Static load test and load transfer mechanism study of squeezed branch and plate pile in collapsible loess foundation

As a special geological phenomenon, the character of collapsible loess foundation is collapsible when penetrated by water. This character leads to the soil losing load bearing capacity largely and may lead to foundation failure. Pile is a popular foundation used in collapsible loess. The squeezed branch and plate pile is a new type of pile developed in recent years and has not be used in a project before. In this paper three squeezed branch and plate piles are tested in collapsible loess after immersion processing. The results may be used for reference in similar construction project, and to provide theoretical references for de- signing of the squeezed branch and plate piles in engineering practice.

Application of bi-directional static loading test to deep foundations

Bi-directional static loading test adopting load cells is widely used around the world at present, with increase in diameter and length of deep foundations. In this paper, a new simple conversion method to predict the equivalent pile head load-settlement curve considering elastic shortening of deep foundation was put forward according to the load transfer mechanism. The proposed conversion method was applied to root caisson foundation in a bridge and to large diameter pipe piles in a sea wind power plant. Some new load cells, test procedure, and construction technology were adopted based on the applications to different deep foundations, which could enlarge the application scopes of bi-directional loading test. A new type of bi-directional loading test for pipe pile was conducted, in which the load cell was installed and loaded after the pipe pile with special connector has been set up. Unlike the conventional bi-directional loading test, the load cell can be reused and shows an evident economic benefit.

Bearing capacity and mechanical behavior of CFG pile composite foundation

CFG pile (i.e., pile constructed by granular materials of cement, fly-ash and gravel) composite foundation is applied in subsoil treatment widely and successfully. In order to have a further study of this kind of subsoil treatment technology, the influencing factors and calculation methods of the vertical bearing capacity of single CFG pile and the CFG pile composite foundation were discussed respectively. And based on the obtained solutions, effects by the cushion and measurements to reduce negative friction area were analyzed. Moreover, the developing law of settlement and bearing capacity eigenvalue controlled by the material strength with the increase of load were given for the CFG composite foundation. The in-situ static load test was tested for CFG pile. The results of test show that the maximum test load or half of the ultimate load is used from all the points of test, the average bearing capacity eigenvalue of single pile is 390 kN, and slightly greater than the design value of bearing capacity. The bearing capacity eigenvalues of composite foundation for 3 piles are greater than 300 kPa, and the mechanical properties of CFG pile composite foundation are almost identical in the case of the same load and cushion thickness. The pile-soil stress ratio and the load-sharing ratio can be adjusted through setting up cushion thickness.

Experiments of Multi-element Composite Foundation with Steel Pipe Pile and Gravel Pile

A set of serf-developed apparatus for foundation physical model were utilized to conduct model tests of the multi-element composite foundation with a steel pipe pile and several gravel piles. Some load-bearing characteristics of the multi-element Composite foundation, including the curves of foundation settlement, stresses of piles, pile-soil stress ratio, and load-sharing ratio of piles and soil, were obtained to study its working performances in silty sand soil. The experimental results revealed that the multi-element composite foundation with steel pipe pile and gravel pile contributed more than the gravel pile composite foundation in improving the bearing capacity of the silty fine sand.

桩基静载过程中OFDR温度补偿试验研究

桩基静载试验是一种准确、可靠的桩基承载力测试试验,通过静载试验中桩身变形的测试,有利于分析桩基变形和承载特性.在桩基静载试验过程中,采用光频域反射(OFDR)技术开展桩基变形测试,考虑到温度的影响,对OFDR技术测得应变结果进行了温度补偿.结果显示:在本次试验中,用于温度补偿的温度传感光纤应变值随着桩顶上部加载出现了较大的变化,其原因是光纤传感器与钢筋笼之间绑扎过于紧密.对比进行温度补偿和不进行温度补偿的桩身应变数据,发现不做温度补偿造成的影响不能忽略,因此应变传感光纤测试结果必须进行温度补偿.研究结果可为桩基变形高精度监测提供科学参考.

Evaluation of ultimate bearing capacity of Y-shaped vibro-pile

Based on Mindlin stress solution, a numerical computational method was proposed to calculate the stresses in the ground induced by side friction and the resistance of Y-shaped vibro-pile. The improved Terzaghi's and ЪерезанцевВГ's methods for ultimate bearing capacity evaluation were proposed by considering the stress strength induced by friction resistance at pile head level of Y-pile. A new method to calculate the ultimate bearing capacity of Y-pile was also proposed based on the assumptions of soil failure mode at the tip of Y-pile and the use of Mohr-Coulomb soil yield criterion and Vesic compressive correction coefficient with the induced stresses in the ground. Based on the comparisons with the field static load test results, it is found that the improved Terzaghi's method gives higher ultimate capacity, while the other two methods shows good agreement with the field results.

