Red sandstone is widely distributed in southern China. Due to the significant difference in mechanical properties before and after hydration and its poor water stability, red sandstone often triggers landslide accidents. In this paper, red sandstone from an open pit slope in Jiangxi Province was taken as the research object. Two variables, namely the initial saturation degree (25%, 50%, 75%, and 100%) and the number of wetting–drying cycles (0, 10, 20, 30, and 40), were set. With the help of nuclear magnetic resonance, the Brazilian disc test, and fractal theory, the relationships among its meso-structure, macroscopic fracture mechanics characteristics, and deterioration mechanism were analyzed. The research results are as follows: (1) Wetting–drying cycles have a significant impact on the pore structure and fracture mechanics characteristics of red sandstone. Moreover, the higher the initial saturation degree, the more obvious the deterioration effect of the wetting–drying cycles on the rock mass. (2) After further subdividing the pores according to their size for research, it was found that sandstone is mainly composed of mesopores, and the deterioration laws of different types of pores after the wetting–drying cycles are different. The porosities of total pores and macropores increase, while the proportions of mesopores and micropores decrease. The fractal dimensions of macropores and total pores of each group of rock samples are all within the range of 2–3, and the fractal dimension value increases with the increase in the number of wetting–drying cycles, showing significant and regular fractal characteristics. Micropores and some mesopores do not possess fractal characteristics. The fractal dimension of rock samples basically satisfies the rule that the larger the pore diameter, the larger the fractal dimension and the more complex the pore structure. (3) Both the type I and type II fracture toughness of rock samples decrease with the increase in the number of cycles, and the decrease is the most significant when the initial saturation degree is 100%. After 40 cycles, the decreases in type I and type II fracture toughness reach 23.578% and 30.642%, respectively. The fracture toughness is closely related to the pore structure. The porosity and fractal dimension of rock samples and their internal macropores are linearly negatively correlated with the type II fracture toughness. The development of the macropore structure is the key factor affecting its fracture mechanics performance. (4) After the wetting–drying cycles, the internal pores of red sandstone continue to develop. The number of pores increases, the pore diameter enlarges, and the proportion of macropores rises, resulting in internal damage to the rock mass. When bearing loads, the expansion and connection of internal cracks intensify, ultimately leading to the failure of the rock mass. The research results can provide important reference for the stability analysis of sandstone slope engineering.

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.

Through-going structures are widely distributed in rock masses of underground engineering, and their dip angles act as the core factor affecting the stress field and mechanical response of surrounding rock. To reveal the mechanical mechanism of rock masses containing through-going structures with different dip angles, this study adopts a combined method of theoretical derivation, indoor model testing and numerical simulation. Firstly, a plane strain mechanical model is established to classify Tectonically-induced Stress, Residual Gravitational Stress and engineering-induced stress, and the theoretical formulas for stress components, stress residual coefficient and stress deflection angle are derived. Secondly, rock-like specimens with through-going structures of various dip angles are prepared and biaxial compression tests are carried out to monitor mechanical parameters such as surrounding rock strain and peak strength. Finally, a large-scale numerical model is built by FLAC2D (version 7.0) software to simulate the whole process of stress equilibrium and excavation unloading of rock mass under a normal stress of 20 MPa. Then the data of principal stress, stress components, stress residual coefficient and deflection angle under different dip angles are extracted. The results show that the dip angle of through-going structure exerts a prominent regulatory effect on the rock mass stress field. With the increase of the dip angle, the Tectonically-induced Stress decreases continuously while the Residual Gravitational Stress rises gradually. The variation trend of stress deflection angle is highly consistent with structural dip angle, and the influence of Residual Gravitational Stress on deflection angle is limited. Due to the differences in loading modes and model sizes between indoor tests and numerical simulations, the evolution laws of stress residual coefficient show opposite trends, but both results verify the dominant effect of structural dip angle. Combined with theoretical, experimental and numerical results, the proposed theoretical system can effectively describe the stress evolution law of rock masses with through-going structures, which provides theoretical reference and technical support for the stability analysis of surrounding rock in similar underground engineering.

To investigate the localized deformation and failure evolution of surrounding rock in which an underground opening coexists with a filled weak discontinuity, biaxial compression tests under low lateral confinement were conducted on sandstone specimens containing a circular hole and a through-going filled fissure. Two one-factor test series were designed to separately examine fissure inclinations of 10°, 15°, and 20° at a constant filling thickness of 2 mm and filling thicknesses of 2, 5, and 10 mm at a constant inclination of 15°, with intact and single-hole specimens used as reference groups. The principal methodological contribution of this study is the simultaneous characterization of global mechanical response, local deformation at the roof and floor of the circular hole, AE activity, and tensile–shear AE-source evolution, thereby linking defect geometry to both overall load-bearing behaviour and localized instability. At a fissure inclination of 15°, the measured peak strengths for filling thicknesses of 2, 5, and 10 mm were 43.16, 48.96, and 47.90 MPa, respectively, while at a filling thickness of 2 mm, the peak strengths at inclinations of 10°, 15°, and 20° were 53.47, 43.16, and 55.61 MPa, respectively. The specimen with a 15° inclination and a 2 mm filling thickness (s-15-2) exhibited the largest local deformation concentration and the earliest local instability around the circular hole, whereas the specimen with a 15° inclination and a 5 mm filling thickness (s-15-5) exhibited a relatively high peak strength, delayed roof instability, and limited floor deformation. Increasing the filling thickness markedly enhanced middle-to-late-stage AE activity and increased the proportions of shear-type AE events to 67.5% and 63.9% for the 5 and 10 mm conditions, indicating that a thicker filled discontinuity increasingly governs damage evolution through interfacial sliding and shear-related activity. These results demonstrate the distinct roles of hole-induced stress concentration and filled-fissure geometry in controlling load-bearing capacity, deformation localization, and AE-source transition, providing an experimental basis for identifying potentially unstable regions around underground openings adjacent to filled weak discontinuities.

As large-scale continuous geological weak planes widely developed in deep mines, through-going discontinuities have a significant impact on the distribution of deep rock mass stress fields and the deformation and failure modes of surrounding rock and are the core factors restricting the safe and efficient extraction of deep resources. In this paper, indoor tests and numerical analysis methods were adopted to study the mechanical strength evolution characteristics and displacement changes of monitoring points of rock mass with prefabricated through-going discontinuities under horizontal and vertical pressure conditions, and the influence laws of different structural thicknesses and distances on the stress characteristics of engineering cavities were obtained. The results show that the introduction of through-going discontinuities can significantly reduce the peak strength and elastic modulus of rock specimens and change the circumferential strain. The thickness, distance, and relative position of through-going discontinuities jointly determine the strength degradation characteristics of the specimens. The larger the structural thickness (3 mm, 5 mm, and 10 mm), the weaker the strength degradation effect; when the through-going discontinuities are located above the cavity, the degradation effect is more significant. The relative position between the structure and the cavity determines the overall level of the stress field, while the structural thickness and distance (20 mm, 25 mm, and 30 mm) regulate the stress distribution and transfer path. When the through-going discontinuities are located above the cavity, the vertical stress of the surrounding rock decreases significantly with the increase of thickness, the horizontal stress remains basically stable, and the overall presents a high stress concentration characteristic. When the through-going discontinuities are located below the cavity, the vertical stress of the surrounding rock is generally low and changes gently; the horizontal stress increases gradually with the increase of thickness, and the overall stress level is much lower than that of the upper working condition.