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.