Experimental study on DX pile performance in frozen soils under lateral loading

Experiments about working mechanism and mechanical characteristics of the DX model pile foundation under lateral dynamic and static loading were conducted by using a model system of the dynamic frozen soil-pile interaction. The horizontal displacement-force relationship of the pile head and bending moment distribution along the body in frozen soils of different temperatures were discussed. According to test results, both the horizontal disp!acement-force relationship of the DX pile head and bending moment distribution of the DX pile body are smaller than that of equal-diameter piles under same lateral loads. The piles with different plate positions show different displacements and bending moments. This phenomenon is mainly related to the soil temperature and bearing plates locations. Thus, dynamic response analysis of the pile foundation should be taken into account.

Numerical analysis of bearing behaviors of single batter piles under horizontal loads in various directions

The horizontal bearing behavior of a single batter pile(SBP)is vital to its application in practical engineering;however,the horizontal responses of SBPs change with the directions of horizontal loads,and this phenomenon is rarely investigated.Therefore,the directional differences in the horizontal bearing behaviors of SBPs are investigated in this study.Four model tests are conducted to preliminarily examine the effects of the skew angle of horizontal loads on the horizontal bearing capacities and distributions of the bending moments of the SBPs.Subsequently,the differences in the responses of the SBPs under horizontal loads in various directions at full scale are analyzed comprehensively via finite-element(FE)analysis.The effects of the skew angle on SBP-soil interaction are discussed.Moreover,an empirical design method is proposed based on the FE analysis results to predict the bearing ratios of SBPs in medium-dense and dense sand while considering the effects of the skew angle,batter angle,and pile diameter.The method is confirmed to be effective,as confirmed by the close agreement between the predicting results with the model test(reported in this study)and centrifuge model test results(reported in the literature).

串珠状溶洞数量影响灌注桩顶部水平受拉特性试验研究

采用室内模型试验,对比研究串珠状溶洞数量对灌注桩顶部水平受拉特性的影响,为岩溶强发育区嵌岩桩的水平承载力设计提供试验依据.结果表明:1)桩顶水平荷载相同时,上部土层段桩身水平位移、桩身弯矩(峰值)、桩侧土抗力随溶洞数量的增多而增大,单桩水平承载力随溶洞数量增多而明显减弱.2)上部土层段桩身水平位移随桩顶水平荷载增大而增大,桩顶水平位移最大,嵌岩段很小甚至为0;弯矩M-深度z曲线整体呈抛物线形,桩身弯矩在土岩界面和上层溶洞与底板界面呈现2个峰值,弯矩的变化仅仅影响到上层溶洞及其底板,第2层溶洞及其以下的弯矩几乎为0,土岩界面弯矩最大、截面最危险;土抗力p-深度z曲线呈“凸肚”形.3)溶洞数量增加,位置越高,p-y曲线越容易收敛,可以采用p/pub=α(y/y50)β拟合土层段的桩身p-y曲线.溶洞数量对α与β值变化幅度的影响较小,α与β值变化幅度体现p-y曲线随深度变化的敏感度,两者敏感度随溶洞数量增多而增大.4)工程桩刺穿串珠状溶洞时,为了提高单桩水平抗拉能力,建议采用注浆法改善上部岩土性质,加大土岩界面处桩身配筋率,并将桩身嵌入上层溶洞底板一定深度,防止基桩在土岩界面处发生弯曲破坏.

钙质砂地基中群桩基础荷载传递特性研究

为研究钙质砂地基中群桩基础承载过程中的性能特征,开展不同桩距条件下群桩基础静载试验.得到不同桩距条件下群桩的Q-S曲线和S-lgt曲线,测得3d、4.5d、6d(d为桩径)群桩在静载条件下的极限承载力F_(u),研究各个桩段的桩身轴力、桩侧摩阻力以及桩端阻力的变化趋势.研究结果表明:群桩的Q-S曲线为缓变型,S-lgt曲线为渐变型,桩基承载变形大致分为稳定、渐进以及破坏3个阶段,相较于3d群桩,4.5d、6d群桩沉降发展更缓慢,F_(u)更高;同等桩距下,钙质砂地基表现出比陆源砂地基更强的群桩效应,钙质砂地基中群桩最佳桩距应在4.5d~6d之间.该研究为群桩在钙质砂地基中的应用提供数据支撑和理论参考.

