| For a long time,coal and gas outburst is one kind of coal mine accidents with serious consequences,complex mechanisms,and difficult governance.Coal roadway headings are the main areas where coal and gas outburst accidents occur,and outstanding accidents have not been fundamentally contained.There are three main difficulties:(1)Compared with the mining face,the geological environment and ground stress state of the excavation face are relatively unclear,and prevention and control are difficult;(2)Compared with the mining face,the pressure relief of the coal seam is not sufficient.The stress concentration phenomenon is more significant;(3)The coal seam gas drainage in the driving face is difficult and the danger is more prominent.In coal roadway excavation,the coal in front of the excavation face is in a state of concentrated stress and the deformation is serious.The instability of the coal seam under stress is an important cause of the occurrence of coal and gas outburst.Judging from the experience of the excavation site of the coal mine,when the speed of excavation is high,there is a noticeable abnormal fluctuation of gas emission,and the dynamic phenomenon of the coal seam is also more severe,which means the increased danger.At present,parameters such as coal's sturdiness coefficient,type of coal destruction,amount of drill cuttings,pressure of borehole gas,and amount of gas discharged from the surface are widely used in the identification and prediction of coal and gas outbursts in excavation roadway widely.However,the mechanism of outstanding start-up and various identification and prediction methods is still unclear.The relationship between the dynamic response law of the coal body in front of the working face and various identification and prediction indicators and stress is also need to be researched.Based on the above understanding,the stress of the working face of the excavation is taken as the main research object,and theories of rock mechanics,damage mechanics,molecular dynamics and elasto-plastic mechanics are applied,and the theoretical analysis,experimental research,numerical simulation and field application are combined to study the mechanics of starting coal and gas outbursts.The main research contents and results are as follows:(1)Established a mechanical model for stress distribution under the condition of improved coal body ahead of the working face.Based on the analysis of the advantages and disadvantages of the mechanical model of the stress state of the coal in front of several typical roadway working face,the results show that the stress state of the plastic zone in the driving face is more consistent with the limit equilibrium equation,and the elastic zone can be based on the elastic equilibrium equation.The combination of the two results in a mechanical model that describes the three-dimensional stress state in front of the driving face.Using the stress state solution model,the analysis of the example under different constitutive hypotheses was carried out,and the influence of different mechanical parameters on the stress state of the coal ahead of the driving face was obtained.Under the assumption of ideal elasticity and plasticity,the higher the excavation roadway is,the wider the plastic zone width is.The larger the internal friction angle of coal is,the smaller the plastic zone width of the coal is,and the larger the concentration stress is;the uniaxial compression resistance of coal is higher.The greater the strength,the greater the concentrated stress value of the coal.With the increase of the stress value of the original rock,both the concentrated stress value and the plastic zone width of the coal are significantly increased.Under the assumption of ideal brittle plasticity,the smaller the residual strength of the coal in the plastic zone,the greater the width of the plastic zone,and the residual strength has little influence on the concentration of stress;the calculated plastic zone width can reach a height of 2-5 times,the stress concentration factor can reach 2 or more,which is more consistent with the results of the assumption of the ideal elasto-plasticity and the on-site monitoring data;the brittle fracture characteristics of the coal body is an important reason for the instability of the coal seam on the face of the face.(2)The calculation method of stress state considering the volume expansion of coal and strain softening is proposed.Based on the analysis of the strain-softening path and the deformation mechanism of coal rock,the stress state solution model considering the coal body expansion and strain softening was established by combining the respective equilibrium conditions of plastic zone and elastic zone of the coal body ahead of the driving face.The model is used to calculate the stress,strain and displacement of the elastic and plastic zones.With coal in the process of loading the dilatancy gradient and plastic strain ratio,elastic strain softening modulus analysis and the deformation and destruction of the "three segment" model,established the heading face of coal body in front of the dilatancy and strain softening deformation of coal seam based on stress distribution calculation model.Finally,according to the model,the numerical expressions of the stress state and the numerical solution of the stress state based on the finite difference are described.Numerical results show that the strain softening parameter increases,driving in front of the plastic zone and fracture zone width is smaller,more stable coal seam;the expansion coefficient is,the more significant coal deformation,plastic zone and fracture zone width is bigger;the softening modulus as a comprehensive index,the greater the value of coal is more prone to smash the destruction,the danger of coal rock dynamic disasters in the greater.(3)Developed a constant-thrust coal seam excavation stress response simulation experiment systemThe permanent thrust driving power system is designed with the rig,guide rails,sliders,pulleys and counterweights as the main components.Using expansive agent as stress source,different amount of expansive agent was added at different positions to simulate the uneven coal bed stress conditions,and similar simulation system for coal road excavation was established.The experimental system can complete the determination of state parameters such as drill cuttings,tunneling speed,rig output power,and coal seam stress change rule during the simulation of coal seam driving under different stresses.Through the analysis of the main influencing parameters of the on-site excavation work,different coal seam stresses,different pressure relief times,and different drilling thrust test plans were designed.