【作者】 贺炜；

【导师】 赵明华；

【作者基本信息】 湖南大学 ， 岩土工程， 2007， 博士

【摘要】 随着我国西部大开发的持续推进与中部崛起战略的提出,超长大直径桩在公路桥梁工程建设中应用广泛。由于该类桩入土深度大,桩周地质条件复杂,其传力体系与普通基桩有较大差别,尤其是桩身压缩变形量大,设计时宜按桩顶沉降控制基桩的竖向承载力。此外,当桩周地基土软弱或自由长度较大时,超长大直径桩往往发生失稳破坏,因此在超长大直径桩的设计与计算中,不仅需考虑地基土质条件、成桩工艺等因素,尚应计入基桩的荷载传递过程、几何尺寸及埋置率等因素的影响。目前,有关超长大直径桩竖向承载性状的研究也有待深入,故本文结合国家自然科学基金项目“基于无网格法的高承台桩基屈曲机理研究”、教育部高等学校博士学科点专项科研基金项目“桥梁桩基屈曲机理的理论与试验研究”以及湖南省交通厅项目“山区超高桥墩桩基础承载机理及优化设计研究”等课题,对此开展深入系统的研究。⑴深入探讨了基桩荷载传递基本方程和荷载传递函数,在此基础上,分别引入双折线模型和双曲线模型,导得了可考虑土体分层特性和桩侧阻力发挥非线性特性的解析解,并提出用边界条件协调法计算基桩荷载-沉降关系,从而实现了按桩顶沉降控制基桩竖向承载力的设计思想。将其应用于湖南茅草街大桥试桩资料分析,能较好地描述基桩荷载传递性状和荷载-沉降关系。⑵基于荷载传递法,改进双折线模型以适应负摩阻力发挥特点,用弹性理论反算地基土沉降曲线,导得了可考虑土体分层特性的桩身负摩阻力的分段解析解,并结合工程实例对桩身轴力分布进行了分析,其效果较好。⑶针对有限元法网格划分带来的局限性,首次将基于微分方程局部弱形式的无网格局部彼得洛夫-伽辽金法引入桩基工程领域,用紧支试函数加权残量法和移动最小二乘法推导了其计算格式,并深入研究了权函数的选取、位移边界条件的处理等问题。在此基础上,引入桩土材料弹塑性模型及材料不连续面处理方法,编制出相应的计算程序,计算分析结果表明,其精度远高于有限元法。⑷将正交试验设计理论引入桩基工程理论分析,用正交拉丁方构造出相应正交表,组合影响基桩竖向承载性状的四个主要因素及其水平,用本文编制的无网格计算程序对各因素影响程度、超长大直径桩荷载-沉降曲线规律及竖向承载力发挥特性等进行了计算分析,深化了对超长大直径桩竖向承载性状的认识。⑸当桩周土较软弱或自由长度较大时,基桩易发生失稳破坏。为此,本文深入探讨了求解基桩失稳荷载的经验法、能量法及幂级数法等,针对基桩稳定问题的大变形特性,结合能量法和变分原理导得了完全埋入桩和部分埋入桩的大挠度非线性微分方程。并首次将摄动理论引入基桩稳定问题,用摄动参数展开桩顶荷载和桩身最大挠度,通过二次摄动技术导得了基桩稳定问题的高阶渐近解,并对基桩失稳荷载及失稳后承载性状进行了多因素分析,获得了一些定性的结论。⑹改进已有试验设备,通过设计不同的桩长、桩径、桩周土材料以及桩头约束等条件,设计并完成了多组室内单桩模型试验。在分析试验数据时,区别模型桩不同的破坏形式,采用相应计算理论进行计算分析,验证了本文方法的合理性。更多还原

【Abstract】 With continuously development of the Western China and strategic opportunity of central rising, the extra-long large-diameter pile is being more and more applied in construction of highways and bridges. For the deep bury-depth and complex geologic conditions around pile, especially the large compressing of pile shaft, the load transfer mechanism of the extra-long large-diameter pile is quite different from used one, and it’s better to limit the vertical bearing capacity design by settlement on pile top. Besides, when pile is surrounded by weak soil or of large unburied length, it usually failed buckling. So the calculating and design of extra-long large-diameter pile should take into account the effect of load transfer mechanism, geometrical size and bury ratio, besides the soil condition, construction method, the particularity of it bring us new challenge. Recently, the vertical bearing behavior of extra-long large-diameter pile is still need to be studied, so systematically work is done in this paper on the basis of national natural science foundation of China“Study on the buckling mechanism of pile foundation with elevated caps for bridge by meshfree method”, Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP)“Theoretical and experimental study on buckling mechanism of pile foundation for bridge”and Project from Hunan communication department“Study on the bearing mechanism and optimization design of pile foundation under extra-high pier in mountainous area”.⑴Based on lucubration on the differential equation and load transfer function of pile, the double-broken-line model and hyperbola model were introduced to deduce the analytical solution of load transfer of pile, which can take the character of layered stratum and nonlinear behavior of side resistance into account. Then, the boundary conditions adjustment method was put forward to calculate the load-settlement relationship of pile and carried out the design theory that limit the vertical bearing capacity of pile by settlement on top. Applied it to analyze the test pile of Mao-cao-jie bridge in Hunan province, and the result indicates that the mehtod could describe the load transfer behavior and load-settlement relationship well.⑵Based on the load transfer method, the two-broken-line model was improved to fit the exertion character of negative skin friction, and the piecewise analytical solution of negative skin friction along pile shaft was deduced, which can take the character of layered stratum into account. Then, an engineering example was analyzed by the method in combination with the Boussinesq solution, the results fits good with the spot measured data.⑶For the restriction of the mesh in finit element method (FEM), the meshfree local Petrov-Galerkin method (MLPG), which based on the local weak form of differential equation, was introduced into the fields of pile foundation for the first time. Then, the calculation format was deduced by weighted residual method and moving least squares approximation, and the related problems, such as the choice of weight functions, treatment of essential boundary conditions were studied. Based on these, the elastic-plastic model of pile and soil, the treatment of discontinous on the interface between different materials were introduced, and relevant program was also worked out. Analysis showed that the MLPG is much better in accuracy than FEM.⑷The orthogonal experiment design theory was introduced into the theoretical analysis of pile foundation, the orthogonal table was constructed by orthogonal Latin matrix to permuted and combined the four main effect factors on vertical bearing behavior of pile with their levels. Then, the effects of the factors, rules of the load-settlement curve on pile top and behavior of vertical bearing capacity were analyzed by the meshless program to deepen the understanding of vertical bearing behavior of extra-long large-diameter pile.⑸The long slender pile can fail in buckling mode, especially in soft soil. For this, experience methods, energy method and power series method for solving the buckling load of pile were firstly discussed, then, in order to analyze the character of the large displacement occurred on pile shaft when buckling, the non-linear large deflection differential equation for entirely and partial buried pile was established respectively by energy approach and variation method. The perturbation theory was introduced into buckling problem of pile for the first time, and through transforming the load on pile top and maximun deflection of pile shaft into power series by perturbation parameters, the higher-order asymptotic solution of load-deflection was founded. And effect factors of the buckling load and the behavior of the pile after flexural buckling was analyzed, some qualitative conclusions were got.⑹A series of model tests in lab were designed and performed by revised model test apparatus, in which the model piles were of various pile length, diameter, constraint conditions at pile top and surrounding by various soil materials. Then the test result was analyzed by various theory according to the failure mode of the model pile to validate the rationality of the methods in this paper.更多还原