Damage Accumulation and Instability of Hard Limestone Under Continuous and Stepwise Confining-Pressure Unloading

Deep underground excavation subjects hard rock to complex confinement-release histories, yet how continuous versus stepwise unloading reorganizes progressive damage remains poorly understood. Here, 30 hard-limestone specimens were tested under uniaxial compression, conventional triaxial compression, continuous confining-pressure unloading, and stepwise unloading at initial confinements of 10, 20, and 30 MPa, with synchronous acoustic-emission (AE) monitoring. Mechanical deformation was evaluated together with AE energy, frequency, RA–AF characteristics, upper-tail amplitude, phase- and cycle-resolved energy partitioning, and failure-proximal activity. Although continuous and stepwise unloading entered the unloading stage from comparable stress states, they produced distinct deformation and energy-release histories. Continuous unloading showed stronger terminal AE-energy localization and greater specimen-to-specimen variability, particularly at low confinement, whereas stepwise unloading distributed AE energy more progressively throughout repeated pressure-drop and hold phases. AE activity persisted during the hold periods, indicating continued internal damage evolution even when the externally imposed confinement was temporarily unchanged. By contrast, the central peak-frequency range, RA–AF characteristics, and upper-tail amplitude varied only modestly between unloading paths, suggesting that unloading mode primarily reorganized the timing and concentration of damage-related energy rather than fundamentally changing the dominant AE signal population. Failure-proximal high-energy activity was also more temporally concentrated under continuous unloading, while stepwise unloading produced a more distributed accumulation history before instability. These results demonstrate that the temporal mode of confinement release is a key control on damage accumulation, energy localization, and the approach to instability in hard limestone, providing a mechanistic basis for interpreting path-dependent failure in deep underground excavation.