Buckling analysis of super-long rock-socketed filling piles in soft soil area by element free Galerkin method

In order to discuss the buckling stability of super-long rock-socketed filling piles widely used in bridge engineering in soft soil area such as Dongting Lake, the second stability type was adopted instead of traditional first type, and a newly invented numerical analysis method, i.e. the element-free Galerkin method (EFGM), was introduced to consider the non-concordant deformation and nonlinearity of the pile-soil interface. Then, based on the nonlinear elastic-ideal plastic pile-soil interface model, a nonlinear iterative algorithm was given to analyze the pile-soil interaction, and a program for buckling analysis of piles by the EFGM (PBAP-EFGM) and arc length method was worked out as well. The application results in an engineering example show that, the shape of pile top load-settlement curve obtained by the program agrees well with the measured one, of which the difference may be caused mainly by those uncertain factors such as possible initial defects of pile shaft and the eccentric loading during the test process. However, the calculated critical load is very close with the measured ultimate load of the test pile, and the corresponding relative error is only 5.6%, far better than the calculated values by linear and nonlinear incremental buckling analysis (with a greater relative error of 37.0% and 15.4% respectively), which also verifies the rationality and feasibility of the present method.

Experimental study and numerical analysis on bearing behaviors of super-long rock-socketed bored pile groups

A centrifuge modeling test and a three-dimensional finite element analysis（FEA）of super-long rock-socketed bored pile groups of the Tianxingzhou Bridge are proposed.Based on the similarity theory,different prototypical materials are simulated using different indicators in the centrifuge model.The silver sand,the shaft and the pile cap are simulated according to the natural density,the compressive stiffness and the bending stiffness,respectively.The finite element method（FEM）is implemented and analyzed in ANSYS,in which the stress field during the undisturbed soil stage,the boring stage,the concrete-casting stage and the curing stage are discussed in detail.Comparisons in terms of load-settlement,shaft axial force distribution and lateral friction between the numerical results and the test data are carried out to investigate the bearing behaviors of super-long rock-socketed bored pile groups under loading and unloading conditions.Results show that there is a good agreement between the centrifuge modeling tests and the FEM.In addition,the load distribution at the pile top is complicated,which is related to the stiffness of the cap,the corresponding assumptions and the analysis method.The shaft axial force first increases slightly with depth then decreases sharply,and the rate of decrease in rock is greater than that in sand and soil.

Effect of sediment on vertical dynamic impedance of rock-socketed pile with large diameter

Based on the fictitious soil pile model, the effect of sediment on the vertical dynamic impedance of rock-socketed pile with large diameter was theoretically studied by means of Laplace transform technique and impedance function transfer method. Firstly, the sediment under rock-socketed pile was assumed to be fictitious soil pile with the same sectional area. The Rayleigh-Love rode model was used to simulate the rock-socketed pile and the fictitious soil pile with the consideration of the lateral inertial effect of large-diameter pile. The layered surrounding soils and bedrock were modeled by the plane strain model. Then, by virtue of the initial conditions and boundary conditions of the soil pile system, the analytical solution of the vertical dynamic impedance at the head of rock-socketed pile was derived for the arbitrary excitation acting on the pile head. Lastly, based on the presented analytical solution, the effect of sediment properties, bedrock property and lateral inertial effect on the vertical dynamic impedance at rock-socketed pile head were investigated in detail. It is shown that the sediment properties have significant effect on the vertical dynamic impedance at the rock-socketed pile head. The ability of soil-pile system to resist dynamic vertical deformation is weakened with the increase of sediment thickness, but amplified with the increase of shear wave velocity of sediment. The ability of soil pile system to resist dynamic vertical deformation is amplified with the bedrock property improving, but the ability of soil-pile system to resist vertical vibration is weakened with the improvement of bedrock property.

Ultimate end bearing capacity of rock-socketed pile based on generalized nonlinear unified strength criterion

In order to study the mechanism of bearing behavior at the tip of a pile embedded in rock, the generalized nonlinear unified strength criterion and slip line principle for resolving the differential equation systems which govern the stress field were applied to derive the ultimate end beating capacity based on some reasonable hypothesis and failure plane model. Both numerical simulation and test results were compared with the theoretic solution. The results show good consistency with each other and verify the validity of the present approach. The depth effect with respective to embedment ratio and other influence factors like geological strength index, intermediate principal stress, overburden factor, and damage on end bearing capacity were discussed in the analytical solution. The results show that the proposed yield criterion can be much better for investigating the ultimate end bearing performance of rock-socketed pile. The end bearing capacity increases with embedment ratio and the increasing degree is influenced intensely by the above parameters. Furthermore, ignoring intermediate stress effect would underestimate the strength properties of the rock material and lead to a very conservative estimation value.

