Study on layout parameters of floor multifunctional roadway in deep ultra-thick coal seams under strong mining disturbance

Deep ultra-thick coal seam mining causes severe disturbance, and the surrounding rock of floor multifunctional roadways exhibits significant deformation and instability. The fundamental reason is the unclear evolution of the stress environment at different depths of the mining-disturbed floor, which leads to the lack of theoretical basis and quantitative criteria for roadway horizon selection. Taking Xiaozhuang Coal Mine in the Binchang mining area as the engineering background, this study systematically investigated the reasonable layout horizon of floor multifunctional roadways using theoretical analysis, similar material simulation, numerical simulation, and field measurement verification. A three-zone classification method for the mining-induced stress environment of the floor and a horizon optimization criterion were proposed. Theoretical calculations showed that the maximum failure depth of the floor was 18.37 m, indicating that the roadway should be arranged at a depth greater than 20 m to avoid the direct mining-damaged zone. Similar simulation tests revealed three distinct zones along the vertical depth of the floor mining-induced stress environment: the strong unloading disturbance zone (＜30 m), characterized by a stress fluctuation standard deviation of 0.018 MPa and loading–unloading frequency of ≥8 times per cycle; the stress fluctuation and concentration zone (30–50 m), characterized by a "loading–unloading–reloading" stress path and potential strain mutation risk; and the stable stress transition zone (＞70 m), where stress fluctuations were less than 0.01 MPa and disturbances were sufficiently attenuated. Numerical simulation results demonstrated that the roadway at the 30 m horizon exhibited severe incompatible deformation, with the roof-to-floor convergence reaching 1 068 mm; the roadway at the 50 m horizon showed asymmetric deformation dominated by floor heave, with a maximum floor heave of 190.5 mm; and the roadway at the 70 m horizon exhibited coordinated overall uplift, with the deviatoric stress ratio reduced to 1.5–1.8, a maximum floor heave of 159.5 mm, roof uplift of 39.7 mm, and net convergence only 11.2% of that at the 30 m horizon. A four-step decision-making logic for horizon optimization of floor multifunctional roadways was established: failure zone avoidance–stress zoning optimization–lithological condition verification–engineering validation. The optimal horizon was determined to be the stable rock layer approximately 70 m below the coal seam floor. Field monitoring data from the southern floor multifunctional roadway of Xiaozhuang Coal Mine showed that the roof-to-floor convergence was less than 100 mm during the entire mining cycle, roof separation was less than 30 mm, fluctuations in the surrounding rock stress were mild, indicating that the roadway achieved long-term stable operation.