Load-bearing characteristics of backfilling solids in deep mining under flexible passive confining pressure: An experimental study

To address the deviation between rigid confining pressure experiments and actual engineering conditions of deep backfill mining, where backfill near the working face has less confining pressure, while that in deep goaf areas is under high confining pressure, this study investigates the load-bearing characteristics of rock granular materials under flexible passive confining pressure. Customized PC molds with varying wall thicknesses and rigid steel molds were used to construct a gradient confining pressure environment. Compression tests were conducted, combined with the characterization of acoustic emission (AE) monitoring, strain measurement, particle sieving, and scanning electron microscopy (SEM) observation. The results show that flexible passive confining pressure divides the particle compression process into three stages that are different from those under traditional rigid constraints, namely the initial compaction stage, the crushing failure stage, and the lateral confinement-dominated stage. AE signals exhibit a bimodal energy distribution, and the time interval between the two can vary by more than 4 times with changes. The failure modes transition from shear to tension. Compared with intact materials, granular materials under lateral confinement maintain continuous volume contraction, and can even maintain a continuous volume contraction trend at least when the strain reaches 8%. And lateral confinement stiffness significantly enhances axial bearing capacity: when the axial strain reaches 30%, the axial stress in the rigid confinement group is nearly 5 times that in the flexible confinement group. Fractal dimension increases from 1.94 to 2.39 as the confinement stiffness rises. This study clarifies the influence mechanism of lateral confinement stiffness on granular mechanics, providing fundamental support for optimizing backfill design based on goaf locations and improving surrounding rock control in deep green mining.