Layout of roadway under goaf based on scale effect of mining cross upper coal pillar

With the condition of contugous double-thick coal seams downward mining as the background, in view of the adverse effects of upper remaining coal pillars on the stability of mining roadway and stope, the field measurement and theoretical analysis are adopted to study the regional characteristics of the deviatoric stress invariant under section coal pillar, the mechanism of strong strata behaviors during mining cross the upper boundary coal pillar and its scale effect. The research shows that: ① When the position of open-off cuts is not even in the upper seam, a transverse translation layout is adopted for the lower mining roadway, which not only place the roadway in a stable stress environment, but also prevent the collapse disaster by transform the whole mining cross the upper coal pillar into twice small-range processes. ② The stress state of floor rock under section coal pillar is analyzed with the second deviatoric stress invariant (J₂) intensity, the third deviatoric stress invariant (J₃) intensity, and the second deviatoric stress invariant (J₂) horizontal gradient. The floor is partitioned by the three indexes, and the minimum horizontal distance between roadway and section coal pillar is determined to be 24 m. ③ During the mining cross the upper coal pillar, the combined interaction of the first or periodical weighting and the sudden instability of roof and coal pillar system is the basic reasons for the strong strata behaviors. Based on the scale effect of mining cross the upper coal pillar, the maximum horizontal distance between mining roadway and section coal pillar is determined to be 36 m. The feasibility and rationality of the transverse translation layout of 30 m in lower mining roadway was verified by the field measurement of typical coal mine.