| As one of the major disasters in coal mines,rockburst has always been a research hotspot of mining science and technology workers.With the deepening of research,the mechanism of rockburst disaster is gradually rich.However,the dynamic disaster of coal and rock mass is a complex dynamic instability phenomenon.Influenced by many factors such as geological environment and working conditions,it shows obvious nonlinear and system instability characteristics from disaster preparation and initiation to occurrence.Many scholars have studied from the aspects of high stress,strong outburst proneness and large geological structure,but the research on structural effect of coal and rock mass and induced rockburst mechanism is still in the exploration stage.Therefore,based on the research method of macro and micro scale change,this paper first makes statistics and analysis of typical rockburst accidents in recent years in China,analyzes the structural evolution law and stress characteristics of rockburst occurrence from the perspective of coal and rock occurrence structure,and puts forward six types of rockburst with typical structural characteristics.Then,starting from the structural effect of coal,the mechanical properties and outburst proneness of different types of coal samples are analyzed by using servo testing machine and CT scanning at the laboratory scale,and the distribution of primary fractures in coal samples is reproduced by using three-dimensional reconstruction technology.On the basis of identifying the main fractures,the samples are loaded and tested for wave velocity,the structural evolution and mechanical behavior of coal samples are mastered.Based on this,by using the theory of stick slip and ultra-low friction effect for reference,the mechanism of structural instability of thick sedimentary coal and rock is explored.It is proposed that the internal mechanism of structural instability is that the interface friction of coal and rock is affected by contact pressure or interface roughness rather than constant,and the transient release of coal body energy caused by unstable sliding leads to rockburst,simplifies the mechanical model of coal face,verifies the ultra-low friction effect of simply supported beam model.At the same time,with the help of physical simulation test system,the mining law of thick sedimentary coal and rock mass is simulated,and the evolution law of structure and stress under normal mining and roof fracturing mining conditions is analyzed.Finally,according to the mine conditions of the typical hard thick layer roof in the western mining area,the high-pressure hydraulic fracture technology is used to fracture the cutting roof of the coal face,and the technological method of controlling the roof structure to prevent and control the induced rockburst under the engineering scale is mastered,which effectively reduces the large-area pressure event.The main conclusions are as follows:(1)This paper analyzes the distribution law of rockburst accidents in China,summarizes the environmental conditions,damage characteristics and induced factors of typical rockburst disasters,refines the classification of rockburst from the respective of structural instability of coal and rock mass,puts forward three kinds of structures and six types: natural structure including fault structure,fold structure,artificial stope structure including island coal pillar and suspension beam roof type,the geological weak plane structure including the coal seam abnormal type and the weak plane between coal and rock type.(2)Based on the structural effect of coal body,the mechanical properties and outburst proneness of different types of coal samples are analyzed by using servo testing machine and CT scanning at laboratory scale.The distribution of primary fractures in coal samples is reproduced by using three-dimensional reconstruction technology.From the reconstruction results,it is concluded that the overall characteristics of primary fractures are obviously different in different mechanical coal and rock media,and their development degree is obvious,which is inversely proportional to the outburst proneness.For the coal and rock with no outburst proneness,the primary fracture structure is highly developed,mainly schistose and reticular fractures,distributed in the whole coal rock medium,with strong connectivity;for the coal and rock with low outburst proneness,the primary fracture structure is higher developed,which is schistose and strip,occupying a lot of space in the medium;for the coal and rock with high outburst proneness,the primary fracture structure is low developed,which is similar to isolated island and schistose,and schistose fissures are mostly similar to horizontal or vertical state.(3)By setting the bedding angle to study the outburst proneness of coal and rock,it is found that the angle between the bedding plane and the loading direction has a significant effect on the mechanical parameters of coal samples,and there is a strong bedding effect on the macro failure mode and micro fracture characteristics.When the angle between bedding plane and loading direction is 90°,the outburst proneness of coal sample is the strongest,followed by 0 ° and the weakest at 45 °.At the same time,the acoustic characteristics of coal samples are anisotropic,the wave velocity of coal samples with axial vertical bedding is 15.1% higher than coal samples with axial syncline bedding.(4)Based on the theory of stick slip and ultra-low friction effect,this paper explores the mechanism of structural instability of thick sedimentary coal and rock,and puts forward that the internal mechanism of structural instability is that the interface friction of coal and rock is affected by contact pressure or interface roughness rather than constant,that is F=?tNt,the transient release of coal body energy caused by unstable sliding leads to rockburst,establishes the mechanics model of coal face,verifies the ultra-low friction effect of simply supported beam model.(5)Based on the comparison of stress and displacement field of surrounding rock in 3-1 coal seam of Bayangol coal mine after normal mining and artificial roof fracturing at the cutting by similarity simulation,it is concluded that under normal mining conditions,when the coal face advances to 66 m,the roof first caving,and the displacement of overburden rock stratum in the mining field mainly collapses downward.After artificial roof cutting,when the coal face advances to 24 m,the first collapse occurs.With the advancing of the coal face,the stress concentration coefficient of the front stress measuring point slowly decreases first,and the stress increases rapidly after the roof collapse to maintain stability,effectively block the stress transmission of the roof,and prevent the sudden outburst collapse of the cantilever roof.(6)Through the inversion analysis of three-dimensional in-situ stress field,the engineering geological environment of Bayangol coal mine is systematically analyzed,the type,size and distribution characteristics of the original rock stress field in the study area are obtained: the 11 panel area is the horizontal main control stress field,the ratio of the maximum main stress to the gravity stress is between 1.44 and 1.90.Combined with the roof thickness,structure and mining height of 3104 coal face,hydraulic fracturing measures were taken to break the cutting roof of the coal face.After the implementation of the fracturing technology,the first weighting interval of the main roof was 30.93 m,which was 61.33% smaller than the main roof without fracturing. |
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