Sponsor(s): China Coal Society
12 issues per year
Current Issue: Issue 05, 2019
Journal of China Coal Society, the 1st in the field of mining engineering, promotes the development of coal theory and practice, as well as academic exchanges at home and abroad to accelerate the transformation from scientific research to real productive forces. It is supervised by China Association for Science and Technology and sponsored by China Coal Society. Launched in 1964, it aims at providing key topics of coal scientific research and mining development. Its scope covers coal geology, geochemistry, geophysics, mineralogy and coal seam formation and evolution; petrophysics of coal; organic matter-rich shales, including mineralogy, formation, transport and storage of gases in coal and shales; unconventional energy systems (e.g. oil shales, shale gas, and other carbon-based fuels); ore deposits of materials and rare metals in coal and coal-bearing strata; and groundwater management. The journal is included in CA, JST, EI, CSCD.
Fan Baoying, Xu Shengyang, Zhu Shuangcheng
Journal of China Coal Society,2019,Vol 44,No. 05
Deep rock exists in the environment with high geostress, high temperature, and high pressure. The influence of mining on it is complex than that on shallow rock. The strong disturbance to deep rock was analyzed systematically by combining the surrounding environment of deep rock and the disturbance characteristics of deep mining. Firstly, the cooperation between static and dynamic disturbance was examined. According to stress paths in deep mining, both the distribution of the initial geostress state and that of the disturbance stress state under different depth ranges were given, which reveal the complicated characteristics of stress paths in deep mining. Thereby a preliminary formula to calculate unloading velocity was proposed based on the rock depth, mining method, rock gravity, residual stress, and excavating speed. According to the type of dynamic disturbance and the law of stress-wave propagation in deep mining, the characteristics of fluid pressure propagation in deep rock were pointed out. Thus it is inferred that the disturbance scope in deep rock was extended and the disturbance time was prolonged. Then the influence of disturbance was investigated according disturbed state concept (DSC). A disturbance function based on energy features was defined by analyzing the energy storage, energy dissipation, and energy release of deep rock, which would be adopted to build a unified constitutive model of deep rock based on DSC. Such function can quantitatively describe the disturbance in deep rock. Lastly, the distribution of an excavation-disturbed zone in deep rock mining and the corresponding stress–strain state were described qualitatively. As suggested, the zone was divided into an initial elastic zone, an excavation damaged zone (EDZ), and an excavation fractured zone, among which the EDZ was divided into inner damage zone, plastic disturbed zone, and unloading damaged zone. A formula was given to calculate the size of an EDZ. Each parameter in the formula was explained and the influence factors were discussed. The study shows that the disturbance to rock in deep mining becomes severer due to the high stress state of the deep rock and the complex multi-field multiphase coupled environment, and thus the range of an EDZ is increased significantly with more complex temporal and spatial evolution. The disturbance function based on DSC and the formula calculating EDZ sizes based on energy analysis can quantitatively describe the extent of disturbance in deep rock. They reflect the increase in deep disturbance excitation and disturbance influence, respectively.
Journal of China Coal Society,2019,Vol 44,No. 05
In China, the shallow mineral resources of the earth have been gradually exhausted. There are many engineering disasters in deep mining at 1 000–2 000 m depths due to the unknown mechanism of a deep mining-induced stress field. At the Ji 15–31030 mining face with an over-kilometer depth in Pingdingshan Coal Mine No. 12, the research was carried out to study the mining-induced stress evolution law at different mining speeds. The in-situ monitoring tests including bolt-stress testing and borehole-stress testing, and fractures in borehole monitoring were also carried out to investigate the behavior of a mining-induced stress field. Results showed that the peak advance abutment pressure increased alternately as the mining face advanced and fluctuated at around 85 MPa due to the great depth when the mining was over 80 m. The stress concentration factor was 3.3, which was higher than that at a shallow mining face. Overlying strata form “three zones” consisting of the caving zone, the cracked zone, and the bending subsidence zone. Meanwhile, the peak advance abutment pressure increased as the mining speed rase gradually; the distance between the peak pressure position and the mining face decreased correspondingly, and the length of the rock formed by the main roof breaking was greater. Hydraulic-support pressure grew periodically as the mining face advances. The bolt stress increased first and then decreased due to stress redistribution and energy release under the influence of mining, which concluded that the mining influence range was about 85 m. Borehole stress increased in the mining influencing range of about 75 m from the mining face and decreased slightly in the advance-abutment-pressure reduction zone. As the mining face advanced, strong mining disturbance resulted in the continuous development of fractures; then, fractures gradually developed into crisscross broken zones and extend upward.
Journal of China Coal Society,2019,Vol 44,No. 05
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 are 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 determination method of dynamic deep areas in deep mining are 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 and the non-linear mechanical response, which are the main causes of the basic state in many states and the associated disasters in deep mining. The deep mining is then defined as a special mining activity in the space of coal rock with high stress and non-linear mechanical response. After the regional tendency of shallow crust and local stress fields of coal mining areas with the depth in China is analyzed, 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 criteria of deep in coal mine areas. Through the combination of the deep mining practice in the eastern and middle part of China, the depth of 850–900 m is appropriately considered as the 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, varying with initial protolithic states. Based on the analysis of the relative change of the different mining geological situations, the result reveals that the softer rock and the larger water content can lead to the shallower visual DCM (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 adopted for the description of a 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 propulsion have the characteristic of the “end effect” changing from shallowing to deepening and then to being normal, and the stress in the central area of working face is at a “shallowing” location (or “shallowing”). The larger mining height can result in the more pronounced “shallowing” state. The “end effect” tends to be gradually slowed down and weaken with the increase in the propulsion distance. The comparison of the critical depth from the typical mining areas shows that the depth in the eastern mining is greater than the reference DCM, similar in the central mining area, and smaller in the western mining area rich in groundwater (Shaanxi, Inner-Mongolia), actually so shallow to 500–600 m. Nevertheless, the local deep mechanical anomaly could be found in the “end effect” area, at the mining depth of 400–500 m with a greater mining height with the large-size modern mining method. Based on the research results of deep rock mechanics and modern coal mining practice, the deep definition methods are proposed and discussed, combined with the characteristics of resource endowment in different mining regions of China and deep mining practice. Through the comparison of the existing related research and practice results, the method is proven to be rational in theory, and its result is reliable. The method is useful for the research and practice of deep coal mining in China.