Influence of reaction piles on test pile response in a static load test

This work presents a new analytical method to analyze the influence of reaction piles on the test pile response in a static load test.In our method,the interactive effect between soil and pile is simulated using independent springs and the shear displacement method is adopted to analyze the influence of reaction piles on test pile response.Moreover,the influence of the sheltering effect between reaction piles and test pile on the test pile response is taken into account.Two cases are analyzed to verify the rationality and efficiency of the present method.This method can be easily extended to a nonlinear response of an influenced test pile embedded in a multilayered soil,and the validity is also demonstrated using centrifuge model tests and a computer program presented in the literature.The present analyses indicate that the proposed method will lead to an underestimation of the test pile settlement in a static load test if the influence of the presence of reaction piles on the test pile response is neglected.

Model tests on XCC-piled embankment under dynamic train load of high-speed railways

Piled embankments,which offer many advantages,are increasingly popular in construction of high-speed railways in China.Although the performance of piled embankment under static loading is well-known,the behavior under the dynamic train load of a high-speed railway is not yet understood.In light of this,a heavily instrumented piled embankment model was set up,and a model test was carried out,in which a servo-hydraulic actuator outputting M-shaped waves was adopted to simulate the process of a running train.Earth pressure,settlement,strain in the geogrid and pile and excess pore water pressure were measured.The results show that the soil arching height under the dynamic train load of a high-speed railway is shorter than under static loading.The growth trend for accumulated settlement slowed down after long-term vibration although there was still a tendency for it to increase.Accumulated geogrid strain has an increasing tendency after long-term vibration.The closer the embankment edge,the greater the geogrid strain over the subsoil.Strains in the pile were smaller under dynamic train loads,and their distribution was different from that under static loading.At the same elevation,excess pore water pressure under the track slab was greater than that under the embankment shoulder.

Simulation analysis for O-cell test of pile and the interaction of upper pile and lower pile

In this paper,the soil-pile system of O-cell test of pile is simplified as an axi-symmetric problem.By using aggregation of quadrilateral isoparametric elements to simulate pile and soil,setting Goodman's elements between pile and soils,a method of numerical simulation analysis on O-cell test of pile is presented with the consideration of nonlinear mechanical behavior of soils and pile-soil interface.The method is applied to the analysis of a case of O-cell test of pile.The load-displacement curves and axial force curves of upper pile and lower pile obtained from the O-cell test of pile are fitted,and parameters of the mechanical model of soils and interface are determined.Analysis results validate that the numerical simulation analysis method put forward in this paper is applicable.Furthermore,the interaction and influence of upper pile and lower pile in the O-cell test are also studied with the method.The result shows that if load box is located in a soil layer with fine mechanical behavior,the interaction of upper pile and lower pile in O-cell test can be ignored generally.

Shaking Table Tests on Bridge Foundation Reinforced by Antislide Piles on Slope

Based on the requirement of seismic reinforcement of bridge foundation on slope in the Chengdu-Lanzhou railway project,a shaking table model test of anti-slide pile protecting bridge foundation in landslide section is designed and completed. By applying Wenchuan seismic waves with different acceleration peaks,the stress and deformation characteristics of bridge pile foundation and anti-slide pile are analyzed,and the failure mode is discussed. Results show that the dynamic response of bridge pile and anti-slide pile are affected by the peak value of seismic acceleration of earthquake,with which the stress and deformation of the structure increase. The maximum dynamic earth pressure and the moment of anti-slide piles are located near the sliding surface,while that of bridge piles are located at the top of the pile. Based on the dynamic response of structure,local reinforcement needs to be carried out to meet the requirement of the seismic design. The PGA amplification factor of the surface is greater than the inside,and it decreases with the increase of the input seismic acceleration peak. When the slope failure occurs,the tension cracks are mainly produced in the shallow sliding zone and the coarse particles at the foot of the slope are accumulated.

Uplift Pile Test in Rocks and Its Bearing Capacity

This paper reports in situ tension test and laboratory model test for large diameter, manually digging anchorage piles in the 2nd Luzhou Changjiang Bridge. Tension behavior, uplift bearing capacity and influence of rock characteristics on bearing capacity are discussed. Proposes are presented with respect to issues related to the construction and design of uplift piles.

