| Coal bump is a kind of dynamic disaster which is induced by mining with underground coal and rock suddenly shocked and pressed out, and the deformation energy of coal and rock will release rapidly. Amounts of coal bumps in China becomes more than ever since the mining depth and strength are increasing and the geological structures in the underground are more complex. The development and happen of coal bumps are in special geological structures and stratum parameters, and it is a consequence of the steady accumulation of energy and it's non-steady release during the homeostasis of mining. So it is a composite result of the physical property of coal and rock, geological condition and mining technique. Meanwhile, it has a temporariness evolution characteristics.Typical coal bumps in Datong coal field generally happened under a hard structure of the combination of coal, roof and floor. In multi-coal seam mining conditions, many coal mines have experienced varies coal bumps happened in shallow mining depth. Under this circumstances, gravity is no longer the determining factor, while the joint of mining stress and tectonic stress joint play a important role for the occurrence of this kind of coal bump.Based on the combination structure of coal and rock and complex geological condition in Datong coal field, hard combination structure of coal and rock and mining stress are chosen as two fundamental factors that lead to coal bumps. In considering the impact of combination structure characteristics of "roof-coal", interaction mechanisms of the physical and mechanical properties of combinations of coal and rock, combination style and mining stress in the dangerous coal bump zone were explored, rules of the stress shift and concentration and accumulation of energy under hard coal, roof and floor in Datong were studied, and comprehensive prevention techniques were discussed. This work mainly contains the following aspects:1. Features of coal bumps in Datong coal field were investigated and analyzed. After investigation and analysis of the coal and rock properties, geological conditions and mining conditions, the effects of different influencing factors for coal bumps were discussed. Combined with coal bump cases, the mechanisms of coal bump under different combinations of factors were analyzed. Mining layout, the hard roof caving, the changes of mining stress, original coal seam geological conditions and geological structures determine the behaviors of coal bumps in Datong coal field together. With the going of the driving of mining activities and prevention practice, nearly 84% of the coal bumps in the newer work faces often happened at the roadways near the gob areas. The accumulation of bending energy in thick and hard roof, significantly brittle shells of hard coal, mining frequently and stress concentration caused by uneven distribution of additional mining stress were considered as the main factors for coal bumps in Datong. While small normal faults with steep dip was not the main factor.2. Considering the combination structures of "roof- coal" for coal bump, uniaxial compression tests with standard combination samples with the size of ?50mm×100mm were taken with the loading speed from 0.006mm/min to 0.12mm/min. By analyzing stress and strain development process and acoustic emission response characteristics under different loading conditions, three basic damage modes and possible mutations conditions were got:(1) When the loading rate were 0.006mm/min and 0.012mm/min, plastic deformation of coal fully developed, roof rock deformation was relatively slow and released uniformly, through of fractures at the peak slip eventually lead to the sample failure. When the loading rate were 0.03mm/min and 0.04mm/min, the plastic cracks are not fully developed in the before peak stage, stress in the coal was in constant adjustment process in the end of the before peak stage and throughout the post-peak phase, coal and rock deformation and damage showed step-wise relaxation and released elastic energy until the structure of layers of crack's final instability. When the loading rate were 0.06mm/min and 0.12mm/min, high loading rate leads to the stress in the coal can't adjust slowly, rock deformation released in the final destruction of the coal and suddenly to brittle failure under the influence of culminating in the upper rock samples and coal energy releasing rapidly.(2) Energy release rate performance for uniform growth and transient spurt stage. damage along the sample crack made the strength of the sample significantly low, and its ultimately destroy was relatively calm. When the loading rate is large, the increase of the cumulative energy rate was relatively slow, and in the peak position the stress can't adjust slowly so there comes stress dips and brittle failure. 50 KHz and 150 KHz were the obvious frequency distribution in the samples, and they were correspond to the main frequency range of the coal and rock fractured respectively.(3) The deformation and failure of the sample have obvious development process, the 2 # surface damage of coal sample is not apparent, eventually damaged along the through crack;And 3 #, 4 # sample develop surface splitting before damage, and coal fracture development led to the loss of bearing capacity of the final structure; 5# sample under larger loading rate develop surface tension damage, with internal bearing structure, eventually suddenly instability as a whole happens. According to the damage characteristics of composite sample, and model of brittle characteristics of the coal-rock combination with damage and mutation is proposed.The three forms of instability of the combination with different energy release intensity are gradual damage, fracturing transfixion crack and surface slip.3. Under different loading conditions, the slip friction characteristics of "sandstone-coal" composite structure were tested, and considering convergence effect under the condition of hard roof and floor, slip coal bumps model was set up. Under the influence of different factors, coal slip danger and overall coal bumps model were analyzed.