Theoretical analysis of a row of piles as passive barriers and an equivalent in-filled trench model

A theoretical study on the ground vibration isolation efficiency by a row of piles as passive barrier in a three-dimensional context was presented. The analysis was accomplished with the aid of integral equations governing Rayleigh wave scattering, used to predict the complicated Rayleigh wave field generated by a number of irregular scatters embedded in an elastic half-space. Then, the passive isolation effectiveness of a row of piles for screening Rayleigh wave was studied in detail. The effects of relevant parameters on the screening effectiveness were investigated and analyzed from the perspective of equivalence with in-filled trench. The results show that using a row of rigid piles as wave barrier is more effective than that of flexible ones, and an optimum reduction of vibration can be achieved either by increasing the size of piles or by decreasing the net spacing between the piles. Finally, based on the derived integral equation for Rayleigh wave scattering, the principle of equivalent modeling of the barrier of piles by an in-filled trench is put forward, which simplifies the analysis of vibration isolation by a row of piles.

Study on the squeezing effect of open-ended pipe piles

During the installation of a pipe pile,the soil around the pile will be squeezed out. This paper deals with this squeezing effect of open-ended pipe piles using the cylindrical cavity expansion theory. The characteristics of soil with different tension and compression moduli and dilation are involved by applying the elastic theory with different moduli and logarithmic strain. The closed-form solutions of the radius of the plastic region,the displacement of the boundary between the plastic region and the elastic region and the expansion pressure on the external surface of the pipe piles are obtained. When obtaining these solutions,the soil plug in the open-ended pipe pile is considered by employing an incremental filling ratio to quantify the degree of soil plugging. Moreover,the effects of the ratio of tension and compression moduli,angle of dilation and incremental filling ratio on the radius of the plastic region and the expansion pressure on the external surface of the pipe pile are investigated. The parametric analyses show that it is necessary and important to consider the difference between the tension modulus and compression modulus,dilation angle and incremental filling ratio for studying the squeezing effect of open-ended pipe pile installation. It is concluded that the analytical solutions presented in this paper are suitable for studying the squeezing effect of open-ended pipe piles.

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。

支盘桩加固既有填砂路基深层软土的模型试验研究

高速公路改扩建中既有填砂路基深层软土加固时存在路面结构层和施工场地空间限制的问题。首先,提出使用单盘挤扩支盘桩方案进行地基处理,通过挤扩法成盘于桩顶,以期实现不破除路面结构层的情况下达到设置桩顶承台或托板的效果。其次,利用群桩地基物理模型试验模拟一个桩土单元体研究加固方案的承载过程,通过分析桩体、盘体和土体的力学及变形指标研究支盘桩加固既有填砂路堤的承载表现和土体变形特征,最后,考虑土体参数和桩体尺寸推导了既有填砂路基深层软土支盘桩加固方法的二维简化模型。研究结果表明,软土在恒载下桩体轴力和土压力随时间变化调整,原因在于软土的压缩性和砂土的散粒体属性;桩顶轴力总和与恒载大小近似线性正相关,同时支盘直径越大桩顶轴力总和越大、桩承荷载占比越高,试验发现支盘桩加固法的桩承荷载占比约70%。另外,利用所建二维简化模型进行参数分析发现填砂路基中的最小盘间净距与上覆荷载、填砂内摩擦角、深层软土强度指标和支盘桩几何参数相关。

埋深和水土含量对松散岩堆抗剪强度的影响

针对松散岩堆剪切力弱会引起滑坡、崩塌等严重自然灾害的问题,运用相似理论,以石英砂为基本岩块、黏性土为填充土,配置相似材料,分别控制不同填充土含量和含水率并进行三轴试验,研究不同埋深下填充土的含量和含水率对松散岩堆抗剪强度的影响。结果表明:填充土的含量和含水率是松散岩堆咬合力和内摩擦角的关键影响因素,咬合力受影响程度更大;填土中的水对于松散岩堆的剪切力起着削弱作用;岩堆剪切力随着埋深的增加而增加,不同埋深处岩堆有相同的最佳填充土含量和含水率;在恒定岩堆土重度情况下,抗剪强度和埋深呈线性关系。