Model Tests of Pile Defect Detection

The pile, as an important foundation style, is being used in engineering practice. Defects of different types and damages of different degrees easily occur during the process of pile construction. So, dietecting defects of the pile is very important. As so far, there are some difficult problems in pile defect detection. Based on stress wave theory, some of these typical difficult problems were studied through model tests. The analyses of the test results are carried out and some significant results of the low-strain method are obtained, when a pile has a gradually-decreasing crosssection part, the amplitude of the reflective signal originating from the defect is dependent on the decreasing value of the rate of crosssection β. No apparent signal reflected from the necking appeares on the velocity response curve when the value of β is less than about 3. 5 %.

Regressive approach for predicting bearing capacity of bored piles from cone penetration test data

In this study, th e least sq u are su p p o rt v ecto r m achine （LSSVM） alg o rith m w as applied to predicting th ebearing capacity o f b ored piles e m b ed d ed in sand an d m ixed soils. Pile g eo m etry an d cone p e n e tra tio nte s t （CPT） resu lts w ere used as in p u t variables for pred ictio n o f pile bearin g capacity. The d ata u se d w erecollected from th e existing litera tu re an d consisted o f 50 case records. The application o f LSSVM w ascarried o u t by dividing th e d ata into th re e se ts： a train in g se t for learning th e pro b lem an d obtain in g arelationship b e tw e e n in p u t variables an d pile bearin g capacity, and testin g an d validation sets forevaluation o f th e predictive an d g en eralization ability o f th e o b tain ed relationship. The predictions o f pilebearing capacity by LSSVM w ere evaluated by com paring w ith ex p erim en tal d ata an d w ith th o se bytrad itio n al CPT-based m eth o d s and th e gene ex pression pro g ram m in g （GEP） m odel. It w as found th a t th eLSSVM perform s w ell w ith coefficient o f d eterm in atio n , m ean, an d sta n d ard dev iatio n equivalent to 0.99,1.03, an d 0.08, respectively, for th e testin g set, an d 1, 1.04, an d 0.11, respectively, for th e v alidation set. Thelow values o f th e calculated m ean squared e rro r an d m ean ab so lu te e rro r indicated th a t th e LSSVM w asaccurate in p redicting th e pile bearing capacity. The results o f com parison also show ed th a t th e p roposedalg o rith m p red icted th e pile bearin g capacity m ore accurately th a n th e trad itio n al m eth o d s including th eGEP m odel.

Shake table test of soil-pile groups-bridge structure interaction in liquefiable ground

This paper describes a shake table test study on the seismic response of low-cap pile groups and a bridge structure in liquefiable ground. The soil profile, contained in a large-scale laminar shear box, consisted of a horizontally saturated sand layer overlaid with a silty clay layer, with the simulated low-cap pile groups embedded. The container was excited in three E1 Centro earthquake events of different levels. Test results indicate that excessive pore pressure （EPP） during slight shaking only slightly accumulated, and the accumulation mainly occurred during strong shaking. The EPP was gradually enhanced as the amplitude and duration of the input acceleration increased. The acceleration response of the sand was remarkably influenced by soil liquefaction. As soil liquefaction occurred, the peak sand displacement gradually lagged behind the input acceleration; meanwhile, the sand displacement exhibited an increasing effect on the bending moment of the pile, and acceleration responses of the pile and the sand layer gradually changed from decreasing to increasing in the vertical direction from the bottom to the top. A jump variation of the bending moment on the pile was observed near the soil interface in all three input earthquake events. It is thought that the shake table tests could provide the groundwork for further seismic performance studies of low-cap pile groups used in bridges located on liquefiable groun.

Field load testing of copper extraction aeration pipes under simulated high heap pile

Recently a research team at Ohio University,USA,conducted a unique full-scale feld load test to simulate the aeration pipe installations at a copper extraction mine operated in Chile.The overliner material taken from the mine was used in recreating the in situ conditions.Electric heaters were utilized to raise the temperature inside each pipe to simulate the essential element of the copper extraction process.The maximum vertical deflection reached by the test pipes was close to 20%,when the simulated heap pile height was 80 m.The plastic pipes and the overliner material were also tested in the laboratory.Based on the results,the maximum heap pile fll depth was recommended for the aeration system.The results indicated that the vertical deflection was the primary performance index for the aeration pipes installed in heap piles at mines.Lastly,the pipe made of polypropylene resin was super.

