At present, the filling mining method is widely used. To study strength evolution laws of cemented tailings backfill (CTB) under different curing ages, in the experiment, mine tailings were used as aggregates, ordinary Portland cement (PC32.5) was used as cementing materials, and different additives (lime and fly ash) were added to make filling samples with the solids mass concentration at 74% and the cement-sand ratios 1:4, 1:6 and 1:8. Based on the nuclear magnetic resonance (NMR) technology, the porosity test of filling samples with curing ages of 3 d, 7 d and 28 d was carried out, and the uniaxial compressive strength test was carried out on the servo universal material testing machine. The relationship between the uniaxial compressive strength and porosity of backfills and the curing age in the three groups was studied, and change laws of the porosity variation and strength growth rate of backfills were analyzed. Based on the variation in porosity, the strength evolution model of the CTB under different curing ages was established, and the model was fitted and verified with test data. Results show that the uniaxial compressive strength, porosity, porosity variation, and strength growth rate of the three groups of backfills gradually increase with the increase of the curing age, the porosity of backfill basically increases with the decrease of the cement–sand ratio, and the porosity of backfill decreases with the increase of the curing age. Porosity variations and relative strength values of the three groups of backfills under different cement-sand ratios obey an exponential function, and the two have a good correlation, indicating that the established filling strength evolution model can well reflect strength evolution laws of the CTB with the change of curing age.

In order to further study the internal relationship between the microscopic pore characteristics and macroscopic mechanical properties of cemented tailings backfill (CTB), in this study, mine tailings and ordinary Portland cement (PC32.5) were selected as aggregate and cementing materials, respectively, and different additives (anionic polyacrylamide (APAM), lime and fly ash) were added to backfill samples with mass concentration of 74% and cement–sand ratios of 1:4, 1:6 and 1:8. After 28 days of curing, based on the uniaxial compressive strength test, nuclear magnetic resonance (NMR) porosity test and the fractal characteristics of pore structure, the relationships of the compressive strength with the proportion and fractal dimension of pores with different radii were analyzed. The uniaxial compressive strength prediction model of the CTB with the proportion of harmless pores and the fractal dimension of harmful pores as independent variables was established. The results show that the internal pores of the material are mainly the harmless and less harmful pores, and the sum of the average proportions of the two reaches 73.45%. Some characterization parameters of pore structure have a high correlation with the compressive strength. Among them, the correlation coefficients of compressive strength with the proportion of harmless pores and fractal dimension of harmful pores are 0.9219 and 0.9049, respectively. The regression results of the strength prediction model are significant, and the correlation coefficient is 0.9524. The predicted strength value is close to the actual strength value, and the predicted results are accurate and reliable.

The risks associated with underground mining at Fankou Lead/Zinc Mine in South China are growing due to the large-scale mining activities there. To recognize mining-induced earthquakes and assess the risk per mining level, a microseismic monitoring system, which is used to record microseismic events, is installed at multiple mining levels in Fankou Lead/Zinc Mine. The purpose of this study is to identify mining-induced earthquakes and to evaluate the risk per mining level by analyzing the spatiotemporal characteristics of microseismic activities in the Fankou Lead/Zinc Mine. In this study, the Gutenberg-Richter (G-R) relationship is applied to compute the b-value, which is used to obtain the maximum magnitude (M (max)) of microseismic event that probably occurs at each mining level. Then, the evaluation of the recurrence period for M (max) and the probability of the microseismic event with the magnitude M (max) is carried out and the M (max) at each mining level is determined based on the recording period of microseismic events. The results show that factors such as the maximum rock vibration velocity, source parameters, displacement, microseismic waveform and energy ratio (ES/EP) can be used to distinguish whether a recorded microseismic event is mining-induced earthquake. Additionally, we propose a method to assess the possibility of mining-induced earthquake at each mining level based on M (max) and predict the recurrence time of microseismic event with the magnitude M (max). The of two years results of microseismic events monitoring demonstrate that the current study is promising for identifying mining-induced earthquakes, assessing the risk of mining-induced earthquakes, predicting the potential maximum microseismic event in a region and estimating its recurrence period and its probability in the Fankou Lead/Zinc Mine.

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.