Experimental Study on the True Triaxial Unloading Mechanical Properties of Cement Tailings Backfill Under Different Intermediate Principal Stresses

Engineering unloading activities during deep mineral resource extraction subject the backfill materials to complex true triaxial stress conditions, where their mechanical behavior and damage mechanisms are critical to stope stability. In this article, a true triaxial testing system was employed to conduct unloading tests under different initial intermediate principal stress (σ2) conditions, aiming to elucidate the influence mechanism of σ2 on strength, deformation, failure modes, and acoustic emission (AE) characteristics of the backfill, and to establish a corresponding damage constitutive model. The results demonstrate that the σ2 governs the mechanical response and failure mode of the filling material. Within the tested range, σ2 nonlinearly enhances both the peak stress, indicating improved load-bearing. As σ2 increases, acoustic emission activity changes from intermittent to continuous high-intensity ringing counts. The transition from brittle to ductile fracture. Model predictions showed high agreement with experimental data, validating its applicability. This study provides a critical theoretical foundation and modeling framework for assessing the stability of backfill structures under deep well mining conditions and guiding engineering design.