不均匀卵石持力层桩端后注浆效果试验研究

我国东南沿海尤其是温州地区广泛存在上覆深厚软土下覆不均匀卵石的地层。该区域的钻孔灌注桩常采用后注浆技术改善其承载性能。为了评价后注浆技术对这类地层中灌注桩承载力改善效果,开展了相应的模型试验,对比了不同注浆量对桩承载力的影响程度;并结合扫描电镜(scanning electron microscope,简称SEM)试验分析了浆液分布特点,探讨了浆液在卵石层中的扩散范围,研究了浆液扩散范围与桩承载力之间的关系。结果表明:浆液能够有效地填充桩端卵石层,注浆量的增加使得填充范围扩大,填充范围为3~4倍桩径时,桩的承载力改善最显著。在不均匀卵石持力层中存在一个最优注浆量,最优归一化注浆量约为2.8,若超过该最优注浆量归一化值,桩的承载力不再显著提高。单桩模型试验确定的最优注浆量与刘金砺公式[1]的预测结果接近。扫描电镜技术有助于评价桩的后注浆技术在上覆深厚软土下覆不均匀卵石的土层中的效果。

考虑竖向荷载作用时液化土中群桩基础水平动力响应

基于Biot饱和多孔介质理论,考虑液化土的流动特性,建立考虑竖向荷载作用的部分埋入群桩水平振动模型,通过分离变量法、算子分解法,引入桩土耦合及位移连续条件,得到复杂条件下液化土中高桩桩间相互作用因子解和群桩水平动阻抗解.通过参数分析,表明液化土特性和竖向荷载对桩间水平相互作用因子、群桩动阻抗有显著影响,指出同一频率下,群桩水平动刚度随着表层液化土厚度的增加而下降,当液化厚度较大时,动刚度随频率上升显著下降,并出现负刚度;桩顶竖向荷载会降低液化土中的群桩动刚度,液化土厚度越大,削弱效果越明显.

黄土区楔形桩-网复合地基承载特性

[目的]针对黄土地区中楔形桩-网复合地基的承载性能进行探究.[方法]开展以几何相似常数为第一基本量、重度相似常数为第二基本量的土工模型试验,研究了不同受荷区域基桩承载能力,并通过有限元计算方法进一步分析其承载特性.比较模型试验结果与有限元计算结果,揭示了黄土区楔形桩-网复合地基的荷载沉降、桩身轴力、侧摩阻力、桩土应力比及荷载分担比等分布与变化规律.[结果]复合地基沉降变化总体趋于均匀,荷载-沉降曲线为缓变型;不同受荷区域基桩的桩身轴力、侧摩阻力呈现明显差异性;楔形桩-土相互作用与荷载和深度呈现正相关关系,但存在一定上限;复合地基中楔形桩随荷载增大能有效分散桩身应力,并将荷载由桩体向桩周土传递、由地基中心向外围扩散.[结论]楔形桩-网复合地基表现出良好的承载能力,直接受荷区桩能更高效地发挥楔形桩承载特性,楔形桩在复合地基中很好地实现桩-网-土协同作用.

不同嵌岩深度下抗拔桩承载特性研究

以广东省江门市某工程为例,通过有限元分析软件ABAQUS,结合地勘报告及现场对桩基进行抗拔静载试验得出的荷载–位移曲线图,建立抗拔桩三维有限元模型进行数值模拟计算分析,研究不同嵌岩深度增量下抗拔桩的承载特性。研究成果表明:增加嵌岩深度可在一定程度上提高桩基抗拔承载力,但二者之间并非呈线性增长关系。当嵌岩深度增量为3D时,相较于原试桩的桩顶位移量减小54.2%,抗拔承载力提高105.2%,对桩基抗拔承载力的提高效果尤为显著。

岩溶区PST管桩复合地基的工程应用

经对某场地高层建筑地基素填土厚度大、下伏基岩岩溶发育、以多层串珠状溶洞为主的地层条件研究,提出一种PST管桩刚性复合地基。采用试验和监测等方法来指导复合地基处理设计与施工,根据场地试桩数据来验证设计的合理性。施工完成后,采用静载荷试验对复合地基进行质量验收检测,并连续进行沉降监测。结果表明:岩溶区PST管桩复合地基是一种处理高层建筑地基很可行的方法,处理后可以满足高层建筑物对地基强度、变形及稳定性的要求。

基于BOTDA的砂土地基预应力管桩抗拔静载试验研究

针对砂土地基抗拔桩受力性能与荷载传递机理研究不足问题,将布里渊光时域分析(BOTDA)光纤传感技术应用于长江下游地区厚层砂土地基预应力管桩原位抗拔静载试验研究,采用特殊设计的桩身刻槽钻孔、光纤粘贴保护、接桩过程连续植纤等技术工艺,实现了预应力管桩在拉拔过程中桩身受力变形状态的分布式测量与数据精确定位。研究结果表明:在上拔荷载作用下,试验桩桩身轴力沿桩身方向逐渐减小;桩侧摩阻力在下桩发挥效果较好,上桩侧摩阻力较小,全桩长侧摩阻力最大值在桩底;抗拔桩随上拔荷载的增加,轴力逐渐向下传递,向下传递的轴力主要由预制管桩侧摩阻力承担;分布式光纤应变传感技术能较好地监测预应力管桩桩身贯入施工因素对抗拔承载特性的影响。研究结果可在其他类型抗拔桩受力特性监测项目中推广应用。