高填方路堤滑坡成因分析及防护对策

以某高速公路互通匝道高填方路堤边坡失稳为例,综合考虑填方区原始地形地貌、地层岩性及施工工艺对路堤边坡稳定性的影响,分析了土−岩二元结构填方边坡的变形特征和破坏机理,认为岩土界面可能为填方边坡的最薄弱层面而非填筑界面。同时,根据滑坡成因分析和稳定性评价结果,借助ABAQUS有限元分析软件对填方边坡的破坏机理做了进一步验证,并对抗滑桩支挡结构的受力状态进行了数值分析。分析可知,滑坡滑面主要在岩土界面处,滑面形态呈折线状;通过开挖台阶、设置宽平台和抗滑桩支挡可有效抑制路堤边坡变形,同时应加强排水;在抗滑桩支挡作用下,高填方路堤边坡潜在滑动面表现为由深层向浅层的演化破坏;滑动面以上桩身应力逐渐减小,在滑面处形成“三角状”高应力区,滑动面以下桩−土接触段桩身应力主要集中在桩前;桩身主要受弯段在潜在滑面以下;桩身主要受推力部位在滑面以上桩长1/5~1/3处,而主要抗力部位在滑面以下桩前部分。

杂填土地层深基坑微型桩-锚-撑组合支护体系受力特性原位试验

为深入研究杂填土地层深基坑桩-锚-撑组合支护体系受力特性,依托青岛市某深基坑工程开展微型桩-锚-撑原位试验,分析不同开挖工况下双排微型钢管桩桩身弯矩与预应力锚索轴力的演化规律,揭示该支护体系下前、后排桩的受力性状、预应力锚索应力分布特征,探讨邻近建筑物、基坑暴露时间及钢支撑拆除对该支护体系内力的影响。研究结果表明:1)在基坑开挖过程中,前排桩在受力中起主导作用;当开挖至基底时,桩身最大正、负弯矩极值呈现增大趋势,且极值点不断下移,开挖面以上桩身弯矩均呈正“S”型分布。2)开挖深度增加引起开挖面上、下1.0 m范围内桩身弯矩显著增大,前排桩桩身的反弯点分别位于钢支撑下方0.5 m、开挖面位置。3)在开挖过程中,锚索轴力沿埋深方向呈现减小趋势,锚固段前端1.5 m之后的轴力基本不变或呈微小波动。4)锚索锚固段应力高度集中在锚固段前端4.0 m以内的区域,约为锚固段长度的44%,锚固段末端基本未产生轴力,可对该段长度进行优化处理。5)邻近建筑物对微型钢管桩桩身受力影响较小;随着基坑暴露时间增加,桩身弯矩呈微小增长趋势;钢支撑拆除后,前排桩的弯矩变化集中在0.38H~0.96H(H为基坑开挖深度)。6)桩-锚-撑组合支护体系能够较好地限制基坑变形,选择合理的支撑预应力是该类基坑设计的关键。

深基坑咬合桩力学性状影响因素及敏感性分析

在淤泥填海地层开挖基坑往往会使围护结构产生严重变形,导致基坑失稳或垮塌。本文依托深圳某地铁明挖区间淤泥填海地层基坑工程,利用理正软件模拟深基坑开挖全过程,根据现场实测数据与模拟数据对比验证了模型的正确性,对该基坑咬合桩围护结构力学性状的影响因素进行分析,并基于正交试验分析其敏感性。结果表明:(1)围护结构嵌入深度与围护结构变形及受力性状密切相关;(2)当地面超载15 kPa、坑外水位7 m、围护结构混凝土强度C35及其嵌入深度10 m、土的黏聚力和内摩擦角均为原来的1.2倍时,围护结构变形及受力最小(I1II3III2IV3V3VI3是最佳组合);(3)对基坑围护结构水平位移、弯矩的最大影响因素分别是土的内摩擦角、围护结构嵌入深度。

通号电化科创大厦超限高层结构设计

通号电化科创大厦结构高度121.35m,采用钢筋混凝土框架-核心筒结构体系。结构设计采用了钢管混凝土外框柱、密肋楼盖及双连梁以达到更优的建筑使用空间。结合地层情况及摩擦型桩基的特点,基础设计采用了考虑桩土共同作用的设计思路,分别通过国内规范考虑承台效应的方法及参考国外文献的方法对桩土分担比例做了计算并按其结果进行了应用。本工程有多项不规则项,为超限高层结构。采用YJK和ETABS软件对结构进行了静力弹性分析,两种软件的分析结果较为吻合且均满足规范要求。采用Perform 3D软件对结构进行了罕遇地震下的动力弹塑性分析,根据分析结果对结构在罕遇地震下的抗震性能进行了评估,结构能够满足罕遇地震下预期的抗震性能目标。钢管混凝土柱与混凝土梁的连接节点为本工程的关键节点,针对该连接节点以往做法存在的问题,本工程设计了一种开大孔的连接节点并进行了节点有限元分析,结果表明节点具有一定的安全储备。最后,对密肋楼盖的建模模拟情况及计算结果进行了讲述。

