Lihui Sun

Rock engineering problems in short-term freeze–thaw zones and short-term freeze–thaw cycles can lead to a reduction in rock strength, thereby inducing engineering disasters. Granite in short-term freeze–thaw zones was selected as the research object. Taking the freezing time (1 h) required for the internal temperature of the rock to reach the target freezing temperature as the reference, freeze–thaw cycle tests with durations of 1 h, 2 h, and 3 h were carried out in sequence. Combined with uniaxial compression acoustic emission (AE) tests, the effects of freeze–thaw duration and freeze–thaw cycle number on the physical and mechanical properties and AE characteristics of the rock were systematically investigated. A multi-index damage characterization system was established. Results show that: (1) Both freeze–thaw duration and cycle number show a positive correlation with the attenuation of peak strength and elastic modulus. (2) With the increase in freeze–thaw duration and cycle number, the AE cumulative ringing count rate and cumulative energy rate show an exponential decay trend. (3) Their proportion shows an increasing trend with the increase in freeze–thaw duration and cycle number. (4) Establishing a multi-index coupled damage variable to replace the traditional single-index characterization can improve the objectivity and reliability of freeze–thaw damage assessment.

Aiming at the technical problem of the section coal pillar width in ultralong fully mechanized mining face under the influence of dynamic pressure, taking 11207 face in Yuandatan Coal Mine as the engineering background, the stress distribution and failure characteristics of surrounding rock under different sections of coal pillar in Yuandatan Coal Mine are obtained through theoretical calculation and FLAC ^3D numerical calculation. The section pillar width suitable for ultralong working face is determined. The research results show that the reasonable width of coal pillar in the ultralong working face of Yuandatan Coal Mine is 29.7 m by theoretical calculation, and the reasonable width of coal pillar is 30 m by numerical simulation. The results of numerical simulation and in situ surrounding rock stress monitoring show that the increase of coal pillar width is conducive to sharing the pressure of overlying rock and further transferring it to deep coal body, thereby reducing the stress peak and its range in coal pillar, and is conducive to maintaining the stability of mining gateway.

Under multiseam mining, the caved strata of upper gob are affected by the lower coal seam mining, and the caved strata of lower gob are affected by the gravity action of the caved strata in the upper gob, resulting in the morphology of caved strata that is more complex than single coal seam mining. In this study, a similar simulation experiment device is used to analyse the morphology, fracture development degree, and subsidence of caved strata in an abandoned gob under multiseam. The physical model of caved strata in an abandoned gob under multiseam is established. Results show that the morphology of caved strata in the abandoned gob is a trapezoid. The caved strata on two sides of the abandoned gob are mainly horizontal fractures, and the middle part is a compacted area. Large number of voids and fractures exist near the start and end mining positions, and they are connected with each other. Because of the influence of the lower coal seam mining, the fracture development degree and subsidence of caved strata in the upper gob increase. The average increase rate of the maximum subsidence and the peak abscission rate is 24·61% and 16·69%, respectively.

The western mining regions of China, known for shallow-buried and high-intensity mining activities, face significant ecological threats due to damage to loose strata and the surface. The evolution of fissures within the loose layer is a critical issue for surface ecological environment protection in coal mining areas. The study employed field measurements, mechanical experiments, numerical simulations, and theoretical analysis, using the ‘triaxial consolidation without drainage’ experiment to assess the physical and mechanical properties of various strata in the loose layer. Additionally, the PFC^2D numerical simulation software was employed to construct a numerical model that elucidates the damage mechanisms and reveals the evolution of loose layer fissures and the development of ground cracks. The research findings indicate that during shallow-buried high-intensity mining loose layer fissures undergo a dynamic evolution process characterized by “vertical extension-continuous penetration-lateral expansion”. As the working face advances, these fissures eventually propagate to the surface, forming ground cracks. The strong force chains within the overlying rock (or soil) layers develop in the form of an “inverted catenary arch”. As the arch foot and the middle of the arch overlap, fissures propagate along these strong force chains to the surface, resulting in ground cracks. The study elucidates the surface damage patterns in shallow-buried, high-intensity mining, offering theoretical insights for harmonizing coal mining safety with ecological conservation in fragile regions.

