Definition of deep coal mining and response analysis

Scientific deep definition,especially the definition method of deep mining suitable for the modern mining practice,is a significant issue in developing the deep mining theory and technical practice. The deep rock,groundwater environment and modern mining methods in coal mining areas were comprehensively considered for the definition of coal deep mining. Based on the analysis of quasi-hydrostatic pressure environment in shallow crust and deep coal mine areas in China,a further study of deep definition of coal mining areas,the relative deep definition in different mining areas with the differences of coal and rock states (lithology and assemblage,water content,etc. ),and the determina- tion method of dynamic deep areas in deep mining were focused. The research shows that the coupling action between the initial state of the original rock and the mining mode determines the mechanical response behavior and state of the original rock. The remarkable features of the deep mechanical state are the high stress environment and the non-linear mechanical response characteristics,which are the main causes of the basic state in many states and the associated dis- asters in deep mining. The deep mining is then defined as a special mining activity in the space of coal and rock mass with high stress environment and non-linear mechanical response. After analyzing the regional tendency of shallow crust and local stress fields of coal mining areas with the depth in China,average lateral pressure coefficient Kav(i. e. the ratio of the average value of horizontal maximum principal stress and minimum principal stress to vertical stress) is selected as a basic parameter for the deep criteria of coal mine areas. By combining the deep mining practice in the eastern and middle part of China,the depth of 850 -900 m is appropriately considered as reference critical depth of deep coal mining in China ( shortly DCM of Hm ). Based on the difference of original rock in different mining areas (rock lithology,structure,water content,etc. ),a comparison model between Hs( i. e. the actual or visual DCM) and Hm is established,varied with initial protolithic states. Analyzing the relative change of the different mining geological situations,the result reveals that the softer the rock and the stronger the water content,the shallower the visual DCM becomes (or “shallowing”),even the depth reduction up to 30% ~ 50% . Based on the phenomenon of “stress arch” in mining,the Kav model and the Hs calculation method are constructed for the description of deep mining stress state. The modeling shows that the local stress and Hs from the cut position of working face to the external section of the pro- pulsion are the characteristic of the “end effect” changes from shallow→deep→normal,and the stress in the central area of working face is in a “lighter” state (or “shallowing”). The higher the mining height,the more pronounced the “lighter” state. The “end effect” tends to be gradually slowed down and weaken with the increase of the propulsion distance. The comparison of the critical depth from the typical mining areas shows that the depth in the eastern mining is deeper than the reference DCM, similar in the central mining area and less in the western mining area rich in groundwater (Shaanxi,Inner-Mongolia),actually so shallow as to 500-600 m. Nevertheless,the local deep mechanical anomaly could be found in the “end effect” area,in the mining depth of 400-500 m with a greater mining height using large-sized modern mining method. Based on the research results of deep rock mechanics and modern coal mining practices,the deep definition methods are proposed and discussed,combined with the characteristics of resource en- dowment in different mining regions of China and deep mining practice. By comparing with the existing related re- search and practice results,the method is proved to be rational in theory and its result is reliable. The method is useful to the research and practice of deep coal mining in China.