开展了相同条件下单桩热力学特性对比试验,探讨水化热作用群桩效应。研究结果表明,水化热作用下,群桩中温度叠加效应并不明显;地层中恒温带的水化热消散速度慢于变温带;混凝土残余应力沿桩深方向分布不均,呈现中间大、两头小的特点,最大残余应力出现在0.6倍桩深处。     

车辆载重与动荷载对X形桩桩-网复合地基动力特性影响的试验研究

基于大比例模型试验,采用半正弦波模拟移动车辆荷载,通过开展不同车辆载重和动荷载作用下X形桩桩-网复合地基的动力特性模型试验,分析桩身动应力、土拱效应、动土压力、垫层应力传递系数、格栅动应变等的变化规律,初步揭示了车辆载重和动荷载对X形桩桩-网复合地基的影响机制。试验结果表明,车辆动荷载对X形桩桩身动应力的影响深度约为2 m;车辆载重和动荷载的增加会导致桩间土承受更多的动应力,并且会降低路肩处土工格栅的作用;但是,路基中心的垫层应力传递系数几乎保持不变;土工格栅的动应变比超过0.048时,格栅累积变形会随着车辆载重和动荷载的增加迅速增大。     

车辆载重与动荷载对X形桩桩-网复合地基动力特性影响的试验研究

基于大比例模型试验,采用半正弦波模拟移动车辆荷载,通过开展不同车辆载重和动荷载作用下X形桩桩-网复合地基的动力特性模型试验,分析桩身动应力、土拱效应、动土压力、垫层应力传递系数、格栅动应变等的变化规律,初步揭示了车辆载重和动荷载对X形桩桩-网复合地基的影响机制。试验结果表明,车辆动荷载对X形桩桩身动应力的影响深度约为2 m;车辆载重和动荷载的增加会导致桩间土承受更多的动应力,并且会降低路肩处土工格栅的作用;但是,路基中心的垫层应力传递系数几乎保持不变;土工格栅的动应变比超过0.048时,格栅累积变形会随着车辆载重和动荷载的增加迅速增大。     

车辆载重与动荷载对X形桩桩-网复合地基动力特性影响的试验研究

基于大比例模型试验,采用半正弦波模拟移动车辆荷载,通过开展不同车辆载重和动荷载作用下X形桩桩-网复合地基的动力特性模型试验,分析桩身动应力、土拱效应、动土压力、垫层应力传递系数、格栅动应变等的变化规律,初步揭示了车辆载重和动荷载对X形桩桩-网复合地基的影响机制。试验结果表明,车辆动荷载对X形桩桩身动应力的影响深度约为2 m;车辆载重和动荷载的增加会导致桩间土承受更多的动应力,并且会降低路肩处土工格栅的作用;但是,路基中心的垫层应力传递系数几乎保持不变;土工格栅的动应变比超过0.048时,格栅累积变形会随着车辆载重和动荷载的增加迅速增大。     

夯实水泥土桩桩身压缩量对承载特性的影响

针对夯实水泥土桩的施工方法,在桩身埋设特制的应变传感器,测定桩身应变。基于试验测试数据,探讨桩身压缩量的计算方法,分析桩身压缩变形的分布规律及其对桩侧摩阻力的影响,并探讨不同桩长、不同水泥掺入比情况下,夯实水泥土桩桩身压缩量及其对承载特性的影响。研究表明,（1）对夯实水泥土桩,桩身压缩主要集中在桩身上部8 d（d为桩径）范围内,且变形速率变化较大,桩身压缩在桩顶位移中占比达80%以上;（2）桩身轴向荷载传递、桩侧摩阻力分布主要发生在此范围内;（3）桩身侧摩阻力分布非常不均匀,上部发挥较为充分,而下部发挥较少;（4）在工程常用水泥掺入比下,桩长大于12 d后,桩长径比和水泥掺入比的变化对桩承载特性影响不显著。     