(4)The dynamic response law of the coal seam stress in the simulated tunneling process is revealed through the experimental study.The stress signal collected from the experiment is first processed to remove the drift value.After the HHT algorithm is used,the signal can be decomposed into IMF components with different frequency bands.The IMF component with lower frequency can be reconstructed to be able to characterize the pressure relief trend during the simulation experiment of coal excavation.The low-frequency stress component and highfrequency signal part using wavelet threshold denoising can reflect the fluctuation characteristics of the stress during coal seam excavation work.During the process of removing the coal wall by removing the movable baffle,the stress of the coal near the exposed coal wall rapidly falls,and the stress of the coal in the middle of the experimental box is significantly increased due to the stress concentration.The coal bodies farther away are basically under the influence of the original stress and are relatively less affected.The experimental results are consistent with the stress distribution law in the coal seam on the driving surface.During the coal seam excavation simulation experiment,the internal stress of the coal body as a whole showed a downward trend.After the completion of a single excavation simulation experiment,the pressure relief characteristics of the coal seam are more obvious.The construction of pressure relief drilling in the excavation roadway is conducive to the release of high stress in the coal seam.Coal seam stress fluctuations and coal seam excavation work procedures can be very good correspondence,according to the order of the signals received by different sensors can calculate the stress fluctuations in the experimental coal transfer rate,the results show that the stress in the coal pressure formed by the soft coal seam The speed of the transfer is much less than the speed of propagation in the raw coal.(5)The relationship between coal seam excavation parameters and coal seam stress is revealed through experimental research.Under the constant thrust,with the increase of the depth of simulated driving,the driving speed and the energy consumption of the drilling rig increase significantly.It is difficult to dig and the amount of cuttings discharged during the excavation is relatively small.This shows that there is a significant stress concentration in front of the excavation roadway.In addition,the characteristics of the excavation state such as the amount of cuttings and the speed of excavation in the simulated excavation process can reflect the internal stress state of the coal seam to some extent.(6)The improved random growth algorithm is proposed to construct the pore structure of the coal seam under the influence of the stress of the heading face.Based on the study of the characteristics of pore fracture structure of coal,the fourparameter generation method of random growth was used to complete the structure of the primary coal structure.The constructed coal body can well characterize the random features of the pore fracture distribution.On the basis of summarizing previous studies on the relationship between stress and permeability,an empirical formula for the relationship between strain,permeability,and porosity was proposed.Combined with the distribution law of the tunneling surface stress,the law of the porosity distribution of the coal on the road surface is given.The fractured coal body is severely broken and the development degree of the pore fissures is the highest.The porosity of the coal seam is the largest,and the porosity decreases gradually with the increase of stress in the plastic softening zone.In the concentrated stress zone,the closed porosity of the original pores of the coal seam is minimized,and the porosity of the coal body in the elastic zone gradually rises to the original porosity.An improved coal fracture structure growth algorithm was proposed based on the stochastic growth four-parameter generation method by adding parameters controlling the structure porosity distribution,pore fracture connectivity and growth direction.The improved stochastic growth algorithm can well construct the coal structure under different stress distribution ahead of the driving face.(7)The distribution characteristics of the gas field in the driving face are obtained through the numerical simulation of LBM.The coal pore structure constructed by the improved stochastic growth algorithm is the boundary of the flow field.The transport characteristics of gas in the coal seam are simulated using the LBM numerical simulation method.The simulation results show that the gas field under the natural drainage of non-bored coal wall has a very small range of influence,gas pressure is only reduced in the rupture zone(pressure relief zone),and the gas in the stress concentration zone and the coal seam behind it has little change.Drilling has a great influence on gas emissions.Whether or not there are high-quality gas channels in the concentrated stress area is a key factor affecting gas emissions.(8)The dynamic evolution law of coal seam stress in the course of tunnel excavation is revealed.On the basis of the calculation of the stress state of the heading face,the evolution law of dynamic stress field of the working face during the heading of coal roadway is studied.Tunnel excavation will lead to higher stress concentration in coal heading face on the front.At great speed in the course of digging,the ultimate stress zone width will decrease,the peak stress increases,and resist stress release in coal body limit equilibrium zone is less and less,the danger of coal and rock dynamic the increase of disaster.According to the theoretical analysis results of dynamic stress field,numerical solution can simulate the variation rule of stress distribution with tunneling time in the process of roadway excavation,and the simulation results coincide with the field experience.(9)The energy index for evaluating the hazard of coal rock dynamic disaster in roadways is put forward,and the calculation method is given.When the tunneling speed is greater than the stress advancement speed,the coal body stress in the elastic zone rapidly increases,accumulating a large amount of deformation potential.Deformation potential energy release and roof subsidence work on the coal body is the main cause of coal body rupture in the concentrated stress area.The amount of accumulated energy and the rate of energy release can be used as indicators for evaluating the risk of coal-fired dynamic disasters.According to the law of accumulated energy and the analysis of the calculation formula,it can be known that the smaller the original rock stress,the smaller the deformation energy caused by excavation and the lower the risk;the smaller the driving speed,the less the unstable energy.The more stable,the smaller the roof subsidence,the smaller the deformation energy of coal accumulation and the less likely the occurrence of dynamic phenomena in the coal seam;the greater the speed of stress advancement,the farther the distance from the concentrated stress area to the driving surface becomes.The stronger the resistance against the driving direction is;the greater the friction coefficient,the slower the release of energy during the unbalanced state and the more stable the coal seam.(10)A method for determining the reasonable driving speed of on-site roadway is proposed.During the excavation operation,the closer the concentrated stress area is to the excavation head,the narrower the rupture zone and the lower the porosity,the smaller the gas emission volume.As the stress relief zone has an increased porosity in the extended rupture zone and increased pressure relief,the gas emission from adjacent shifts will increase significantly.The stress evolution in the process of roadway excavation will cause the fissure field in the coal seam to change,and then affect the gas emission from the coal wall in the heading.The stress state of the coal in front of the heading faces can be inferred from the characteristics of the change of gas emission from the coal roadway,and the reasonable speed of the roadway can be determined. |
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