The Shaking Table Test on the Performance of Cement-mixed Piles in Liquefiable Railway Foundations

Cement-mixed piles,as countermeasure against liquefaction of silt and sand ground,can improve the shear strength and bearing capacity of foundation soil,meaning cement-mixed piles are capable of resisting displacement when an earthquake happens. However,investigations of cement-mixed piles by experimental methods such as the shaking table test is few and far between. It is especially true for the seismic performance of cement-mixed piles in liquefiable railway foundations in high seismic intensity regions. To this end,a cross-section of the Yuxi-Mengzi railway was selected as the prototype and studied by the shaking table test in this study. The results showed that composite foundation of cementmixed piles was not liquefied when the seismic acceleration was lower than 0. 30g. In the process of acceleration increasing from 0. 30g at 2Hz to 0. 60g at 3Hz,the upper middle silt outside slope toe was partly liquefied. The foundation soil under the shoulders and center of subgrade was far from the initial liquefaction criterion during the test. Cementmixed piles can effectively reduce the embankment settlement and differential settlement. It can be concluded that, the design of cement-mixed piles can ensure the seismic performance of the subgrade,and satisfy the seismic design requirements of the YuxiMengzi railway in areas of VIII degrees seismic fortification intensity.

In-Situ Test on Fatigue Characteristics of Top-Mounted Dividable Pile-Board Subgrade for High-Speed Railway

To simulate the fatigue characteristics of the pile-board structure under long-term dynamic load, using the in-situ dynamic testing system DTS-1, the forced vibration loading was repeated one million times at different cross-sections of the pile-board structure for high-speed railway. The dynamic deformation, permanent deformation and dynamic stress of main reinforcements were measured. The test results show that the dynamic responses of the pile-board structure almost did not vary with the forced vibration times under the simulated trainload. After one million times of forced vibration, the permanent deformations of the midspan section of intermediate span and midspan section of side span were 0.7 mm and 0. 6 mm, respectively, and there was no accumulative plastic deformation at the bearing section of intermediate span.

砂土中能源桩承载力受热冷循环影响的离心机试验

为研究砂土中能源桩在热冷循环温度作用下的承载能力,开展不同密实度奉浦砂土中细长能源桩的离心机模型试验.试验中进行20次热—冷温度循环作用,获得能源桩轴力、侧摩阻力、单桩承载力等的变化规律,并进行对比研究.试验结果表明:随着温度循环次数增加,能源桩桩身轴力均逐渐衰减并趋于稳定,且中密砂中能源桩最大轴力衰减值远高于密砂中能源桩;在热冷温度循环过程中,中密砂中能源桩桩身中下部存在中性点,在冷循环过程中中性点以上存在正的附加侧摩阻力,下部存在负的附加侧摩阻力,而在热循环过程中中性点以上存在负的附加侧摩阻力,下部存在正的附加侧摩阻力;密砂对能源桩下部存在明显的约束作用,使其在冷循环过程中全桩身相对桩周土体存在向下的位移趋势,产生全桩身正的附加侧摩阻力,而热循环过程中产生负的附加侧摩阻力;长期循环荷载作用使得能源桩的桩基承载力发生折减,与相应原型桩相比,埋设于中密砂和密砂中的能源桩承载力分别减小了7.3%和15.6%;密砂中的原型桩及能源桩承载力均高于中密砂中原型桩约11%;当实际工程中能源桩处于不同密实度的砂土层中时,需采取合理的措施,以满足能源桩的承载要求.

桩-震陷土层耦合作用下变截面单桩动力响应

为研究不同类型地震波作用下大直径变截面钢管混凝土复合单桩的动力响应规律,依托厦门第二东通道翔安大桥工程,通过室内振动台试验,选取地震动强度为0.15g的5010波、1004波、Kobe波及El‐Centro波,研究大直径变截面钢管混凝土复合单桩的桩身加速度、水平位移、弯矩及桩基损伤等变化规律。试验结果表明:不同类型地震波由于其频谱特性不同,大直径变截面钢管混凝土复合单桩的动力响应特性存在差异;桩顶加速度最大值、桩顶水平位移最大值、桩身弯矩最大值均在1004波作用下最大,在Kobe波作用下最小;桩身弯矩最大值均未超过桩基设计抗弯承载力;在地震力的作用下对桩基础的抗弯承载能力进行设计时,应重点考虑软硬土层的分界面处抗弯能力。

玉米沉降形成粮堆时的分级试验研究

为减缓粮食入仓时的分级现象,以玉米为对象,研究粮堆分级特性。以粮堆分级指数M为指标,在单因素试验的基础上通过响应面试验研究不同因素对玉米沉降运动形成的交互作用。结果表明:粮堆分级指数M随着沉降高度、粮堆直径、质量流量的增加,均有上升的趋势;粮堆分级指数M随着排料口直径的增加而减小。其中沉降高度、粮堆直径,质量流量3个因素对粮堆分级指数M的影响大小依次为沉降高度>粮堆直径>质量流量。响应面试验结果表明模型拟合效果良好,可靠性高。

