Creep-Induced Time-Dependent Deformation and Width Optimization of Slice Drifts in Underhand Backfill Mining: Analytical, Numerical, and Field Investigations

The stability of slice drifts is critical to the safety and sustainability of mining operations. In underhand cemented backfill mining, slice drifts may be subjected to a special condition in which both the roof and sidewalls are composed of cemented backfill. Therefore, evaluating their long-term stability is of significant engineering importance. To analyze the time-dependent deformation of this special structure, the roof and sidewalls were idealized as a fixed-end beam and a fixed-end wall, respectively. Burgers creep compliance was then incorporated to derive time-dependent expressions for roof subsidence and sidewall convergence. The effects of slice-drift width, in situ stress conditions, and creep influence range on deformation were then analyzed. Subsequently, a three-dimensional FLAC3D 6.0 model incorporating the actual excavation, backfilling, and creep processes was established. The Mohr–Coulomb and Burgers constitutive models were used to investigate the evolution of stress and deformation in slice drifts with different widths. The results show that increasing the slice-drift width significantly increases the roof tensile stress and sidewall convergence rate. Under the extreme condition in which four adjacent drifts on both sides are excavated simultaneously, roof tensile stress concentration and drift deformation are further intensified. Considering both the roof tensile stress and the allowable convergence rate during the stable creep stage of the sidewalls, a slice-drift width of 5.0 m is recommended. Finally, field convergence monitoring was conducted at eight monitoring points in two slice drifts with widths of 4.8–5.08 m. The monitoring results show that the average convergence rates within this width range were lower than 0.25 mm/d, satisfying the stability requirement. These observations support the reasonableness of the numerical results and the recommended slice-drift width. The findings provide a reference for long-term stability assessment and width design of backfill-formed slice drifts in underhand backfill mining.