Quantitative assessment of drilling stability in backfilled soils for sustainable urban building redevelopment

Urban redevelopment in densely populated areas often involves removing existing building foundations and backfilling boreholes, followed by installing new cast-in-place piles. Inadequate backfilled soil consolidation can lead to drilling deviations, compromising building structural integrity and environmental sustainability. This study develops a quantitative framework to evaluate drilling stability in horizontally heterogeneous soils, comprising cylindrical backfilled zones and surrounding native soil, to enhance safe and resilient urban construction practices. Employing three-dimensional elastoplastic finite element analysis integrated with the shear strength reduction method, the research examines how strength contrasts induce asymmetric shear failure and surface deformations that misalign drilling equipment. Parametric analyses vary the backfilled-to-native soil strength ratio from 0.8 to 1.2 in sandy and clayey soils. At a ratio of 0.8, effective plastic strain on the backfilled side reaches 2.6 times that on the native side in sand and 3.5 times in clay, resulting in surface displacement asymmetries up to 0.09 m. Strong correlations (R 2 > 0.94) between strength ratios and deformation metrics enable predictive modeling. A critical strength ratio of 1.1 is proposed to minimize asymmetry, supporting low-impact construction and reducing remediation needs. This methodology provides rational guidelines for backfilling specifications, promoting sustainable building performance, resource efficiency, and urban environmental resilience in foundation renewal projects.