| Xinjiang is one of the largest billion-ton coal bases in China.A large number of coal mines are located in the vicinity of the Tianshan Mountains,where melt snow,combined with tectonism,erodes the surface to form a valley with a depth up to one hundred meters,imposing a gradient loading on the coal seam.When coal mining faces are approaching these gradient loading areas rock bursts can occur,resulting in a restriction to the efficient production of mines.this thesis applies the methods including field investigation,case analysis,physical simulation,numerical simulation,theoretical analysis and engineering practise,to study Kuan Gou Coal Mine of Xinjiang Energy Co.LTD,involving occurrence mechanism and prevention measures of rock burst.The research achievements are as follows:(1)To analyse the characters of the geography and geomorphology and summarize the apparent characteristics of coal seam mining under gradient loading.The results show that the valleys in the Kuan Gou coal mine are well developed,with a maximum drop up to 300 m.The mountain regions characterized by a large surface drop(the gradient load area)have a concentrated distribution of microseismic energy density and hydraulic support stresses,and more fractures on the surface mountain,inducing many rock burst accidents.In contrast,the microseismic energy density and support stresses with the flat surface is uniformly distributed,leading to fewer macro-energy mine earthquake accidents and support stresses extremum.(2)To implement physical simulation and obtain the characteristics of the movement of overburden rock under gradient load.As the mining face is advancing in the mountain and fixed support point of deflection of overlying rock is moving,the mountain deflects to different directions and creates an "open-close effect".When coal seams under gradient loading is in mining,the degree of activity of overlying rock masses are ranked as the following order:in-slope phase>out-slope phase>flat-slope phase.The existence of critical area of the coal seam plays a key role in the movement of the overlying rocks,and when it is mined,the overlying rocks will experience temporary,large-scale,destructive movements,accompanied by a big jump in stress.(3)To establish a 3D refined numerical model and study the distribution characteristics of the situ stress field under gradient loading and the evolution of the stress field during the mining process.The results show that the distribution of the situ stress field is coincided with the shape of the gradient load,and the advanced stress in the mining process of coal seam is influenced by the gradient load.The greater the drop of the surface mountain,the more pronounced the gradient"pressing " stress on the coal seam underneath and the greater the influence on the situ stress field and the advanced support pressure at the mining face.However,the influence decreases as the depth of the coal seam increases.The maximum influenced depth is around 3.6 H for a mountain with a drop H.In detail,0～1.6H is defined as strong influence zone,1.6～2.4 H is medium influence zone,2.4～3.6 H is weak influence zone and greater than 3.6 H is zero influence zone.The main influence factors of stress distribution in coal seams under gradient loading are the height of the mountain and the gully drop,while width and slope of gully are regarded as minor factors.(4)To establish the theoretical formulae of static load pressure stress and dynamic load transfer stress and revel the mechanism of pressure-transferring coupling induced rock burst when the working face is approaching the gradient load area.The mountain with large drop "presses" the coal seam to create a critical condition with high static load,while the deflective motion and "open-close effect"of the mountain with the movement of the fixed joint point generates dynamic load.The coupling of static load and dynamic load induces rock burst.In the stage of working face entering and leaving the mountain,the peak value of the advanced support pressure is close to the critical value of burst,and the rotary stress reaches the maximum value.These two stresses couple to induce rock burst.When the mining face is right under the mountain,the peak value of the advanced support pressure is close to the critical conditions of burst and the top of the mountain"presses" the overlying coal seam to a hazard state of near critical burst.In such case,smaller rotary stresses can induce rock bursts with mainly compressive and supplementary rotary stresses.(5)To propose the method of identifying burst hazard zones for coal seam mining under gradient loading and identify burst hazard zones of the working face by using the membership function to incorporate subfactors including the mountain height,drop of gully and the slope and width of the gully.By assessment,four and five regions with high rock-burst danger were added to the working faces 1010203 and 1010206 respectively,and the results of the traditional rock-burst danger assessment method were provided.Finally,a regional-localised measure was proposed and the validity of the assessment results and the effectiveness of the measures are verified.The results of the study have significant guiding meaning in theory and engineering application for the prevention and control of rock burst under gradient loading. |
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