Theoretical analysis of water inrush from goaf in floor and mining stability for upward repeated mining

The method of "upper longwall-lower pillar" is adopted to re-mines the upper coal seam on the water-logged goaf of lower coal seam,which brings serious problems of safety and water inrush. In this study,the goaf overburden supported by coal pillars and confined water accumulation was simplified as a semi-infinite plane to establish a reasonable mechanical model. Through the elastic mechanics solution,the stress field distribution of the overburden was obtained. The maximum failure height of the overburden was obtained by introducing Mohr-Coulomb criterion and solving the extreme value. Combined with the failure depth of floor on the upper coal seam mining,the stability of interlayer strata was analyzed under repeated mining,and the corresponding discrimination indexes were put forward. Considering the interlayer water-resisting control strata and water head loss,the water inrush criterion under the different stability of interlayer strata were proposed,and the effect of horizontal tectonic stress on the stability of repeated mining was revealed. Comprehensively considering the stability of repeated mining,the water inrush from goaf in floor,and the bearing stability of the coal pillars,the evaluation methods and prevention measures under different working conditions were given,including reasonable drainage and pressure reduction measures and the index of "pillar-side backfilling" for the coal pillars. The results show that the roof stress field is related to mining depth,horizontal in-situ stress,coal pillar width,single goaf span and water pressure,and the failure form of the roof in the goaf is roughly "wavy". When the safety factor T ≥ 1,the stability of repeated mining is poor,and the longwall caving method cannot be used in the upper coal seam without taking sufficient measures. The greater the water pressure is,the less the coal pillar bears,and the coal pillar is under the condition of high confining pressure. Therefore,the stability of the coal pillar is higher,but the water inrush from the goaf in the floor is easy to occur under high water pressure. The larger the width of the coal pillar,the smaller the average supporting force,the lower the peak value of σ_y stress,the smaller the σ_x stress,and the less likely it is to be damaged. If the lateral pressure coefficient is greater than 1,it can effectively improve the feasibility of upward repeated mining. The water pressure p₁ after the drainage should meet the conditions of avoiding water inrush and coal pillar instability at the same time, that is, p_m < p₁ < max (p_f, h_d/100 + p_max). Otherwise,the water shall be completely drained,and then the "pillar-side backfilling" should be adopted by the size index W_m0. The research can provide useful guidance for the safe re-mining of the upper coal seam by "upper longwall-lower pillar" on the water-logged goaf,preventing water inrush from goaf in floor,improving resource recovery rate and reducing resource waste.