桩端下有软弱下卧层的群桩沉降分析

群桩沉降由桩身压缩和桩端沉降组成。本文采用桩身线弹性的假定计算单桩桩身压缩,用单桩的桩身压缩近似代替群桩桩身压缩,并用承台下的平均附加应力乘以桩端荷载传递系数后作为桩端的附加应力,推出了桩端下有软弱下卧层的群桩沉降计算方法,同时与其他计算方法和工程实测值进行了比较。     

素混凝土桩复合地基支承路堤变形破坏模式

为了分析素混凝土桩复合地基支承路堤沉降变形特征和失稳破坏机制,建立了3组不同桩间距的素混凝土桩复合地基支承路堤离心试验模型及其数值模型。结果表明：在路堤填土自重、轨道和车辆荷载作用下,改变桩间距对素混凝土桩复合地基支承路堤沉降变形、桩体应变、加筋垫层和桩体破坏模式具有显著的影响;当桩间距不大于4倍桩径时,加筋垫层整体基本保持完好,路堤下素混凝土桩复合地基沉降能逐渐趋于稳定,而桩间距达到6倍桩径后,桩顶刺穿加筋垫层,加筋垫层对桩土变形协调和传递荷载作用失效,素混凝土桩复合地基支承路堤沉降持续增大;当桩间距达到4倍桩径时,素混凝土桩最大应变值发生随上部荷载的增大反而减小的突变现象,最靠近坡脚的素混凝土桩最先产生弯曲破坏而不是剪切破坏,当桩间距增大至6倍桩径时,桩体弯曲破坏逐渐往路堤中心方向发展。     

动力沉桩桩基拉应力分布及其施工控制试验研究

采用“现场沉桩试验、室内桩基模型试验、现场沉桩实例验证”的技术路线,通过对16根桩的动测试验结果的全面介绍和分析,揭示了打桩过程中桩身拉应力的分布规律,即桩身最大拉应力通常发生在桩的上部,特别是打桩初期,桩尖位于软土层或桩周为软土地基时,桩身最大拉应力点发生在距桩顶1/4l（l为桩长）附近,拉应力占同点压应力的50 %左右,这是桩身产生垂直桩轴线方向横向裂缝乃至发生桩身断裂现象的根源。试验亦表明,采用碟簧桩帽沉桩能有效地减小锤击拉应力,克服了采用老式桩帽辅助沉桩时预应力混凝土（管）桩桩身拉应力过大而造成桩身断裂这一缺陷,控制实测拉应力在允许拉应力（5 MPa）范围之内,以确保沉桩桩身完整,这是防治桩身拉裂的有效措施。     

冻融作用下三类纤维加筋固化土的抗压抗拉性能

合成纤维、矿物纤维和植物纤维加筋土,增强了土的强度和抗变形性能。开展冻融作用下的无侧限抗压试验和劈裂抗拉试验,研究聚丙烯纤维、玄武岩纤维和棕榈纤维加筋石灰固化土的抗压和抗拉性能随冻融次数的变化规律。结果表明：未冻融和冻融环境下,聚丙烯纤维加筋固化土、玄武岩纤维加筋固化土和棕榈纤维加筋固化土的最优质量加筋率分别为0.2%、0.2%和0.4%。随冻融次数增加,三类纤维加筋固化土的抗压强度和抗拉强度均呈阶段性下降,纤维加筋固化土的破坏应变均大于石灰固化土。冻融作用下,聚丙烯纤维加筋固化土的抗压强度、抗拉强度和抗变形性能均优于另两类加筋固化土。纤维与土颗粒间的界面作用力和纤维对土的空间约束作用,增强了土的冻融耐久性。对比三类纤维加筋固化土的试验结果,聚丙烯纤维加筋固化土的抗冻融性能最优。     

Towards a secure basis for the design of geothermal piles

Using pile foundations as heat exchangers with the ground provides an efficient and reliable energy source for the heating and cooling of buildings. However, thermal expansion or contraction of the concrete brings new challenges to the design of such structures. The present study investigates the impact of temperature variation on the mobilised bearing capacities of geothermal piles. The mechanisms driving the variations and redistribution of mobilised bearing forces along geothermal piles are identified using Thermo-Pile software. The EPFL and Lambeth College test piles are modelled and analysed as real-scale experiments. Three simple representative cases are used to investigate the impact of over-sizing geothermal piles on their serviceability. It is found that the mechanisms responsible for the variations and redistribution of mobilised bearing forces along the piles are unlikely to cause geotechnical failure, even if the ultimate bearing force of a pile is reached. Furthermore, over-sizing geothermal piles compared to conventional piles can have a negative impact on their serviceability.     

