Abstract: Using four-fold line strain softening model which considers the structure damage, according to the effect of squeezing soil when pipe sinking, the improved cylindrical cavity expansion theory is deduced; and it is compared with the existing models; the results indicate that the simplified four-fold line model is proximate with the typical compressing curve; and the method for determining each line and relevant factors is put forward. The formulas of stress and strain for each region around pile and the method for determining ultimate softening radius, ultimate destructive radius and ultimate expanding press are obtained by using Mohr-Coulomb yield criterion and associated with flow rule. On the basis of engineering case study, the improved cylindrical cavity expansion theory using four-fold line strain softening model is compared with the existing models; the results indicate that the displacement calculation of four-fold line model is the closest to the measured value; so the model can satisfy the practical engineering of squeezing soil more effectively.

Key words: soft soil structure, damage, cylindrical cavity expansion, pipe pile, four-fold line model