Synergistic mechanism of coupling bearing of backfill-coal pillar under lateral confinement condition

To ensure the stability of coal pillar in underlying goaf during the process of mining above mined-out area, the coal pillar-backfill couple bearing compression test under lateral confined conditions was conducted in this paper to study the mechanical characteristics of coal pillar under different backfill strengths and backfilling ratios as well as the interaction mechanism of coal pillar-backfill. In the meantime, the influence law of backfill strength and backfilling ratio on the compression characteristics, failure mode, failure strength and circumferential strain of coal pillar was analyzed, and the radial strain characteristics of elastic modules inside backfill with different heights and strengths were studied. On this basis, the active and passive constraint analysis models under roof and non-roof contact states were established. As can be seen from the research results, after the failure of coal pillar, the stress-strain curve of the combination with 90mm high backfill shows a “cascade” change, while that with 70 mm and 50 mm backfills shows a “sharp” decline, respectively. The failure strength of coal pillar increases non-linearly with the increase of backfill height and strength, and increase with the decrease of the difference between the uniaxial compressive strength of coal pillar and backfill. The failure mode of coal pillar gradually transits from shear failure to splitting one with the increase of backfill strength. The increase of backfill height can significantly improve the integrity of coal pillar upon backfill damage, and lengthen the bearing process of coal pillar when backfill wrap is damaged. Under roof contact state, the backfill produces radial expansion after axial compression to provide active lateral constraint for coal pillar. Meanwhile, it also bears the passive lateral constraint from the backfill which is squeezed by the coal pillar. By contrast, the backfill under non-roof contact state only provides passive lateral constraint for the coal pillar. Based on the stress analysis between the backfill under roof contact state and coal pillar, the equation for the active restraint stress provided for the coal pillar under roof contact state was obtained, which solves the problem that it is difficult to distinguish the tensile strain and the compressive strain caused by the active and passive constraints in the backfill after being offset. Finally, the failure strength equation of coal pillar under the condition of roof free contact was obtained by using the Mohr-Coulomb criterion.