Research on Mechanical Mechanism of Instability in Overlying Strata–Abandoned Coal Pillar Groups in Strip Mining of Inclined Coal Seam

In mining methods such as the “three-underground” shortwall strip mining and other methods involving coal pillar retention, research on the instability mechanism of overburden–coal pillar groups considering the rheological properties of coal and rock is of great significance for ensuring the long-term safety and stability of the coal pillar system, the safe and efficient production of mines, and the safety of surface structures. Based on the interaction between the coal pillar and the surrounding rock, a plane strain model of residual coal pillars in inclined coal seams is established, and analytical expressions for the stress and displacement fields of the coal pillar are derived using the Ritz method. Combined with the actual conditions of strip mining in a certain village, a three-dimensional numerical calculation model for coal pillar group stability is established, which incorporates the rheological characteristics of coal-rock media. The influence of time effect on the long-term stability of coal pillar group is investigated. The impact of coal pillar width, mining depth, and coal seam dip angle on the plastic zone, stress distribution, and deformation characteristics of coal pillar groups is further studied. Research findings indicate that under the long-term action of the gravity of the overlying strata, the two sides of each coal pillar entered the plastic yielding state after 1 day of creep, and present a shear failure form. With the width of coal pillars increasing, both the extent of plastic zones and stress and deformation within each pillar decrease to varying degrees. When the pillar width reaches 40 m, the disparity in plastic zone coverage between pillars diminishes from a maximum of 21.14% to 10.16%. As mining depth increases, the rate of growth in plastic zones, stress, and deformation across coal pillars gradually accelerates, with the range of plastic zones expanding from 3.55% to 11.36%. An increase in coal seam dip angle similarly accelerates the development of the plastic zone within the coal pillar group. However, the vertical stress and vertical displacement of individual coal pillars exhibit varying degrees of reduction, while the maximum shear stress and horizontal deformation of the coal pillar group increase more rapidly.