深厚杂填土地基挤密桩加固效果试验研究

在深厚杂填土场地地基处理时,挤密桩由于较好的适用性和经济性而得到广泛应用。但是,在挤密桩的加固效果计算上尚有诸多不足。在夯扩挤密桩试验研究中,通过设置观测标点,研究在以粉土、砂质粉土、黏土及碎石素填土为主的深厚杂填土场地夯扩挤密桩地基处理的效果,判断夯扩挤密桩挤密影响范围,并采用干密度试验比照分析,对定位点测量的结果进行验证。通过综合分析得到挤密桩较精确的地基加固范围,并分析竖直方向加固效果的变化,为类似工程设计提供参考。

双排桩与扶壁式挡墙组合结构在高填方边坡加固中的应用分析

陕北某住宅小区位于填沟造地形成的填方地基上,采用桩基础穿透填土层支撑于下部中风化砂岩层以满足竖向承载力要求,而小区外侧高填土边坡(高度20m)未进行防护,现状稳定性较差,不能满足规范要求。根据现状边坡勘察结果,分析高填方边坡变形失稳机理及现场施工条件等因素,采用刚度较大的双排桩与扶壁式挡墙组合结构加固高填方边坡,并对桩周进行双液法压密注浆加固,取得良好加固效果。本文就双排桩与扶壁式挡墙组合结构加固机理、设计要点及加固效果检验进行论述,希望对类似的高填方边坡加固工程提供参考。

临岩堆边坡填方高速公路稳定性评价研究

西南地区地质条件较差,广泛存在松散岩堆边坡,天然岩堆呈极限平衡状态,具有潜在失稳风险。在岩堆边坡新建填路,在外力作用下岩堆边坡极限平衡打破,填方道路可能出现失稳,填方边坡薄弱面发生局部或整体滑动,威胁高速公路工程建设。本文以云南岩堆边坡填方高速公路为典型实例,通过收集资料岩堆内部沉积结构特征,反演分析岩堆边坡稳定状态和参数,进一步分析运用数值模拟分析得到填方道路稳定性与变形破坏之间的关系,验证了加固岩堆边坡填方道路稳定特性,证明施工方案的可靠性。

柱锤冲扩桩技术在杂填土路基处理中的应用

随着城市化发展,生产建设过程中产生的大量建筑垃圾堆积形成的杂填土层不仅造成了严重的环境污染,也因其可压缩性高、成分复杂和结构松软严重影响公路工程建设。合理地利用这些建筑垃圾并且增强杂填土层的承载性是目前城市发展和工程建设的重要课题。依托山西省阳城县滨河东路延长线坡底段综合工程项目,对K0+000-K0+650路段路基下的深厚杂填土层展开研究,通过室内实验获取场地各岩土层物理力学性质,在此基础上针对不同的路基加固方案根据其在本工程中的适用性进行比选,结果显示柱锤冲扩桩作为一种在土桩挤密法和强夯技术基础上发展来的加固技术在本工程中的适用性较高。进一步对柱锤冲扩桩法的设计方案和施工工艺进行说明,最后通过单桩静荷载试验和复合地基静荷载试验验证路基加固效果,结果表明采用柱锤冲扩桩法加固后的路基,其承载力得到提高且满足设计要求。本次研究为杂填土路基的处理设计和施工提供了科学依据,同时也对柱锤冲扩桩法在杂填土路基中的应用效果进行了验证。

桥梁桩基工程泥浆护壁旋挖成孔灌注桩施工技术研究——以广西梧州市苍容浔江大桥为例

文章以广西梧州市苍容浔江大桥为例,就桥梁桩基工程泥浆护壁旋挖成孔灌注桩施工技术展开了深入研究。首先,研究泥浆护壁灌注桩旋挖成孔工艺,同时详细设计了旋挖成孔的过程。其次,进行水下混凝土的配制与灌注,确保桩基工程的质量符合要求,进而完成桥梁桩基工程泥浆护壁旋挖成孔灌注桩施工技术的设计。最后,实例分析结果表明,随着孔深的增加,孔中心位置倾斜程度增大,但是该工程的测试结果均符合施工要求,证明该技术具有较高的可行性。

路基高填方施工工艺及技术质量控制研究

为了提高武汉汉南区通航产业园通江五路项目高填方路基的稳固性,文章提出对该路段的路基采用水泥搅拌桩处理加固工艺,以解决高填方路基施工质量低的问题,避免路基沉降,提升高填方路基的耐久性和稳定性。研究结果表明,此方法可以提高施工质量和效率,研究结果可应用于公路施工工程领域,为道路施工单位提供一定的支持。

济南某石灰岩区高速桥桩底沉渣缺陷处理研究

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