Jianchun Ou

Bedding angles (BAs) in coal mining promote shear failure and can trigger rockbursts. Using Particle Flow Code (PFC) direct shear simulations on coal with BA = 0°, 30°, 60°, and 90°, we quantified BA effects on mechanical behavior and cracking. Increasing BA reduces shear strength and shear modules, reaching minimum of 4 MPa and 0.7 GPa at 90°. Failure modes shift from progressive, bedding parallel shearing at 0° to mixed paths at 30–60°, and abrupt brittle failure at 90°. Crack density and orientation evolve systematically: dense bedding parallel shear at 0°; more dispersed, lower-density mixed shear tension at 30–60°; and reconcentrated, high-density cracking causing premature shear at 90°. Corresponding force chain patterns aligned at 0°, dispersed at 30–60°, and realigned at 90° govern these outcomes by modulating stress transfer across bedding interfaces. Overall, BA is the first-order control on coal shear instability; the quantified thresholds and mechanisms provide actionable guidance for excavation orientation, support design, and targeted monitoring to reduce shear out and rockburst risks in coal mines.

In deep coal mining, fault slip-type rockbursts occur frequently. Understanding the shear mechanical properties of bedded coal seams and their intrinsic mechanisms is crucial. This study used PFC^2D7.0 numerical simulation to systematically investigate the shear mechanical behavior and micro-mechanisms of bedded coal under different normal stresses (1, 2, 3, 4 MPa). The research results show that: (1) The shear stress-displacement curves of bedded coal show three stages: elastic rise, strain softening, and residual stability. Both peak and residual shear strengths increase with the rise in normal stress. The peak strength shows nonlinear growth, while the residual strength exhibits a good linear relationship. Higher normal stress significantly reduces the strength reduction rate and effectively inhibits the brittleness of coal. (2) The failure mode consistently manifests as shear failure along the preset weak bedding plane, forming a distinct shear zone. Crack evolution analysis shows that shear cracks within the bedding are the primary form of damage, with minimal contribution from tensile cracks. (3) Force chain analysis shows that an increase in normal stress significantly enhances the density and connectivity of compressive force chains within the shear zone. It also effectively inhibits tensile force chains, with the bedding plane consistently serving as the primary area for stress concentration and transfer. This study provides important theoretical references for understanding the shear instability mechanism of bedded coal, predicting its mechanical response, and preventing fault slip-type rockbursts in deep coal mines.

In longitudinal ventilation tunnel fires, the thermal characteristics become more intricate due to the presence of blockages. This phenomenon becomes more complex when multiple blockages occur, which results in a unique interaction between the fire and longitudinal ventilation through gaps between the blockages. Most of the previous studies have only considered single obstacles or have only performed qualitative analyses and have not obtained predictive models. To fill this research gap, we conducted numerical simulations using the Fire Dynamic Simulator (FDS) to study the effects of vehicular blockages in three lanes and two fire locations. Our study highlights the differences in the flame behavior, maximum temperature rise, and smoke back-layering length in the presence of multiple blockages and reveals that as the ventilation velocity increases, the flame bifurcation angle increases and the smoke back-layering length decreases. Additionally, when the fire is in the side lane, the flame tilts towards the sidewall, leading to higher maximum temperatures compared to those in the middle lane. Based on these findings, we have developed modified formulas that predict the maximum temperature rise, smoke back-layering length, and maximum temperature ratio at different fire locations and blockage rates, which are linearly related.

