| Rock bursts always happen abruptly,are difficult to forecast,and wreak massive damage,so they are among the typical coal and rock dynamic disasters,seriously influencing the normal and safe production of coal mines.Subvertical coal seams represent a special occurrence state under intense crustal movement,for which the coal seams and roof and floor strata generally have high strength and large thickness,so a lot of elastic energy is likely to accumulate there.Under such condition,strata behaviors even occur during mining in shallow parts.In recent years,numerous severe rock burst disasters have happened in coal mines of subvertical extra-thick coal seams,seriously threatening safety of underground personnel.Therefore,studying the mechanical behind rock bursts in subvertical extra-thick coal seams and corresponding early-warning and prevention measures is a major safety related issue that needs prompt solutions in coal mines under similar geological conditions.To solve the problem,the research used methods including numerical simulation,theoretical computation,laboratory tests,and engineering practice to investigate the mechanism underlying rock bursts during combined mining of subvertical extra-thick coal seams.The main conclusions and achievements of this paper are as follows:1)The characteristics of caving and impact breaking of overlying strata during combined mining of subvertical extra-thick coal seams were analyzed.Besides,physical similar simulation tests on the combined stoping of subvertical extra-thick coal seams at horizontal sublevels were carried out.In this way,the breaking characteristics of the roof,stress change law,and acoustic emission(AE)response characteristics of coal and rock to damage were obtained.The stress distribution of rock pillar is an obvious cantilever beam stress state.With the increase of the mining depth,the value and range of stress concentration of rock pillar increase continuously,and reach the peak value before the fracture.Each sublevel mining of upper and lower coal seams has certain damage effect on rock pillar,and the damage position of rock pillar is consistent with the sublevel mining position of upper and lower coal seams.Before impact failure of rock pillar,stress concentration occurs in the vicinity of the breaking position,accompanying with release of microfracturing energy.During breaking of rock strata,violent impact shock occurs and triggers large-scale damage of coal and rock structures in the coal mine.2)The evolution laws of stress and displacement of the coal and rock strata during combined mining of subvertical extra-thick coal seams were studied.By using the3DEC numerical simulation software,3D models for combined mining of subvertical extra-thick coal seams with dip angles of 65°,70°,75°,80°,and 85°were established.The stress evolution of coal and rock stratas and the rule of overburden stratas fracture in the process of combined mining of subvertical extra-thick coal seams are simulated.Simulation results indicate that the caving range of roof is expanding with the increase of mining depth in the process of mining in upper coal seams.The stress variation of roof strata of the upper coal seams is mainly caused by the mining of the upper coal group,while the stress variation of the middle rock pillar is mainly caused by the mining of the lower coal seams.The fracture of roof stratas of upper coal seams and intermediate rock pillar is caused by local stress reaching tensile strength.However,the maximum compressive stress reached more than 80%of the compressive strength before the roof stratas and rock pillar were broken.The variation of coal seams'stress is shown as the vertical stress of lower coal seams is released after mining in upper coal seams,and the increase of horizontal stress in mining section and under section is mainly caused by mining action.With the sublevel mining,coal stress near the working face of upper and lower coal seams increases,and the inclined range of stress rising is mining section and lower section(i.e.25 m above and below the working face).The trend range of increased stress in the upper coal seams is 80 m?115 m in front of the working face,and the trend range of increased stress in the lower coal seams is 70m?95m in front of the working face.3)The mechanism behind impact breaking and rock bursts of rock pillar during the combined mining of subvertical extra-thick coal seams was revealed.Large areas of rock pillar between coal seams are suspended,and rock pillar themselves form stress concentrated zones in the vicinity of the sublevel under mining,which separately pry and extrude the upper and the lower coal seams.As a result,the horizontal stress on the coal is increased.As the coal is under high stress,the high-energy impact shock occurring during breaking of the rock pillar triggers rock bursts of surrounding rocks of the mining space in the upper and the lower coal seams.Based on the elastic theory,the fracture condition and elastic energy calculation formula of rock pillar are derived.The greater the inclination angle?of the rock pillar,the longer the strike length of the breaking range b of the rock pillar will be,and the longer the breaking time will be.The impact elastic energy V_?released when the rock pillar is broken decreases with the increase of inclination angle?The breaking of rock pillar occurred during the mining of the third and fourth sections.When mining in the third section(a=75 m),the broken energy value of the rock pillar is 1.027×10~9 J?1.385×10~9 J.When mining in the fourth section(a=100 m),the broken energy value of the rock pillar is 4.698×10~9 J?6.645×10~9 J.The broken energy value of