• Flexible passive confining pressure induces three distinct compression stages in granular backfill materials, differing from traditional rigid confinement.
• AE signals show bimodal energy distribution with time intervals varying by over 4 times, indicating a transition from shear to tensile failure modes.
• Lateral confinement stiffness significantly enhances axial bearing capacity; at 30% axial strain, rigid confinement yields nearly 5 times higher axial stress than flexible confinement.
• Fractal dimension of particle breakage increases from 1.94 to 2.39 with rising confinement stiffness, reflecting enhanced particle fragmentation.