Thermal runaway (TR) is a serious thermal disaster that occurs in lithium-ion batteries (LIBs) under extreme conditions and has long been an obstacle to their further development. Water mist (WM) is considered to have excellent cooling capacity and is widely used in the field of fire protection. When used in TR suppression, WM also exhibits strong fire-extinguishing and anti-re-ignition abilities. Therefore, it has received widespread attention and research interest among scholars. However, most studies have focused on the cooling rate and suppression effect of TR propagation, and few have mentioned the effect of WM on flame heat transfer, which is a significant index in TR propagation suppression. This study has explored the suppression effect of WM released at different TR stages and has analyzed flame temperature, heat release, and heat radiation under WM conditions. Results show that the flame extinguishing duration for WM under different TR stages was different. WM could directly put out the flame within several seconds of being released when SV opened, 3 min after SV opening and when TR ended, and 3 min for WM when TR was triggered. Moreover, the heat radiation of the flame in relation to the battery Q_E could be calculated, and the case of WM released 3 min after SV opening exhibited the greatest proportion of heat radiation cooling η (with a value of 88.4%), which was same for the specific cooling capacity of WM Q_m with a value of 1.7 × 10⁻³ kJ/kg. This is expected to provide a novel focus for TR suppression in LIBs.

The occurrence of rock burst hazards is closely related to the brittleness of rocks. Current research has paid less attention to the in-depth relationship between rock brittleness and acoustic emission (AE) signal characteristics and precursor information caused by rock fracture. Therefore, in order to further improve the accuracy of the AE monitoring of rockburst hazards, uniaxial compression tests were carried out and AE were monitored for rocks with different brittleness (yellow sandstone, white sandstone, marble, and limestone) in this paper. The relationship between the mechanical properties and the time-frequency characteristics of the AE was analyzed. In addition, the multi-fractal theory was introduced to further deconstruct and mine the AE signals, and the multi-fractal characteristics of AE from rocks with different brittleness were investigated. The results show that the stronger the brittleness of the rock, the higher the main frequency and main frequency amplitude of the AE. Brittleness is positively correlated with the multi-fractal parameter Δα (uniformity of data distribution) and negatively correlated with Δf (frequency difference between large and small data). In addition, the dynamics of Δα and Δf provide new indicators for AE monitoring of rock stability, and their abrupt changes can be regarded as precursors of failure. The weaker the brittleness of the rock, the earlier the failure precursor is and the more significant it is. This has potential engineering application value, which can help identify rockburst precursors and take timely protective measures to ensure engineering safety.

In recent years, with the continuous increase in the depth and intensity of coal mining, coal and gas outburst disasters pose a severe threat to the safe production of coal mines. Thus, this experiment studied the characteristics of electromagnetic radiation, acoustic emission, and electric potential signals during gas adsorption, stress loading, and the entire outburst process. The results indicate that during the adsorption process, different parts of the coal body exhibit variations in electric potential signals, electromagnetic radiation, and acoustic emissions. During the loading process, the consistency between the acoustic–electric signals and the load change rate is good, and at the moment of outburst, the acoustic–electric signals significantly increase with the ejection of coal and gas. Outbursts generally occur during the decline in electromagnetic radiation and acoustic emission signals, with the internal electric potential signal strength first decreasing then rapidly increasing and the surface electric potential directly rising. The closer to the outburst opening, the greater the change in signal amplitude. Based on the above experimental results, the outburst can be monitored through the acoustic–magnetic–electric precursory signal changes during the adsorption and loading processes, which is of great significance to the safety production and rapid excavation of coal mines.

This study explores the intricate behaviors of smoke flow, temperature distribution, carbon monoxide (CO) levels, and visibility dynamics within complex underground spaces during fire incidents. A key revelation is the profound impact of ventilation speed, with the identification of a critical range between 2 and 3 m/s that consistently proves to be instrumental in curbing smoke-related hazards and ensuring the safe evacuation of personnel. Furthermore, this paper underscores the influence of accelerated longitudinal winds on temperature profiles, particularly under high HRR conditions, underscoring the importance of accounting for wind effects in comprehensive fire response strategies. Regarding CO concentration, which is a critical safety concern, this study demonstrates that higher ventilation speeds effectively reduce hazardous gas levels, thereby fortifying overall safety measures. The visibility is analyzed, with the findings indicating that elevated ventilation speeds enhance visibility, albeit with considerations about potential drawbacks on personnel evacuation due to excessive wind speed. In conclusion, this paper offers a comprehensive understanding of the pivotal role played by ventilation speed in underground space safety by encompassing smoke control and temperature management.