论现行“桥规”中的桩基理论误区和如何设计超长大直径桩基问题

Theoretic Errors of Pile Foundations in Current "Bridge Codes" and Ways of Design of Very Long and Large Diameter Piles

结合几座大桥钻孔桩基础承载力出现不足的实例,对现行"桥规"中有关桩基承载力计算方法进行剖析,揭示现有设计与施工技术方面的错误观点与理念,指出现行"桥规"中单桩承载力计算基本理论存在不足,建议修改现行"桥规"。在对桩-土相互作用和桩在土中的荷载传递原理进行详细分析的基础上,提出要重视临界深度与有效桩长的概念、摩阻力与端阻力相互影响、地基土的抗剪强度并不是桩的承载力、桩上表皮摩阻力沿桩长分布不是矩形分布等观点;提出可用于计算超长大直径桩单桩承载力的"混合荷载传递法"和"公式直接计算法",供工程实践中试用。

With reference to the practical examples of insufficient carrying capacity problem occurring in the bored pile foundations of several bridges, the relevant calculation methods for the carrying capacity of pile foundations in the current ＂bridge codes＂ are analyzed, the wrong views and ideas in relation to the existing design and construction techniques are revealed, the deficiency of the fundamental theory for calculation of the carrying capacity of single pile in the ＂bridge codes＂ is pointed out and the corresponding proposals that the ＂bridge codes＂ should be modified are made. On the basis of the detailed analysis of the principles of pile-soil interaction and load transferring of piles in soil, the views that importance should be attached to the concept of critical depth and effective pile length and to the mutual influence of frictional and pile tip resistance are put forward and the views that the shear strength of ground soil does not mean the carrying capacity of the piles and the distribution of frictional resistance of the upper surface part of a pile along the pile length is not the rectangular distribution are given as well. Finally, the ＂mixed load transferring method＂ and the ＂direct formula calculation method＂ that can be used for calculation of the carrying capacity of very long and large diameter single pile are provided for tentative application in engineering practice.