Mechanism of disaster induced by dynamic instability of coal pillar group in room-and-pillar mining of shallow and close coal seams

In the western part of China,mine disasters such as support failure,surface stepped subsidence and mine earthquake,are prone to occur when lower coal seam is extracted which is closely beneath the coal pillars left after room-and-pillar mining with shallow overburden. In this paper,physical modelling and numerical simulation method were adopted to investigate the dynamic instability process of upper coal pillar group and the mechanism of support failure under the influence of lower coal seam mining. According to the measured statistics in some mines after coal mining of Yuyang District,only when the proportion of elastic core area of the coal pillars is greater than 31% can the room coal pillars keep long-term stability. The simulation result after mining lower coal seams shows that the failure form and instability sequence of the upper coal pillars have close relations with the positions relative to the lower coal seam working face. The damage and instability of room coal pillar occur in the positions of open-off cut,working face, and the middle of goaf successively. Furthermore,the coal pillars in the open-off cut and working face mostly show hor- izontal shear failure,while the coal pillars in the middle of the goaf show vertical fracturing damage. According to the numerical simulation results of Shigetai Coal Mine,the peak abutment pressure of coal pillar in upper 2-2 coal seam in- creases from in-situ stress 2. 8 MPa to 12 MPa after coal mining with the stress concentration factor at 4. 28. After the lower 3-1 coal seam working face mined,the peak abutment pressure of the retained pillar in upper 2-2 coal seam con- tinually increases to 30 MPa with the stress concentration factor up to 10. 71. The open-off cut side of the lower coal seam working face showed uneven horizontally bearing with the coal pillar directly above the lower coal seam working face and the influence of horizontal tensile deformation are the main reasons for the diagonal failure mode of the boundary coal pillars. After the instability of two sides coal pillars,the roof instantaneously occur overall shear break- age,causing mine earthquake,and multi-layered strata in the roof settled suddenly in the form of " rigid body motion" and hit against the floor,leading to the collapse of the room coal pillars in the middle of goaf. At the same time,the huge impact force further leads to the full cutting of the strata between the upper and lower coal seams,which results in the roof cutting and support failure in the lower coal seam working face. It was found in the experiment that there was only about 0. 45 seconds from the failure of the first coal pillar in the overlying coal pillar group to the overall unstable movement and the final stability,among them,the full cutting of the strata between the upper and lower coal seams on- ly took about 0. 05 seconds.