the rock pillar above is greater than 1×10~9 J.Therefore,breaking of rock pillar between coal seams is the uppermost inducement triggering serious rock bursts in coal mines.4)According to the law of AE energy release during the loading process of coal and rock mass,the prediction model of the energy release law of coal and rock mass is established.Through mechanical loading tests and AE monitoring of coal and rock specimens,the corresponding relationship between the loading stress and AE energy release law of coal and rock specimens is attained.According to the theory of particle mechanics,the"strength-volume density"of particle element of coal and rock mass is determined,and the prediction model of energy release regularity of coal and rock mass is established.Based on AE energy monitoring data and combined with the prediction model of energy release law,the energy release law of coal and rock mass is studied quantitatively.The results show that the impact energy of coal and rock is not only related to macroscopic mechanical properties of materials,but also to the strength distribution characteristics of mass units in materials.Besides,the energy release law prediction model was used to predict the energy release law of coal and rock composite specimens,and the rationality of the prediction model was verified by comparing the measured AE energy of coal and rock composite specimens.The prediction model of the energy release law of coal and rock mass establishes the quantitative correspondence of“the loading stress-the fracture-the fracture energy”,which revealing the dynamic evolution mechanism between the fracture energy and the load of coal and rock mass,and providing a theoretical basis for the formulation of the rock burst warning method for combined mining of subvertical extra-thick coal seams.5)A warning method for rock burst in combined mining of subvertical extra-thick coal seams was proposed,which includes the monitoring and warning of regional rock burst microseismic and the monitoring and warning of local rock burst geoacoustic in front of the working face.The relationship between the energy release rate of rock mass and the loading stress is analyzed.Compared with the loading stress,the energy release rate increases more significantly on the eve of rock mass impact failure,and the failure warning features are more obvious.Therefore,the stress and failure states of coal and rock can be judged and impact failure can be early-warned according to the energy release rate during loading.In accordance with the idea,the relationship between the energy release rate from coal and rock and the stress was established using the calculation model for energy release law before rock failure,and the warning values of the impact energy of coal seams and rock stratas are calculated respectively.As to the local impact to the coal seam in front of the working face under the mining effect,the frequency of high-energy earth-sound signals in coal before impact failure was calculated by analyzing characteristics of underground sound signals during local impact failure of coal.Furthermore,the warning conditions for rock burst in underground sound monitoring were determined.6)The prevention and control technology of rock burst in combined mining of subvertical extra-thick coal seams is put forward.The prevention technology includes deep hole blasting precrack technology of rock seams and water injection weakening technology of coal seams.By taking deep hole blasting precrack measures to prevent rock bursts in the rock strata,a fractured zone can be formed in the strata,so as to eliminate the stress concentration and reduce influences of dynamic impact loads on the roadway and working face.However,because the hard rock stratum(especially the middle rock pillar of coal seams)plays a key role in the stability of overburden strata structure,the blasting precrack measures should not destroy the supporting function of the the excavation space's hard rock stratums.Based on the above considerations,deep hole blasting precrack schemes for the roof of B3-6 upper coal seams and the middle rock pillar are designed respectively.According to the calculation formula of the bursting elastic energy of the middle rock pillar in the coal group,the bursting elastic energy of the rock pillar after the implementation of the precracking scheme is checked.The results show that the impact-elastic energy released when the rock pillar is broken is 2.616×10~5 J,and the precracking measures can effectively reduce the energy accumulation of the rock pilla.The coal seams water injection scheme also needs to reduce the intensity and stress concentration area of coal seams to avoid impact damage and ensure the stability of surrounding rock of coal mining face and coal seam roadway.For this reason,the weakening scheme of local water injection of coal seams is formulated,and the weakening range of water injection is the upper part of mining subsection.The above rockburst prevention and control technical scheme effectively reduces the stress concentration and energy accumulation of coal and rock mass,so that the monitoring values of microseismic energy and underground sound energy are always at a low level.The roadway surrounding rock integrity is good,and the roadway deformation does not change significantly.Therefore,deep hole blasting precrack technology of rock seams and water injection weakening technology of coal seams reduce the risk of rock rock rock burst and ensure the stability of the excavation space structure.There are 112 figures,16 tables,and 167 references in this dissertation. |
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