【作者】 吕一平；

【导师】 陈云敏； 凌道盛；

【作者基本信息】 浙江大学 ， 岩土工程， 2003， 硕士

【摘要】 如何分析软弱地基中桩筏基础的内力和沉降一直是岩土工程中的一个重要课题，也是工程设计中非常关键的一部分，许多岩土工作者对此进行了系统的研究。本文对桩筏基础的工作性状进行了研究，着重分析了新规范中用Geddes解分析桩筏基础内力和沉降的方法，探讨和解决了在实际应用过程中还存在的一些不完善的地方，并将它与传统的几种分析方法进行了比较。 群桩应力的确定是整个分析的基础。本文采用了Geddes对桩侧摩阻力分配形式的简化假定和对Mindlin解的二次积分公式来分析下卧层的附加应力，弥补了传统的实体墩基法不能分析长短桩等较为复杂的桩基形式的不足。对于这种方法在分析桩端应力时出现应力集中的现象，对Mindlin解重新积分来修正附加应力系数公式，以避免这种现象；由于弹性半空间和实际情况之间的模型误差会带来附加应力差距，通过大量的计算分析提出了修正系数m、n，并给出了m、n随桩径、计算点深度变化的函数关系，最终使得应力的计算结果与有限元的计算结果十分吻合，而计算相对比较简便。 桩端摩阻力分配系数α、桩侧均布摩阻力分配系数β是确定附加应力的关键。本文分析了α、β对下卧层附加应力的影响并讨论了荷载分配系数α、β的计算方法。由于传统的计算荷载分配系数的方法本身存在缺陷，造成α、β值不唯一，而且不能考虑桩端下土层的性质，与实际情况存在相当大的差距。本文认为桩侧阻力分布形式对下卧层附加应力的影响不大，从桩的剪应力和桩体位移间的对应关系方程上求解了桩端荷载分配系数，相对比较合理，在大多数情况下是可以为工程所接受的。还讨论了从原位试验出发确定荷载分配系数的方法。 建立了桩筏基础的整体分析模型。基桩对基础板的作用被等效为一个点弹簧，用16节点实体退化等参元对筏板进行分析。通过单桩的荷载沉降曲线确定群桩中单桩的刚度，并可用逐次逼近的方法求解桩顶反力和基础沉降，避免了对刚度的非理性调整。 最后，将本文的方法用于分析上海的一幢高层建筑，并将它与新规范上的实体墩基法及其他的分析方法相比较，分析结果具有符合实际的特点，具有广阔的应用前景。更多还原

【Abstract】 The internal force and the settlement of pile raft foundation in soft soil are very important problems in geotechnical engineering,and they are also key technologies in engineering design. Many researchers study these problems carefully. In this paper,the design method of pile raft foundation in soft soil is studied,especially the method for analysis of pile raft foundation by using Geddes’ stress formula,some problems which exist in application are discussed and resolved. The paper also compares this method with traditional National Code of Pile Foundation Design ( NCPFD ).The distribution of vertical stress is basic in analyzing. The paper analyzes the vertical stresses in soil based on Geddes’ assumption and stress formula,because the equivalent deep foundation is not appropriate when the lengths of piles are different. But the stress of bottom of pile is not accurate,so this paper modifies the fomula by integrating the Mindlin formula. Considering the model of elastic analysis method is different from the practice,correction factors nu n are introduced to get more accurate stress in soil. The result of this method agrees well with that estimated by FEM.Load ratio coefficients a and P are important to calculate vertical stresses. In this paper the influence of a and P is presented and the methods of calculation are discussed. Since there are some deficiencies in the traditional way,the result differs greatly from the practice. Based on the assumption that the form of skin friction is not significant,the paper calculate the coefficients through the corresponding equation between shearing stress and displacement,it is appropriate under many conditions.The model of pile foundation is developed based on the above theory and method. The pile is simplified as a nonlinear elastic spring. The raft is simulated by the 16-node degenerated element. Single stiffness is presented through Q-S curve,and the load of the top of pile is computed by circulating many times,so it can avoid man-made rectification.At the end of the paper,the application of the method is introduced on a project. It is shown that,compared with others,the design method is close to the practical situation.更多还原