Mechanism of roof presplitting in a nonpillar mining method with entry automatically retained and its influence on the strata behaviors

The directional roof presplitting technology is the core of the nonpillar mining method with entry automati- cally retained. The effects of roof presplitting directly influence the stability of the retained entry. Based on the engi- neering practice in Ningtiaota coal mine,China,the theoretical analysis method was used to investigate the mechanism of directional roof presplitting,and the influence of roof presplitting on the sur-rounding rock behaviors was studied by discrete element numerical simulation and field experiment. The results show that the cracks propagated in random di- rections in the entry roof under the conventional blasting condition,and the integrity of the entry roof was easily de- stroyed. However,under the energy-accumulated tensile blasting condition,the directional crack could be generated under the guidance of original penetrated crack. To achieve ideal roof cutting effects,the charging amounts and blast hole distance should be harmoni-ously designed. The roof cutting parameters had an important influence on the deform- ation and stress distri-bution in the entry surroundings. Within a certain range,the increase of roof cutting height could increase the bulking volume of the gob roof,reduce the deformation of the entry roof and reduce the stress concentra- tion in the integrated coal rib. However,when the cutting height was too large,the construction difficulty increased,and it was not conducive to the stability of the retained entry. When the cutting direction was vertical to the roof,the de- formation of the entry surroundings was the largest. When there was a rational angle (10°-20°) between the roof cut- ting and vertical direction,the deformation of the entry surroundings was significantly reduced. Field test shows that when the roof cutting height was 9. 0 m,roof cutting angle was 10°,the distance between blast holes was 600 mm,the charging amount of a single hole was 3 200 g,the gob roof could fully collapse and the final entry retaining effects were satisfactory in the Ningtiaota coal mine. It is concluded that the roof cutting height affects the bulking gangues and its action on the short arm roof rock structure,whereas the roof cutting angle affects the dynamic dropping force and steady supporting force acting on the short arm roof structure during caving and stabilizing processes. The key of the roof split- ting in the new nonpillar mining method is cutting off the connection of the gob and entry roof. The roof splitting effects and interactive relationship between the gangues and short arm roof rock structure should be comprehensively consid- ered when the roof cutting parameters are designed.