Study On The Design Method Of Pile-raft Foundation Based On Geddes' Stress Formula

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.