【作者】 喻君；

【导师】 张忠苗；

【作者基本信息】 浙江大学 ， 岩土工程， 2007， 硕士

【摘要】 如今的大型基础设施的建设都离不开桩基础，但是桩基础的沉降计算一直以来都是一个难题。而对大量的桩基沉降的原位试验和离心试验都表明桩基的侧摩阻力在不同应力水平下呈现很明显的非线性，于是很有必要将非线性考虑到单桩或群桩的沉降计算中。本文针对侧摩阻力非线性的多样性，包括侧阻硬化，侧阻软化和弹塑性模型，建立了改进的荷载传递法——统一三折线模型对单桩和群桩的沉降计算做了解析方法和数值方法上的研究。对荷载传递法中的一些容易引起误解的概念做了对比分析，首先发现桩侧摩阻力与桩土相对位移的关系和桩侧摩阻力与桩身位移的关系是相同的，其原因是桩土相对位移和桩身位移在桩侧表面实际上是同一个物理量。其次明确了荷载传递的双曲线模型有经验双曲线与理论双曲线之分，以及侧摩阻力为加工硬化时临界位移值是如何确定的。通过建立统一三折线模型(可以统一表示侧阻硬化、侧阻软化和侧阻弹塑性特性)，导出了考虑极限侧摩阻力沿深度线性增加时的单桩荷载传递的解析解，并给出了荷载传递过程中桩身塑性段和滑移段的计算方法。利用该解析解分别对侧阻弹塑性，侧阻软化和侧阻硬化三种情况进行计算，并和已有文献算例的计算结果及桩基离心机试验结果进行了对比，发现吻合较好。采用该模型，分析了桩身几何参数、桩顶荷载水平和桩周(端)土特性对单桩承载变形性状的影响，并得出端阻比随桩身参数的变化规律。将统一三折线模型与杆系有限元耦合，建立了一种简便的矩阵位移法，从而将统一三折线模型扩展到成层土中单桩的沉降计算。该法通过统一三折线将桩侧摩阻力化为等效非线性弹簧，而将桩身离散成杆系单元，采用初应力法求解非线性方程组，同时可将桩身混凝土的弹塑性以及桩侧摩阻力对桩端沉降的影响计入到最终的单桩沉降计算值。利用该法对文献已有算例，离心试验以及工程实测中的单桩沉降进行计算对比，发现与实测结果以及解析法的计算结果比较吻合。利用该模型研究了高水平荷载作用下，混凝土弹塑性的分布规律以及侧阻对桩端沉降影响的适用范围。将三折线模型的线弹性段荷载传递关系通过群桩混合法的概念引入到群桩沉降计算中。通过和Randolph模型的类比，得出该线弹性段刚度的理论表达式，并将该表达式应用到群桩计算中，进行了相关的参数研究。分析了在弹性半无限空间中不同群桩的基桩荷载沉降比和各基桩的荷载分布随长径比、距径比、桩土刚度比的变化规律；分析了有限厚度土层以及桩端土模量与桩周土模量之比对群桩沉降和荷载分布的影响；最后还分析了成层土各土层的分布形式对群桩荷载分布的影响。将统一三折线模型通过群桩混合法引入到群桩沉降计算中，从而能够将侧摩阻力的非线性计入到群桩沉降的计算。用该方法对离心试验和工程实例中的群桩沉降进行计算，并将计算结果与实测结果进行对比，验证了方法的可行性。同时用该法研究了群桩端阻比和桩端沉降随承台顶荷载的变化规律，以及软化程度对群桩承载变形特性的影响。更多还原

【Abstract】 Nowadays, pile foundations are needed in almost every large infrastructure construction, but the calculation of pile foundation settlement is always a difficult problem. A great many in-situ and centrifuge model tests of pile foundation indicate the skin resistance of pile represents obvious nonlinearity at different stress level; therefore, it is necessary to incorporate the nonlinearity into calculation of pile foundation settlement. Based on the variety of skin resistance nonlinearity, including material stiffening, softening and elasto-plastic model, a modified load transfer method with unified trilinear model is established to study on the settlement of single pile and pile group analytically and numerically.Comparing some concepts easily to be understood in the load transfer method, it is firstly found that the relationship of skin resistance and relative displacement of pile and soil is just the same as the one of skin resistance and shaft displacement, because the relative displacement and the pile shaft displacement at he interface of pile shaft are actually the same. Secondly, it is found that the hyperbolic model of load transfer can be divided into empirical and theoretical curves and how the critical displacement of stiffening model is determined.Through setting up the unified trilinear model, which can express the stiffening, softening and elasto-plastic properties of skin resistance, the analytical solution of load transfer of single pile is deduced, considering the limit skin resistance increasing linearly along the depth. Then the way to calculate the length of plastic and sliding segment is also suggested. With the analytical solution in this paper, the cases in publications and the centrifuge model test of pile are used to calculate the settlement of single pile, including the stiffening, softening and elasto-plastic model. The predicted values are in agreement with those of publications and results of the test. Then the analytical model is employed to study the influence of geometric parameters of pile, the load level at pile top and the properties of soil around pile on the behavior of load-deformation of single pile and the changing regularities of bearing ratio with pile parameters.Coupling the unified trilinear model with rod element in Finite Element Method (FEM), a convenient Displacement Matrix Method is established, which enlarge the application of unified trilinear model to the settlement calculation of single pile in layered soil. In this model, the skin resistance is transformed into equivalent nonlinear spring, and the pile shaft is divided into many rod elements, then the initial stress method areemployed to solve the nonlinear equations. Meanwhile the elasto-plasticity of pile concrete and the effect of skin resistance on pile end settlement can also be considered. The case in publications, centrifuge tests and case history are studied with this model, it shows the calculated values are well in agreement with measured values and analytical solution. Then the elasto-plasticity of pile concrete under heavy load and application scope of the influence of skin resistance on pile-end settlement are also studied.With concept of hybrid method for pile group, the linear elastic load transfer relationship in trilinear model is introduced to pile group settlement. By analogy with Randolph model, the theoretical expression of stiffness of linear elasticity is obtained. Then the expression is taken into pile group and carried on parametric study. The changing regularities of the load settlement ratio of each pile in different pile group and load distribution of each pile with slenderness ratio, space ratio and stiffness ratio in elastic half-infinite space are studied. Then the influence of finite thick layer and ratio of the modulus of pile-end soil to that of pile side soil on the settlement of pile group and load distribution are analyzed. Finally, the effect of configuration of layered soil on the load distribution of each pile in group is also investigated.The unified trilinear model is introduced to the calculation of pile group settlement through the hybrid method, and then the nonlinearity of skin resistance can be taken into pile group settlement. The centrifuge test and case history are analyzed by this method, and the calculated values of pile groups are compared with measured values, which show well agreement, hence the rationality of this method is also verified. Meanwhile, by this method, the changing regularities of the end bearing ratio of pile group and end settlement with loads at cap and the effect of softening degree to the behavior of pile group load-deformation are studied.更多还原