Dynamic evolution of bed separation volume and strata migration in coal seam groups under repeated mining

To enhance mining safety in coal seam groups underlying ultra-thick sandstone aquifers, this study investigates the Yanjiahe Mine's geological conditions through physical simulation, numerical modeling, and in-situ monitoring. We analyze the evolution patterns of bed-separation space in No. 5^- 1, 5^- 2, and 8 coal, revealing volume expansion characteristics across mining methods and strata migration mechanisms. Firstly, through physical simulation of upward mining of No. 8 and 5^- 1 coal and downward mining of No. 5^- 2 coal, it is determined that the bed-separation volume of coal seam group mining has the expansion characteristics of Fracture Type (FT) → Cavity Type (CT) → Cavity + Fracture Type (CFT)→ Fracture Type (FT). Secondly, through numerical simulation of upward mining of No. 8 and 5^- 1 coal and downward mining of No. 5^- 2 coal, it is determined that the bed-separation strata have the migration laws of Lower Type (LT) → Higher Type (HT) → Lower Type (LT). Finally, by observing the overlying strata of 8105, 5105, and 5205 working faces through borehole peeping, the evolution laws of bed-separation are obtained as Lower Fracture Type (LFT) → Lower Cavity Type (LCT) → Lower Cavity + Fracture Type (LCFT) → Higher Cavity Type (HCT)→ Higher Fracture Type (HFT)→ Lower Fracture Type (LFT).