Study on overlying strata movement and surface subsidence of coal workfaces with Karst aquifer water

The overlying strata layers of coal workfaces with karst aquifer water normally causes serious safety problems due to the precipitation, drainage and water inrush, such as a wide range and long term of surface subsidence. In this study, by taking 10,301 working faces of the Daojiao coal mine in Guizhou Province as the engineering background, the numerical model of water-bearing strata with fluid-solid coupling was established by using UDEC to illustrate the laws of overlying strata movement and surface subsidence. A theory model was proposed to calculate the surface settlement caused by the drainage of aquifer based on the principle of effective stress modified by the Biot coefficient αb. The results showed that the corresponding maximum value (0.72 m) and the range of the surface subsidence with the occurrence of karst aquifer water were larger than that of the overlying strata without karst aquifer water (e.g., the maximum value of surface subsidence with 0.1 m). Moreover, the surface subsidence caused by the drainage of aquifer accounted for 17.8% of the total surface subsidence caused by coal mining. According to the field monitoring of surface subsidence in 10,301 working faces, the maximum value was 0.74 m, which was highly consistent with the results of numerical simulation and theoretical analysis. It verified the accuracy and reliability of the numerical model and the theory model in this study

Effect of crack-like defects on the fracture behaviour of Wire + Arc additively manufactured nickel-base Alloy 718

The fabrication of large components using a high deposition rate, near-net shape process like Wire + Arc Additive Manufacturing (WAAM) is a promising option for many industries, due to the potential for reduction in material wastage and shorter lead times in comparison to conventional methods. Specialist materials like nickel-base superalloys, which are typically used in high temperature and corrosive environments, are particularly attractive options due to their high raw material costs. Although nickel-base Alloy 718 seems well suited to the process due to its good weldability, process-induced defects can arise from unfavourable deposition conditions and elimination of these defects may not always be possible. In WAAM Alloy 718 deposited under such conditions, crack-like defects with planar morphology and hot cracking characteristics were observed. These defects were observable using conventional non-destructive testing techniques and displayed directionality relating to the deposition path. The fracture behaviour of WAAM Alloy 718 containing these defects was “semi-stable” – a mixture of fracture instability and stable crack extension. The apparent fracture toughness of WAAM Alloy 718 containing these defects was found to be anisotropic, which can be attributed to the interaction of the notched crack with pre-existing defects. WAAM Alloy 718 displayed an apparent fracture toughness comparable to that of wrought Alloy 718 when notched perpendicular to the defects; but only half that of wrought when notched parallel to the defects. Therefore, careful consideration of defect orientation and their effects on mechanical properties is important in assessing the fitness-for-service of WAAM Alloy 71

Study on Overburden Movement and Fissure Evolution Law of Protective Layer Mining in Shallow Coal Seam

This study aims to effectively solve the problem of the destruction of the coal roof and floor overlying rock after mining the protective layer, and determines whether the gas in the protected layer can be effectively released. We do this based on the engineering background of the Weng’an Coal Mine; research and analysis of the movement of the roof and floor overlying rocks; the evolution of cracks and the pressure relief characteristics of the protected layer after mining the protective layer; and through theoretical analysis and similar simulation experiments. Through numerical simulation, it was found that the protected layer was depressurized due to the mining of the protective layer, and the decompression rate of the protected layer was 0.2–0.8. In addition, the overall expansion rate of the protected layer was greater than the requirements of the “Detailed Rules for Prevention and Control of Coal and Gas Outbursts” for coal mines, and the investigation of the residual gas pressure and content of the protected layer revealed that the protected coal seam had been mined in the upper protective layer coal face. The gas pressure dropped to 50.7% of the original coal seam gas pressure, the rate of decrease was 49.3%, the residual gas content dropped by 68.67%, and the gas concentration in the return airway was 0.31% on average, meeting the national regulations that require its value to be less than or equal to 1%. The study comprehensively demonstrates that mining of the protective layer is beneficial to the release of gas from the protected layer, and provides a practical reference for coal and gas outbursts in mines

Optimization of Pre-Splitting Blasting Hole Network Parameters and Engineering Applications in Open Pit Mine