Numerical analysis of seasonal heat storage in an energy pile foundation

The use of concrete geostructures for energy extraction and storage in the ground is an environmentally friendly and easy way of cooling and heating buildings. With such energy geostructures, it is possible to transfer energy from the ground to buildings by means of fluid-filled pipes cast in concrete. By injecting thermal energy in summer and extracting it in winter, the ground in the area of a building’s piles can be used for seasonal energy storage, as long as the underground water flow in the storage remains low. This paper is a contribution to the improvement of the knowledge in the field of energy geostructures. The behaviour of a multi-pile seasonal storage system subjected to thermo-mechanical loading is examined numerically from both thermal and mechanical perspectives. The purpose of this paper is (i) to propose a thermo-hydro-mechanical 2D solution to the 3D problem, (ii) to explore the thermal behaviour of this type of storage and (iii) to evaluate its structural consequences. Coupled multi-physical finite element modelling is conducted. The efficiency of the storage is not dramatically affected by an increase in the annual mean temperature of the storage. It is shown that induced mechanical loads are less important when considering a wholly heated pile structure than when considering a single heated pile in a foundation. The evolution of stresses in the piles and in the soil during heating–cooling cycles also reveals possible critical phenomena.     

On the understanding of cyclic interaction mechanisms in an energy pile group

Integrating ground heat exchanger elements into concrete piles is now considered as an efficient energy solution for heating/cooling of buildings. In addition to the static load of buildings, the concrete piles also undergo a cycle of thermal deformation. In the case of single energy pile, calculation methods already exist and permit to perform a proper geotechnical design. In the case of energy pile group, the thermo‐mechanical interactions within the group are more complex. Very few experimental results on the energy pile group are available so that numerical analysis can be an interesting way to provide complementary results about their behavior. This paper deals with a numerical analysis including a comparison between a single energy pile and an energy pile group with different boundary conditions at the pile head. In order to take into account the stress reversal induced by the thermal expansions and contractions, a cyclic elastoplastic constitutive model is introduced at the soil–pile interface. The analysis aims to give some insights about the long‐term cyclic interaction mechanisms in the energy pile group. Based on this qualitative study, some guidance can be brought for the design of energy piles in the case where group effects should be considered. Copyright © 2015 John Wiley & Sons, Ltd.     

Study of thermal migration and induced mechanical effects in double U-tube energy piles

This paper investigates two geothermal energy piles using thermal response tests (TRTs). A set of parameters including pile wall temperature, ground temperature and strain are monitored at four different depths. The thermally induced mechanical behavior of the energy piles are then analyzed based on the monitoring data.The results show the following: (1) The temperature at the pile wall clearly varies throughout the heating and cooling cycle, and the ground temperature distribution shows a delay compared to the TRT stages. (2) The thermally induced mechanical effects are influenced by both the temperature and restraint conditions.     

Experimental and numerical investigations of the behaviour of a heat exchanger pile

The geothermal use of concrete geostructures (piles, walls and slabs) is an environmentally friendly way of cooling and heating buildings. With such geothermal structures, it is possible to transfer energy from the ground to fluid‐filled pipes cast in concrete and then to building environments. To improve the knowledge in the field of geothermal structures, the behaviour of a pile subjected to thermo‐mechanical loads is studied in situ. The aim is to study the increased loads on pile due to thermal effects. The maximum thermal increment applied to the pile is on the order of 21°C and the mechanical load reached 1300 kN. Coupled multi‐physical finite element modelling is carried out to simulate the observed experimental results. It is shown that the numerical model is able to reproduce the most significant thermo‐mechanical effects. Copyright © 2006 John Wiley & Sons, Ltd.     

