New framework predicts drilling stability in redeveloped urban sites

Urban redevelopment projects in densely populated areas often involve replacing aging structures, which necessitates the removal of existing foundation piles. This process leaves cylindrical voids that must be backfilled before new piles can be installed. However, achieving the same ground properties as the original undisturbed soil in these backfilled zones is challenging due to space limitations, depth constraints, and compaction difficulties. Strict construction schedules further hinder adequate consolidation of the backfill material.

These differences in ground properties between backfilled and native soil can lead to significant geotechnical issues during the installation of new piles. When drilling equipment operates near or through these backfilled zones, insufficient consolidation can result in inclined drilling. This deviation occurs because of the strength disparities between the backfilled soil and the surrounding original ground, creating unbalanced forces on the drilling equipment. Field observations have noted pile inclinations exceeding 10 millimeters per meter of depth.

Such inclined piles compromise structural integrity and bearing capacity, posing safety risks and incurring substantial economic consequences. Corrective actions, such as redrilling and pile reinstallation, can cause significant project delays and increase material costs. In congested urban environments, remediation may be impractical, potentially requiring a complete redesign of the foundation system. Current methods for preventing these issues are largely empirical, leading to specifications that are either overly conservative or insufficiently robust.

To address this challenge, a research team led by Professor Shinya Inazumi at Shibaura Institute of Technology in Japan has developed the first quantitative framework to predict drilling stability in backfilled ground during urban redevelopment. This framework utilizes finite element analysis integrated with the shear strength reduction method to clearly illustrate how strength differences between backfilled and native soils can cause drilling equipment misalignment. The study, published in Results in Engineering, aims to transform experience-based judgment into a measurable design problem.

The proposed framework involves parametric analysis by systematically varying backfilled ground strength, considering ratios from 0.8 to 1.2 relative to native soil strength, and examining both sandy and clayey original soils. It employs a three-dimensional elastoplastic finite element analysis combined with the shear strength reduction method, where soil shear strength parameters are reduced until failure is detected in the model. The framework allows for analytical evaluation of heterogeneous ground conditions, considering vertical drilling loads through both backfilled and original ground.