Abstract: The formation of a mixed pile foundation, integrating new and existing piles, is an effective approach for reusing old piles. Based on an elasto-viscoplastic model, this study analyzes the evolution of mechanical properties of the soil surrounding piles during long-term service, and introduces the quasi-overconsolidation ratio to characterize changes in soil shear modulus. By introducing a pile interaction coefficient, we establish an interaction model for old and new pile foundations of different lengths and propose a variable stiffness leveling design method for mixed-pile systems. Furthermore, the effectiveness of this method is verified through comparison with finite element analysis results. Findings indicate that after 30 years of service, the void ratio of soil around old piles reduces by 10.5%, and the shear modulus increases to 3.04 times the initial value due to creep. In addition, increases in the in-situ overconsolidation ratio, decreases in the secondary consolidation index, and prolonged creep time necessitate greater pile lengths during leveling. When the length of new piles increases from 30 m to 37.8 m, the interaction between short piles and long piles decreases, and the settlement difference decreases from 2.07 cm to 0.09 cm.

Key words: new and old pile groups, creep, differential settlement, variable stiffness leveling, interaction