Optimising gangue-containing backfill strength in gob-side entry retaining: From particle distribution morphology to release parameters

This study explores the substantial influence of coal gangue particle distribution morphology on the strength of gangue-containing backfill in gob-side entry retaining, while addressing the challenges associated with controlling it. Viscosity testing and CT scanning were conducted to quantify the temporal variations in the solidification viscosity of high-water material (HWM) slurry and the suspension depth of gangue particles. A particle migration equation within HWM slurry was derived for various curing ages, while a settlement coefficient ( S c ) was introduced to characterize the distribution morphology. Then, gangue motion and suspension characteristics were simulated using PFC2D coupled with Fish language scripting. Results indicated that S c increased with release time but decreased with release flow rate, height, and particle size, and a uniform particle distribution ( S c ≈ 1) was found to maximize the backfill strength. Furthermore, analyses of stress distribution, crack propagation, and energy evolution revealed that stress concentration within gangue particles and inadequate inter-particle bonding are responsible for premature failure in the HWM matrix, ultimately resulting in backfill damage. A framework based on GBM, optimized via GWO (Forward algorithm) and NSGA-II (Inverse algorithm), achieved efficient and accurate feedback prediction among backfill strength, S c , and release parameters. These findings provide insights for solid waste utilization and stability control in gob-side entry retaining.