刚性桩复合地基的载荷试验问题

《建筑与市政地基基础通用规范》(GB 55003—2021)对刚性桩复合地基承载力验收的条文是:复合地基承载力的验收检验应采用复合地基静载荷试验,对有粘结强度的复合地基增强体尚应进行单桩静载荷试验。在实际工程中,可能会遇到这样的情况:(1)复合地基静载荷试验合格,单桩静载荷试验不合格,复合地基承载力真的不合格吗?(2)复合地基和单桩静载荷试验都合格,复合地基承载力就一定合格吗?本文对此进行了分析讨论,认为与地质条件有关,规范方法主要是针对单一均匀地基的情况,对于多层土地基,规范方法有局限性,由于试验压板尺寸小,压板试验还不能反映深层土的影响。对于大尺寸的基础,还要进行科学的分析,应对实际尺寸的基础沉降进行计算分析,才能判断和保证工程的安全,仅靠小尺寸的压板载荷试验是不够完善的。

基于界面本构模型的砂土中单桩荷载−沉降响应预测方法

基于界面本构模型提出了一种新的砂土中单桩荷载沉降响应的预测方法。首先,从土−结构界面本构模型出发推导了严格的桩−土界面非线性荷载传递模型,该模型承继了界面本构模型特征,能够模拟桩−土界面上发生的应变硬化/软化、剪胀与应力路径依赖性等行为。此外,采用双曲线荷载传递模型模拟桩端−土相互作用的非线性应力−位移关系。上述荷载传递模型所需参数可以通过室内界面剪切试验和土工试验进行校准。继而,基于荷载传递法,提出了单桩荷载沉降响应分析的一维计算模型,并采用迭代算法进行数值求解。最后,将理论解答与已报道的模型试验、自主开展的模型桩试验以及数值模拟结果进行比较,以验证所提出的理论方法的正确性。试验结果表明,预测值与实测值吻合较好,且该方法能够很好地预测非位移桩与位移桩的荷载沉降响应。提出了一个基于界面本构模型的单桩荷载沉降响应分析框架,为竖向荷载下砂土中单桩优化设计提供了理论参考。

大直径嵌岩灌注桩竖向承载特性试验研究

为了探讨泥岩地基大直径灌注桩竖向承载性能,依托青岛某机场桩基工程,对嵌入泥岩的9根大直径灌注桩进行了竖向抗压静载试验和桩身应力监测,分析大直径嵌岩灌注桩的竖向抗压性能,明确大直径灌注桩的荷载传递规律。结果表明,试桩桩身完整性较好,桩身质量可靠;试桩荷载沉降(Q-s)曲线均为缓变型,加载初期存在明显的压密段,桩顶沉降量为10.31 mm,最低回弹率大于50%;最大荷载作用下,各试桩的竖向抗压极限承载力特征值均不小于6480 kN,试桩竖向抗压承载能力仍有较大潜力;试桩均表现出端承摩擦桩的特性,轴力自桩顶到桩端逐渐衰减,桩侧摩阻力峰值发生在中风化泥岩段,桩端阻力占桩顶荷载的10.55%~34.48%。

复杂地质条件下抗滑桩水平承载特性影响因素分析

抗滑桩是治理滑坡时经常采用的一种有效的边坡防护工程措施,其水平承载力是评价边坡加固效果的关键。为研究复杂地质条件下边坡抗滑桩的水平承载特性,以卡拉水电站上田镇滑坡体防护工程为例,采用现场试验和数值模拟相结合的方法研究边坡抗滑桩的水平承载特性。基于抗滑桩现场单桩水平静载试验,验证数值计算模型的准确性,在此基础上进行抗滑桩水平承载力影响因素分析。结果表明:前后排桩受水平承载力呈现不均匀性,在相同水平承载力作用下后排桩桩顶最大弯矩大于前排桩桩顶最大弯矩;不同桩布置对群桩水平承载特性影响较大,其中双排交错布置方式桩顶弯矩最小,单排交错次之,双排水平桩顶弯矩最大;相同水平荷载作用下,与1.2 m和1.3 m桩间距相比,1.4 m桩间距的群桩效应较为明显;桩入岩深度对群桩水平承载能力影响较小,只在土-岩体分层界面有明显差异。