In order to optimize the parameters of a pre-splitting blasting hole network, taking an open-pit mine in Inner Mongolia as the engineering background, the numerical models of different pore sizes and hole spacing were established by LS-DYNA software. The stress wave propagation law, peak stress change and rock fracture state under various working conditions were analyzed. The optimization formula of a hole network relationship was proposed and verified on site. The results show that the shock wave generated by the explosion propagates rapidly upward from the explosion source and forms a plastic flow zone around the two boreholes. The energy consumption is the largest at this stage. With the propagation of the stress wave, energy dissipates, and its waveform gradually attenuates to a compression wave and seismic wave. In each working condition, a 110 mm aperture first cracked in the stress wave superposition area compared with other working conditions, while a 120 mm aperture delayed evolution to the seismic wave compared with different borehole aperture, and the energy attenuation rate is the slowest. Meanwhile, the fastest energy attenuation rate is with the 130 mm borehole aperture. With the attenuation of the propagation energy of the stress wave, among the four measuring points set at the center of the connection between the two boreholes, the Y-direction stress of the observation points B, C and D is stable between 2.3 and 3.5 MPa, and the Y-direction stress of the observation point A is strenuous between −1.3 and 1.2 MPa. The B, C and D observation points of 90–130 mm aperture conditions showed rock cracking at 7–9 times, 7–9 times, 7–10 times, 7–11 times, and 7–11 times hole spacing, respectively. The cracks of the two boreholes were interconnected. The optimal hole network relationship is obtained by fitting: y = 1.12 + 0.076x, where y is the optimal hole diameter and hole spacing multiple, and x is the hole diameter, which is verified by engineering. After blasting, the slope is smooth and smooth, and the half-hole rate is guaranteed to be above 90%

Mine pressure monitoring and data analysis of Shanjiaoshu Coal Mine

The paper introduced general situation of a coal face in Shanjiaoshu Coal Mine and its mine pressure monitoring purpose, content and problems that need attention, and gave a scheme of mine pressure monitoring, collected and analyzed mine pressure data. Effective treatment measure can be adopted by monitoring and analysis to ensure safe-mining, and results of mine pressure monitoring and analysis has a positive significance for understanding and knowledge of mine pressure behavior, as well as reasonable support selection and improvement of other mining faces in the same mining area

Study on Overburden Movement and Fissure Evolution Law of Protective Layer Mining in Shallow Coal Seam

This study aims to effectively solve the problem of the destruction of the coal roof and floor overlying rock after mining the protective layer, and determines whether the gas in the protected layer can be effectively released. We do this based on the engineering background of the Weng’an Coal Mine; research and analysis of the movement of the roof and floor overlying rocks; the evolution of cracks and the pressure relief characteristics of the protected layer after mining the protective layer; and through theoretical analysis and similar simulation experiments. Through numerical simulation, it was found that the protected layer was depressurized due to the mining of the protective layer, and the decompression rate of the protected layer was 0.2–0.8. In addition, the overall expansion rate of the protected layer was greater than the requirements of the “Detailed Rules for Prevention and Control of Coal and Gas Outbursts” for coal mines, and the investigation of the residual gas pressure and content of the protected layer revealed that the protected coal seam had been mined in the upper protective layer coal face. The gas pressure dropped to 50.7% of the original coal seam gas pressure, the rate of decrease was 49.3%, the residual gas content dropped by 68.67%, and the gas concentration in the return airway was 0.31% on average, meeting the national regulations that require its value to be less than or equal to 1%. The study comprehensively demonstrates that mining of the protective layer is beneficial to the release of gas from the protected layer, and provides a practical reference for coal and gas outbursts in mines

Coal and gas outburst features in Guizhou mining areas and its prevention countermeasures

With collection and analysis of outburst indexs in 381 mines and 695 seams and 214 coal and gas outburst accidents occurred in 120 mines in Guizhou mining areas, a study on coal and gas outburst index distribution features and outburst features in Guizhou mining areas was conducted, and relative outburst prevention countermeasures were proposed. Enhanced coal seam outburst index measurement, developing strict management system to mining areas with high outburst strength and more outburst such as Qianbei, Zhina, Liuzhi, Panjiang and Shuicheng, enhanced outburst prevention work of coal roadway heading face, and enhanced blasting management, dynamic phenomenon monitoring and geological structure detecting could be applied to reduce coal and gas outburst accidents occurred in Guizhou mining areas