A Finite Element Approach For Axisymmetric Contact Problems And Its Application In Pile Foundation Engineering

As the interface between pile and geotechnical media effects the mechanical behaviors of the pile foundation significantly, a reasonable model is critical in the calculation and analysis of pile. So far, though finite element method has been widely used in pile foundation engineering, it still remains an intractable problem to simulate the interface ideally in numerical analysis. The author deems that it is a practical way to treat this problem as contact problem in solid mechanics. The main intension of this paper is to develop a finite element approach for axisymmetric contact problems and apply it to pile foundation.So far, in the FEM analysis, the contact interfaces usually are taken as the joint element, which was introduced by Goodman early in 1968. Nevertheless, in the model, the contact interfaces are charactered simply with stiffness Kn and Ks, whick is not perfect and rigorous in the view of solid mechanics and rather rough for the description of the behavior of the interfaces. In contrast, taken as a contact problem, the principal characters of the interfaces can be reflected: (1) the contact interface is of shear strength which implies that a generic pair of contact points on the interface will displace coincidentally if the tangential force at this point has not reached the limit resistance, else, relative slide between the points will occur along the tangential direction. (2) No matter the relative slide occurs or not, the contact bodies cannot invade each other in any wise.There are considerable numbers of research concerning finite element method on contact problem and a lot of techniques such as mathematics programming, penalty method and so on. The method depends on the 'Freedom Degree' technique, which was developed in [2], is an effective method to solve contact problems. In this Paper, on basis of the theory, the anisymmetric isoparametric FEM formulae wereconstructed with the principle of virtual work. Moreover, the implementation of these formulae on the computer is also discussed in detail. Also, the similarity and difference between the method and conventional method are compared.Most of the present finite element programs are written in the structural language, such as FORTRAN, C and so on. As the size of programs grows, the defects of the structural language, which are hard to maintain and update, are exposed. The object-oriented programming (OOP) that appeared in 1980's can solvethis problem well. So far, though OOP has been applied to the field of FEM programming, it rarely be used to solve contact problems. In the paper, the programming of contact FEM code with technique of OOP is discussed and the program frame with object-class is established.Lastly, two axially loaded bored piles in some building site near Xichang city was calculated with the method and some useful data and curves including load-settlement curves and distribution of friction resistance etc. are presented. The accordance between the results of calculation and in-situ test verifies the validity of the method developed in the paper.