Improvement to the fictitious pile method for pile-soil interaction problems

Abstract

Dynamic pile-soil interaction analysis is often a key task in many civil engineering problems and several methods have been developed and applied. To many researchers, the fictitious pile method is a good candidate because of its high computational efficiency compared with the three-dimensional finite element method (FEM) and boundary element method (BEM). However, due to the assumptions made in the method, application of the method to dynamic pile-soil interaction at sufficiently high frequency may be problematic. A study on this issue is performed in this paper by comparing with FEM. It is found that the assumption that the vertical load (termed the patch load) applied by the fictitious pile to the ground is uniformly distributed on the cross-section is an important factor invalidating the method at high frequencies. Upon this founding, the original fictitious pile method is then improved by replacing the uniformly distributed patch load with a series of ring loads which make the vertical displacements at the intermediate radii of the ring loads identical. A set of linear algebraic equations are established to determine the ring loads. Results from both the improved and original fictitious pile methods are compared with those from FEM as benchmark. The comparisons show that the improved fictitious pile method has a much better accuracy (or for a given accuracy, works to higher frequencies) than the original one but the calculation efficiency remains similar. Although for the cases studied in this paper the improved fictitious pile method is much more accurate than the original one, the effectiveness of former may be still unsatisfactory for other cases. Therefore, the effect of parameters, including soil Young's modulus, pile radius and pile length, on the accuracy of the fictitious pile method is also investigated. It turns out that the pile radius and length have adverse effects on the accuracy of the fictitious pile method while stiffer soil makes the method work better. Based on the parameters of the pile and soil, a critical frequency is derived below which the fictitious pile method can be well applied.