Numerical investigation of geomechanical responses of pillar during sequential stope-and-fill operations in squeezing...

Mechanical performance of the intervening pillar critically influences extraction efficiency and operational safety in multiple stope-and-fill mining operations. However, the time-dependent interactions between backfill and creeping rocks and the resulting impact on pillar behavior remain poorly quantified, posing significant challenges in scheduling optimization, designs of pillar and backfill. This study employed FLAC3D to investigate the time-dependent geomechanical responses of pillar during sequential stope-and-fill operations, explicitly incorporating volumetric hardening behavior of consolidated backfill and creep behavior of rock mass. Results indicate that a short stope sequencing interval enables the consolidated backfill in the first stope to provide immediate lateral confinement to the pillar upon excavation of the second stope, improving stability. Conversely, extended intervals allow greater rock creep and enhance backfill densification and swelling pressure that eventually causes a beam-like bending of the pillar after adjacent excavation. It further leads to an asymmetric stress state characterized by tensile minimum principal stress σ₃ in the outer region (adjacent to the new stope), and high differential stress and elevated brittle shear ratio BSR in the inner part (against the consolidated backfill), marking the most critical state of pillar instability. An explicit dual-indicator framework based on σ₃ and BSR were proposed to capture this asymmetric mechanical response. Substantial parametric analyses were conducted to analyze the effects of stope geometry, pillar width, mine depth, backfill compression and swelling indexes, and rock viscosity on the results. Notably, pillar stability significantly recovered once the second stope was backfilled. These insights provide a mechanistic basis for optimizing the extraction strategy and the design of backfill and pillar in squeezing ground to minimize dilution and enhance operational safety and efficiency.