Advanced backfilling technology for maintaining pile installation precision in urban redevelopment

The accelerating pace of urban redevelopment has significantly increased the demand for pile removal, presenting critical challenges in maintaining ground stability and ensuring construction accuracy. This study presents a comprehensive methodology for achieving controlled, uniform strength characteristics in backfilled zones following pile extraction. This methodology combines the simultaneous penetration and injection of a cement-bentonite (CB) slurry with mechanical mixing using an optimized auger system. Through field implementation involving the removal of 350-400 mm diameter, 20 m deep prestressed concrete piles in cohesive clay ground conditions, the methodology achieved a mean unconfined compressive strength of over 100 kN/m 2 at 87% of the sampling locations. Cores sampled 26 days after backfilling and tested 28 days after backfilling achieved a characteristic strength (mean minus one standard deviation) of 85-95 kN/m 2 depending on the location. This demonstrates improved uniformity (coefficient of variation 12-19%). Laboratory mixing tests determined the optimal water-to-cement ratio to be between 500 and 600 percent, based on the pile body-to-perimeter soil ratio. Field validation with 45 new pile installations (1,000 mm in diameter) adjacent to backfilled zones maintained positional accuracy within acceptable tolerance limits (≤100 mm). The highest eccentricity values were observed in the approximate 3-8% overlap range. However, given that only 5 piles exhibited partial overlap, no statistical threshold can be established from this dataset. This observation is purely descriptive and indicative and is intended solely as a practical caution for future projects, not as a validated design criterion. This methodology provides a reliable solution for urban redevelopment projects involving cohesive clay ground conditions, with pile diameters ranging from 350 to 1000 mm and depths up to 20 m. New piles should be installed no earlier than 28 days after backfilling. Long-term performance beyond 90 days remains to be verified. • Developed controlled CB-slurry backfilling for pile removal in urban redevelopment • Achieved 28-day strength of 85-95 kN/m 2 (COV 12-19%) in backfilled zones • Optimized mixing to 350 rev/m to eliminate unmixed pockets to 20 m depth • Verified pile head eccentricity ≤100 mm for 45 new piles near backfilled zones • Provided practical QC parameters and design recommendations for cohesive clay sites