Angle effect of the roof stress transmission path in longwall mining of steeply dipping coal seam

The key to safe and efficient mining of steeply dipping coal seam is an effective control of surrounding rock.Discovering and revealing the spatial time evolution characteristics of mining stress and the angle effect are the basis for the stability control of surrounding rock.Taking No.25221 working face of a mine in Xinjiang,China as engineering background,this paper adopted the comprehensive research methods of field monitoring,theoretical analysis,and numerical simulation.On the basis of comprehensively analyzing the general laws of abutment pressure and its causes,the spatial time evolution characteristics of the mining stress transmission path of roof and the angle effect were studied.The results show that during the mining of steeply dipping coal seams,the dynamic evolution process of roof stress and the abutment peak pressure has experienced a growth period and a stable period with the advancement of the working face.Affected by the angle of coal seam,the roof stress presents the characteristics of three dimensional transmission and evolution that are symmetrical along the strike and asymmetrical along the inclination.In the inclined section,the transmission path of the roof stress is an asymmetrical arch shape.From top to bottom,the stress deflection position gradually shifts from the left side to the right side of the working face.The load of overburden strata is transferred to the upper and lower coal seams with the stress deflection boundary as the boundary,which causes the concentration coefficient of the upper to be larger than that of the lower.In the strike section,the stress transfer path of the surrounding rock is a flat arch shape.The load of overburden strata in the semi arch area of the flat arch is not only transferred to the coal bodies on both sides along strike,but also to the upper and lower coal bodies along inclined direction.The load of overburden strata in the middle straight area of the flat arch is mainly transferred to the upper and lower coal bodies along inclined direction.As the angle of coal seam increases,the asymmetric characteristics of the stress transfer and evolution of surrounding rocks increase significantly.The distance between the stress deflection boundary and the central axis of the working face gradually increases,and the height of the intersection of the two shows an evolution trend of increasing decreasing increasing.Affected by this,the concentration coefficient of abutment pressure of strike and the upper along inclined direction show an evolution trend of increasing decreasing increasing.The asymmetrical spatial time evolution of the roof stress causes significant differences in the mechanical properties and behavior of the roof in different areas of the stope space.This difference leads to significant differences in the loading and instability characteristics of the supports in different areas,and this phenomenon will become more obvious with the increase of angle.