Study on evolution and deformation of section coal pillar abutment pressure under repeated mining in overlying remnant coal pillars based on optical fiber sensing technology

Based on the background of close distance coal seam group mining in western mining area, combined with theory and field test, the influence of overlying residual coal pillar on the underlying section coal pillar under repeated mining is explored. The spatial and temporal dynamic decoupling of stress field and strain field of coal pillar is realized in No.2-2 coal seam of Huojitu well in Daliuta coal mine by using distributed optical fiber and fiber grating cooperative monitoring technology, and the evolution law of bearing structure and mechanical response characteristics of section coal pillar under repeated mining are revealed. The results show that with the advancement of the working face, the internal stress of the coal pillar has undergone the dynamic change process of the original rock stress zone-stress increase zone-stress release zone-stress increase zone-stress recovery zone. In the stress-increasing zone, the coal walls on both sides of the coal pillar respond faster to the abutment pressure and the accumulated energy will be released in advance. In the stress recovery zone, the stress in the middle of the coal pillar will reach a stable zone earlier than that on both sides of the coal wall. During the initial mining, the section coal pillar forms a three-zone bearing structure of ' elastic zone-plastic zone-broken zone ', with an average development length of 9.63 m, 3.53 m and 1.83 m, accounting for 64.2 %, 23.53 % and 1.22 % of the total width of the coal pillar, respectively. After repeated mining, the fracture zone of the section coal pillar has expanded by 80.88 %, and the overall deterioration of the coal pillar has become a bearing structure dominated by 'plastic zone-fracture zone ', and the overall fracture degree of the coal pillar is more serious.