Reasonable gangue section length for disposing gangue pollutants in the new green mixed workface

Liberating layer mining is often used in high gas and low permeability coal seam, but this mining method will produce a large number of gangues. If these gangues are stacked on the surface ground, they will not only pollute the environment but also increase the transportation cost for the gangue. Backfilling method is an effective way to solve the problem of waste stacking. For this reason, Chinese scholars put forward a mixed workface which combines one part of the goaf that is managed with a fully mechanized gangue backfilling method (FMGBM) while the other part is dealed with the traditional caving method (TCM). The FMGBM is used for one part of the mixed workface, and the TCM is used for another. The two methods work together to ensure the coal output of the mixed workface. For this kind of mixed workface, the length of the backfilling section, length must be determined and well designed to provide a basis for support equipment design selection and mining pressure management. In order to solve this problem, it was firstly analyzed that the mining technology of mixed workface and the ABAQUS is used to establish the numerical simulation for the length optimization of the backfilling section in the mixed workface. By analyzing the influence of backfilling section length on roof vertical stress (i.e., vertical stress at the bottom of immediate roof, which represents the vertical pressure of immediate roof on coal body, caving rock, or backfilled gangue) and coal wall failure depth in backfilling section, caving section, goaf central position, and abutment stress area of the mixed workface, it is concluded that when the backfilling section length is more than 120 m, the roof vertical stress and coal wall failure depth are relatively small. Considering the cost of backfilling hydraulic supports, the backfilling section length of gangue is finally eventually determined to be 120 m. Finally, the industrial test was carried out in Ji(15)-31010 mixed workface of Pingdingshan No. 12 Coal Mine. The roof vertical stress in 5-m front of the workface coincides with that of the numerical simulation, which verifies the rationality of the 120 m length of the gangue section.