基于能量桩现场试验的土体综合热导率系数研究

能量桩技术兼具支撑上部荷载和浅层地热能换热器双重功能;作为一种节能减排技术,近年来在国内外获得了一定的发展。然而,目前简单套用基于传统地埋管换热器获得的土体综合热导率系数,无法准确计算能量桩换热效率。依托河南理工大学某低承台3×3能量桩群桩工程应用,开展基于能量桩的土体综合热导率系数测试现场试验和数值模拟研究,分析加热时长、加热功率、流速及桩长等因素对土体综合热导率系数的影响规律,继而探讨能量桩在群桩中的布置形式对土体综合热导率系数的影响规律。研究结果表明,基于传统地源热泵测试所发展起来的土体综合热导率系数线热源分析方法,并不适用于分析基于能量桩现场实测所获得的相关数据;有必要推导一套考虑桩径影响的、适用于能量桩的土体综合热导率系数测试与计算分析方法。     

循环温度场作用下PCC能量桩热力学特性模型试验研究

PCC能量桩是河海大学岩土所开发的一种新型能量桩技术。在常规桩基静载荷模型试验基础上,将PCC能量桩放置在南京典型砂土中,并通过导热管内水体的循环对模型桩体施加温度场,以模拟PCC能量桩在实际运行过程中的承载力特性与受力机制,PCC能量桩先加载至工作荷载（极限荷载的一半）,再施加热-冷循环一次,最后加载至极限荷载,测得不同温度下PCC能量桩的荷载-位移关系曲线、桩身应力-应变关系曲线等变化规律。试验结果表明,能量桩换热过程中,热量更容易从桩体传向土体（即夏季模式的热循环）;热循环及制冷循环都明显改变了桩顶位移值,且往复循环作用下产生的塑性变形不能完全恢复,其积累变形可能危害上部结构安全;桩身受温度场作用产生的热应力相对较大,且不同约束条件下其变化值有所差异;在制冷循环下,桩底部甚至可能产生较大拉应力。     

U型、W型和螺旋型埋管形式能量桩热力学特性对比模型试验

能量桩是一种可以节省地下空间和施工埋管费用的新技术,目前针对其在供暖和制冷过程中土体和桩基之间的相互作用机制研究却相对较少。基于模型试验方法,系统地研究了饱和砂土中单U型（绑扎和预埋形式）、W型和螺旋型等4种不同埋管形式情况下的能量桩热力学效应、传热和承载特性,测得桩体和桩周土体温度、桩端阻力、水平土压力、桩顶位移以及桩体应变随时间的变化规律。试验结果表明,在相同输入功率情况下W型埋管形式桩体的温度、土体压力、应力和桩顶位移均较螺旋型和U型埋管形式的情况大。     

Determination of ground thermal properties for energy piles by thermal response tests

Thermal properties of ground heat exchanger (GHE) such as effective thermal conductivity and borehole thermal resistance are commonly measured in the field by thermal response tests (TRTs). TRT has been proved to be a consolidated method to determine thermal properties of traditional borehole heat exchangers (BHEs). However, there is still lack of data for adopting TRT on energy piles with often a large diameter and deficiency in validation of TRT results with geological materials. In this study, ground thermal properties for typical configured GHEs of energy piles are investigated. Three TRTs are conducted and the obtained results are analyzed. Effective thermal conductivity, λ_eff, of the ground derived by following the traditional linear source model shows large deviation as compared to the thermal conductivity of the geological materials. In order to determine λ_eff properly, the linear source model is modified and an equivalent radius, r_eq, of energy piles is considered. The λ_eff estimated by the modified model shows a good agreement with thermal conductivity of the in situ geological materials. In addition, there has been no obvious correlation between borehole thermal resistances and thermal efficiency due to heat transport of energy piles that depends not only by borehole thermal resistance but also by the pile’s diameter and ground conditions. The findings drawn from this study indicate that the modified model is reasonable and useful in determining thermal properties of energy piles.