This study examines frequent disasters, including large-scale deformation and collapse, caused by underground mining in weakly cemented strata in Western China. The weakly cemented rock’s unique characteristics, including low strength and easy disintegration, demonstrate a different damage pattern than that traditionally seen in the central and eastern regions. Using Fast Lagrangian Analysis of Continua-Particle Flow Code (FLAC2D-PFC2D) coupling, we model the strata, focusing on the 3-1 coal seam roadway at Hongqinghe mine. This study investigates the damage–rupture–destabilization progression in the peripheral rock under varying levels of moisture content. Our findings indicate that a water content of ω = 5.5% is the threshold for roadway damage, and moisture content 5.5% abruptly increases cracks and shifts the rock’s force chain, causing significant deformation and affecting the ceiling the most. Moreover, higher levels of moisture content weaken the anchor solid’s performance, with two primary failure modes: anchor interface slippage (comprising five stages: elasticity, elasticity–shear hardening, elasticity–shear hardening–decohesion, shear hardening–decohesion, and decohesion) and shear damage. These insights are vital for improving numerical simulations of underground mining, obtaining a more accurate understanding of mineral pressure disasters in weakly cemented strata mining regions in Western China, and developing a solid foundation for the better control of such strata.

Taking the weakly cemented sandstone of Ordos, China, as the research object, the evolution law between the relative stress of weakly cemented sandstone and the multiparameters of the acoustic emission under different dry-wet cycles was explored, and the critical failure identification mode of weakly cemented sandstone under dry-wet cycle was established. The results show that as the number of dry-wet cycles increases, the wave velocity loss rate gradually increases. Overall, the longitudinal wave loss rate is larger than the shear wave loss rate, indicating that the longitudinal wave is more sensitive to the degradation of weakly cemented sandstone. With an increase in the number of dry-wet cycles, the crack is mainly caused by the main crack penetration failure, and the secondary crack is significantly reduced. The fractal dimension decreases with an increase in the dry-wet cycles and reaches its maximum at 0 dry-wet cycles, which means that 0 dry-wet cycles witness the most complex morphology of fractures within the weakly cemented sandstone. This finding indicates that the dry-wet cycle inhibits the generation and expansion of fractures. The event rate appears to be close to 0 before the rupture, and then the platform oscillates, followed by a sudden increase. The acoustic emission b value is relatively high during the initial stage and then decreases, which is the initial damage process. The elastoplastic phase rises again, the peak stage decreases rapidly, and the weakly cemented sandstone undergoes unstable damage. The change in the acoustic emission entropy value is exactly the opposite of the b value change law. When the weakly cemented sandstone reaches the critical failure state under different dry-wet cycles, the relative stress value is 95. The test results provide new methods and a basis for the damage evolution mechanism and fracture prediction of weakly cemented sandstone under dry-wet cycles.

Bond strength is one of the most important parameters and can affect the macroscopic mechanical properties and the damage state of the rock to some degree. The coarse-grained sandstone with strength of less than 40 MPa was studied by the controlled variable method. The influence of parallel bond strength on the peak strength and failure mode of coarse-grained sandstone was simulated, the evolution law of peak strength and failure mode of bond strength were comprehensively analyzed. The results show that the peak strength of rock was positively correlated with the bond strength, the difference value between tensile and shear crack was negatively correlated with tensile bond strength and positively correlated with shear bond strength. Tensile-shear bond strength ratio less than 0.5, the peak strength of the rock was usually stable at the certain extreme value under a constant tensile bond strength. Tensile crack was negatively correlated with the tensile-shear bond strength ratio, shear crack was positively correlated with the tensile-shear bond strength ratio. The failure mode of coarse-grained sandstone is shear failure. The research results can be used to guide the ground control of other mine stopes or roadways with weak cementation lithology.