Estimation of Axial Pile Bearing Capacity According to Shear Strength Parameters of Soil

At pesent, it is very popular to estimate pile bearing capacity by use of empirical formula and physical indexes of soil provided in the design codes for civil construction in China. This paper attempts to apply mechanical indexes of soil and semi-empirical formulas, which are based on soil mechanical theories and were summarized and presented by Meyerhof in 1976, to calculate the axial pile bearing capacity. Loading test results of 24 single piles in Tianjin area have been collected and compared with the proposed calulation approach.

基于DMT的孟加拉国帕德玛大桥桩基承载力预测研究

依托孟加拉国帕德玛多用途大桥,采用扁铲侧胀试验(Flat Dilatometer Test,简称DMT)分析大直径钢管桩基的竖向极限承载力,并与现场静力载荷试验和室内三轴试验的计算结果进行对比。结果表明:相对室内试验参数,基于3种不同DMT试验参数的计算结果误差均在10%以内,整体预测较可靠;魏晋攀方法的预测精度最高,Marchetti法次之,Campanella&Robertson法略低估了桩基的竖向极限承载力;桩端和桩侧压浆可有效提高桩基承载力,基于DMT测试的预测结果可为桩基压浆效果提供可靠经济的评估依据。

螺牙高度对螺纹桩竖向承载特性的影响

为探明竖向荷载作用下螺牙高度对螺纹桩承载特性及桩周土体的影响,通过室内半面模型桩试验,结合数字图像相关技术(DIC),研究了不同螺牙高度、不同荷载条件对桩身承载性能、桩周土体破坏特征、桩周土体位移场的影响。采用ABAQUS数值模拟方法对模型试验的结果进行验证,并扩展了螺牙高度的取值范围,分析螺纹桩桩身极限承载力与材料利用率、桩周土体应力场分布随螺牙高度变化的规律。结果表明:增加螺牙高度能提升桩身承载力,增大桩周土体位移影响范围,延缓桩侧土体剪切破坏的发展;随着桩身沉降量增加,桩顶荷载以桩侧阻力承受为主,逐渐演变为以桩端阻力承受为主;螺牙高度的增加虽然能提升桩身极限承载力,但桩身材料利用率存在峰值。

Pile-up现象对材料本构关系反演计算的影响

针对商用纳米压痕测试中忽略凸起(Pile-up)现象的问题,开展了Pile-up现象对材料力学参数和本构反演计算结果影响的研究。推导能量法理论,选取5种不同材料进行了纳米压痕测试,利用商用Oliver-Pharr(O-P)法和能量法分别进行硬度和弹性模量计算,并比较测试误差率。推导极限分析法理论并进行反演计算,得到考虑材料Pile-up现象的应力应变曲线,并根据计算结果对纳米压痕过程进行仿真。结果表明:当纳米压痕测试中压痕残余深度与压入深度比值大于0.7时,Pile-up现象对商用O-P法测试影响明显,弹性模量和硬度值误差率达到20%以上;对发生塑性变形较大材料进行本构关系反演计算时,理论计算值和数值模拟结果均证明了Pile-up现象对应力应变曲线计算有一定的影响。研究结果可为纳米压痕测试本构关系的反演计算方法提供参考。

多分层地基条件下单桩水平承载特性试验研究

随着海上风电的深远海和风电机组大型化发展趋势,固定式单桩基础的直径随之增大,现有的理论方法及简化模型已无法准确地指导设计,多分层地基条件下的大直径单桩承载特性亟需科学评估。根据3个风场的五根试桩现场水平承载力试验数据,对有限元模型的参数化输入进行率定,并对水平试桩荷载位移曲线的工程数据进行拟合,推荐有限元模型的砂土弹性模量应取压缩模量的1倍,黏性土取3倍。

地铁上盖土方开挖施工及水位动态控制技术

某工程地铁上盖结构施工,研究通过降水井降水试验,分析了该既有地铁上部结构施工区域现有地下水位、渗漏系数、降水影响半径等参数;进而确定了现场降水井直径以及降水井与降水井之间设置间距要求,施工过程中的水位标高动态控制要求等,分析了上盖结构土方开挖施工过程中的要点和重点施工保护措施,形成了一套完整的分析方法和控制措施,以实现提高质量、降本增效的目标。