Remnant coal pillar stress distribution and cross-pillar roadway layout control technology in extra-thick coal seam

The paper aims to determine the reasonable layout and stability control scheme of the mining roadway under the influence of remnant coal pillars in the multi-slice longwall mining of the extra thick coal seam in Laogong Yingzi Coal Mine. Combining field monitoring, theoretical analysis and numerical simulation, the stress distribution characteristics of the remnant coal pillar and the coal in the lower slice are studied to compare and analyze the stress states of the surrounding rock of roadways at different locations, the roadway layout across the remnant coal pillar is proposed, and a mechanical model of the floor slip line field is established. The model takes the width of the stress increasing area as the width of the bar-shaped foundation, the roadway layout scheme of 4 m horizontal distance from the remnant coal pillar iss determined. Combining with the roadway layout characteristics, the active and passive combined support scheme of “I-beam steel sets + cable bolts + shotcrete + shallow hole low-pressure grouting” is proposed. The results show that there is a double peak stress concentration both within the remnant coal pillar and the lower coal slice after mining on both sides of the upper slice. Inside the coal pillar is the vertical stress concentration zone, and at the edge of the coal pillar is the shear stress concentration zone.  External spaced overlapped roadway is influenced both by vertical and shear stresses. The stress concentration of external spaced overlapped roadway is the highest followed by vertical overlapped roadway and increases with the increase of the outer staggered distance. The influence of the shear stress is more obvious for vertical overlapped roadway. The influence of the vertical stress and shear stress on the roadway layout across the remnant coal pillar is significantly reduced, and the peak value of the vertical stress stabilizes at 34.1 MPa. The peak shear stress stabilizes at 6.7 MPa, and the stress concentration is the lowest and decreases with the increase of the interval of the two roadways. Field application results show that the deformation of roadway surrounding rock across the remnant coal pillar is small, and there is no intensive pressure observed, which meets the requirements of safety production.