| Based on many research methods, such as theoretical analysis, laboratory tests,numerical simulation, field tests and et,al, using the knowledge of elastic mechanics,plastic mechanics, damage mechanics, fracture mechanics, rock mechanics,multivariate statistics, neural network and et,al, the main controlling factor system of water inrush from coal seam floor was established. The relationship between shallow coal seam floor failure depth and deep coal seam floor failure depth was calculated and the cut-off point of the deep and shallow coal seam floor failure rules was defined. The mining failure mechanics model of deep coal seam floor was built by the semi-infinite body theory and the slip line field theory. Based on the damage and fracture theory and unified strength theory, the unloading failure mechanism of deep coal seam floor was analyzed by grey correlation analysis. The characteristics of rock permeability coefficient varied at the different stress state with the reloading and unloading progress were analyzed by using TAW-3000 servo controlled rock three triaxial test machine.The mathematical model of deep coal seam floor failure depth was built. The PSO-SVM model for predicting the water inrush risk of deep coal seam floor was constructed. Finally, taking the measured results of the coal seam floor failure depth of panel 112145 in the deep part of Xin’an coal mine in Fengfeng mining area into consideration, the deformation and failure mechanism of the deep coal seam floor and the prediction model of water inrush risk were verified. The main contents and conclusions of this paper were as follows.1. The main controlling factor system of water inrush from coal seam floor was built. The system mainly included the water resisting property of effective coal seam floor and the strength of water inrush from coal seam floor, which including 17 aspects.The effective resistance property of water-resisting layer of coal seam floor was related to the thickness and rock lithology of aquifuge, the failure depth of coal seam floor, the original lifting height of confined water of the coal seam floor, the geological structure.The influence weights of main controlling factors of coal seam floor damage depth were analyzed. 32 kinds of orthogonal tests scheme were designed, which contained nine factors(eight main controlling factors and one blank)and four levels. The weighted order of 8 main controlling factors of coal seam floor damage depth was analyzed. The relationship between 8 main controlling factors and coal floor damage depth was studied.2. 74 sets of measured datas of coal Carboniferous Permian coal seam floor seam floor damage depth were collected. The augmented matrices of deep and shallow coal seam floor failure depth and its influence factors were established. The correlated coefficient vector of shallow and deep coal seam floor depth was analyzed by extracting the vector of the correlated coefficient matrix. The cut-off point of the deep and shallow coal seam floor failure rules was defined. The stability of the depth of the partial boundary points was verified by means of increasing the data method. The rationality of the depth of the partial boundary points was verified by means of fuzzy C means clustering method.3. In the process of coal seam mining, the deep coal seam floor had experienced five stages, which were mining compression stage, shear slip stage, mining unloading stage, goaf reloading stage, goaf compaction stage. Shear slip stage, mining unloading stage and goaf reloading stage were tended to water inrush, thus analyzed in detail.(1)Shear slip stage. The stress distribution of the end of stope was analyzed by using the fracture theory, and the plastic zone of the end of stope was determined by Griffith criterion. Based on this, the distribution rule of abutment pressure of stope was analyzed by semi-infinite element theory, and the active damage area of the coal seam floor under the influence of abutment pressure was determined. The mechanical model of mining failure of deep coal seam floor was constructed by slip line field theory.Considering actual parameters of the coal seam floor failure depth of panel 112145 in Xin’an coal mine in Fengfeng mining area, the relationship between the maximum failure depth of the coal seam floor and its influencing factors was analyzed. Studies had shown that the deep coal floor damage depth was positively related to advancing abutment pressure of panel. With the certain areas of panel plastic zone, the coal floor damage depth was positively related to internal friction angle of coal seam floor, and negatively related to cohesion of coal seam floor.(2)Mining unloading stage. On the basis of analysis of water inrush failure mechanism of deep mining floor unloading effect by Mohr diagram and fracture mechanics theory, taking the saturated pressure into consideration, the damage threshold of rock bridge failure and the calculation of plastic fractured region of branching crack tip were established by damage fracture mechanics and unified strength theory. The mechanical model of unloading failure of deep coal seam was bulit. Analysis of crack crack tip plastic zone influence on the crack damage fracture energy GI was carried out. The relationship between the crack damage fracture energyGI and its affecting factors was studied. The results showed that the stress intensity factor of crack tip under the maximum principal stress complete unloading state was larger than the biaxial stress state with the side pressure coefficient λ=0.5,and the mining floor was prone to failure. Taking the influence of the plastic fractured region of branching crack tip into account, the crack damage fracture energy was larger,compared with taking no consideration of the influence of the plastic fractured region of branching crack tip. The crack damage fracture energy was positively correlated with crack half-length, ratio of crack connected area to the crack total area, crack seepage pressure and minimum principal stress, whereas the crack damage fracture energy exhibited a negative correlation with friction coefficient of crack surface and modulus of rock mass.(3)Goaf reloading stage. According to the actual geological conditions of panel112145 in Xin an coal mine, the laboratory test plan is designed. The coal seam floor of panel 112145 in Xin’an coal mine was drilled, and the axial strain, radial strain,axial pressure, confining pressure, water pressure and water outlet with a stable flow of seepage flow was recorded by using the TAW-3000 servo controlled rock three triaxial test machine. The various characteristics of rock permeability coefficient of the deep coal seam floor at the progress of reloading and unloading stage was analyzed. The results had shown that the rock permeability coefficient with the progress of unloading at the elastic stage and reloading was similar to the normal condition. The rock permeability coefficient with the progress of unloading at the plastic stage and reloading was contributed to the formation of penetration resistance of rock. The rock permeability coefficient with the progress of unloading at the post peak stage and reloading enlarged the rock permeability.4. Combined with the measured data of Carboniferous Permian deep coal seam floor failure depth, the mathematic model of deep coal seam floor failure depth was constructed by using multiple linear regression theory. The correlation coefficient of regression results and measured results was 0.9415, indicating a high fitting degree.Based on this, combined with 19 cases of water inrush of deep coal seam floor in North China, considering the effects of length of panel, buried depth, coal seam floor confined aquifer pressure and thickness of aquifuge, the program was carried out by MATLAB, which penalty factor and kernel function parameter of SVM was optimized by PSO. The PSO-SVM model for predicting the water inrush risk of deep coal seam floor was constructed.5. The coal seam floor failure depth of panel 112145 in the deep part of Xin’an coal mine in Fengfeng mining area was simultaneously observed by drilling water injection method and borehole stress strain method. The mechanical model of mining failure of deep coal seam floor and mathematic model of deep coal seam floor failure depth were validated. The mechanical model of unloading failure of deep coal seam and PSO-SVM model for predicting the water inrush risk of deep coal seam floor were verified by the safety mining of panel.The results of this study provided an important reference for the failure mechanism of deep coal seam floor and evaluation method of water inrush mining above confined aquifer. |
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