The shaft load transfer factor is calculated through the shear displacement method. Based on linear load transfer function to pile-base soil

using the elastoplastic model

a set of analytical equations for the settlement of vertically loaded pile are obtained. The theoretical model considering the strength of soil varying along the depth and the slip between pile and soil will be more realistic in practice. It is extended to determine the interaction factor between two piles and application to the analysis of pile groups is also presented. The proposed approach overcomes the limitation of elastic theory. The validity of the proposed method is verified through the comparisons of finite element method and field tests for the settlements of single pile and pile groups. General good agreement lends confidence in the applicability of the proposed method to practical problems.