6.Conclusions
The excavation of coal and gas outburst-prone coal seams in shafts is one of the most serious safety problems in shaft construction. As mining depths increase, the risk of coal and gas outbursts in coal seams in deep shafts also increases and technical difficulties are intensified. In the paper, we determined that a funnel-shaped pressure relief zone existed under the working face and that a stress concentration zone surrounding the shaft was the critical area for gas outburst disasters.
To ensure the continuity and safety of the entire process of coal excavation, we divided the process of coal uncovering into four stages of management and applied three types of gas prevention and control measures: the primary technical measures (e.g., drainage or extraction boreholes), accelerated gas drainage measures (e.g., coal sluicing and hole cleaning) and safety protection measures (e.g., coal seam strengthening using metal skeletons and grout injection). Specific procedures for drilling were also provided, including circular orbit drilling and the use of a concave water pocket. The coal seams in the shafts of the Taoyuan and Haizi coal mines in the Huaibei mining region occurred at great depths and with high gas pressure and contents. All related investigation indexes were reduced below the critical values after adopting safety technologies for coal excavation, and fast and safe coal uncovering was realized. The technical systems developed in this paper would allow coal excavation procedures for deep shafts to be programmed, which were set up as new regulations. And the regulations would ensure the safety of coal excavation in shafts and serve as a reference for uncovering outburst-prone coal seam in shafts.
Acknowledgements
The authors are grateful to the National Science Foundation of China (No. 51004106), the China Postdoctoral Science Foundation (No. 2012M510145) and the Research Funds for the Youth of China University of Mining & Technology (No. 2009A005).