Impact of cemented paste backfill on mechanical properties and stability of coal pillars in open pit highwall mining

During highwall backfill mining, the backfill substantially impacts the mechanical properties and stability of coal pillars. Analyzing the mechanical properties and failure characteristics of coal pillars under cemented paste backfill constraints is crucial for managing slope stability and enhancing resource recovery rates. The mechanical behavior of the backfill-coal pillar-backfill (BCB) was systematically examined under varying conditions of backfill ratios and strengths through a series of confined compression tests. The stress-strain curves, failure strength, and failure modes of BCBs were determined. Numerical simulations were used to evaluate the effectiveness of backfill in controlling slope deformation. The findings showed that the stress-strain curves of BCBs can be delineated into five stages: pore compaction, elastic bearing of the coal pillar, crack development and coalescence, failure of the coal pillar, and coal pillar bearing under backfill constraint. With low backfill ratios and strength, the failure strength of coal pillars was less than that without backfill. Increasing the backfill ratio and strength improved the failure strength of the coal pillars, changing the failure mode from localized shear failure in the upper unconstrained sections to a global shear-tensional composite failure. Once roof contact was made, the backfill provided active confinement and supported part of the load, greatly enhancing the coal pillars' load-bearing capacity. Through numerical simulation, the efficacy of backfill control was quantified, further revealing a non-linear variation. The backfill transitioned from providing passive confinement to engaging in active, cooperative load-bearing when the backfill ratio increased from 90% to 100%, effectively eliminating the plastic zone on the slope surface and minimizing slope deformation.