(1) According to the characters of the evolution of shear stress and displacement of monitoring points and the phase change of acoustic emission characteristic value of composite sample, slip friction could be divided into compression storage stage, slip starting stage, whole slip stage. Slip displacement of the development and improvement of shear stress and acoustic emission parameters were consistent with the change of each stage, and slip starting process was with particle breakage in interface and the release of elastic energy, along with the rapid growth of accumulative release energy and acoustic emission peak amplitude was at this stage. 5# sample slipped in the start phase and appeared obvious stick-slip instability. "sandstone and coal" stable friction coefficient was about 0.36, while the friction coefficient of stick-slip instability was only 0.11, high axial compression under the condition of stick-slip instability risk are dangerous factors of coal bumps.(2) For the two influence factors of coal bumps hazard, roof caving and strength distribution of coal seam, this paper discussed the different plastic zone distribution and high stress state under different condition of parameters and vertical stress. The plastic zone length changed with mining depth were 0.5 m, 1.5 m, 2.4 m, 2.8 m, the peak stress also gradually changed from 11.2 MPa to 11.2 MPa, the disturbed zone length also increased. As the strength weakening, the plastic zone length 1.5 m, 1.8 m, 2.3 m, 2.9 m, the peak stress from 21.5 MPa gradually reduced to to 14.1 MPa, with disturbance zone length unchanged. With the increase of the lateral pressure coefficient, the plastic zone and the disturbance zone length range increased linearly.(3) The chain coal bumps of floor impact of roadway was simulated, after the coal was prominent weakened, the vertical pressure relief lead to a 300 mm increase of floor convergence of roadway, coal seam and roof and floor were all influenced in coal bumps, and coal seam slip and floor impact were prominent coal bumps forms, while thick layer of hard roof generally does not occur damage.4. In view of the two influence factors of coal bumps hazard: roof caving and the distribution of coal seam strength, practical method for coal bumps hazard assessment in the process of working face mining under the condition of different combination of coal and rock structure were studied, and through the division of dangerous area, simulation analysis of the two kinds of prevention methods were compared in effect and applicability:(1) Along with the increase of lateral pressure coefficient, coal stress levels increased, the coal pillar and working face coal body in front of the scope of the plastic zone along with the increase of confining pressure decreased, but the stress level was increased, and the units amount of high stress range, the peak stress increased with basic trend.(2) Under the condition of the roof caving does not reach the designated quality, the stress of coal working face side significantly increased, and by extrusion influence, coal pillars stored more elastic energy, which increased the stress level of coal body in front of the working face, and the position of stress peak became nearer to the shallow position of coal wall.(3) With the increase of the adjacent cavity hanging plate width, the stress level of coal, the range of plastic zone in coal pillars and working face in front both decreased, but the stress level increased, the amount of high stress units area decreased, but the elastic region of central pillar under higher clamping accumulated a lot of energy, increasing the roadway side to drift out of coal pillar and potential lane for coal slip instability risk.(4) With the coal strength weakening, the plastic zone increased significantly, the peak stress reduced and moved to deep in coal seam. Under the effect of different factors combination form, stress distribution and energy distribution state directly determines the high stress distribution of plastic zone.5. Hard roof caving characteristics over time of Datong mining area determined the development process of coal destruction, which includes both above the working face of hard roof caving, and panel as the scale which develops more slowly and the release being more severe overall caving, this process was proved by means of monitoring and early warning.6. Dividing the dangerous zone reasonably and accurate preliminary classification are the basis of the implementation of monitoring and early warning. Hazard degree along with mining development, in the time when the "square" and the end stage of mining are when special attention should be paid to, and on the space features mainly divided the three areas:(1) mining stress concentration area;(2) coal seam occurrence anomaly area;(3) the area influenced by tectonic structure. Mining pressure observation method, drilling cuttings method, seismic and electromagnetic radiation monitoring measures formed the coal bumps monitoring and early warning system of Datong mining area. Rational support mining method and optimization design were the basis of prevention system, and the powerful artificial caving was the key of keeping Datong mining area from coal bumps hazard, and weakening of coal seam linked the overall control system. By effect examining and selection of effective and feasible measures, initially formed a dynamic system for the prevention and control of coal bumps of Datong mining area. |
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