| Thick coal seams with thick and hard roof often produce mining tremor in the process of roof fracture caused by mining.The seismic wave generated by mining tremor may be transmitted to the roadway to induce rockburst disaster.The disaster mechanism of mining tremor-induced rockburst is becoming the key scientific issue in studying the dynamic disasters of deep coal mine roadway.Taking HQH coal mine in Ordos mining area entering deep mining,and the high occurrence of mining tremor induced rockburst as the research object.Comprehensively applying theoretical analysis,numerical simulation,similar simulation methods and field monitoring,this paper studies the preparation law of mine tremor,focal mechanism,source radiation effect,mining tremor seismic wave propagation path,site effect and the dynamic load response characteristics of roadway,reveals the disaster mechanism of mining tremor induced rockburst in gob-side roadway,and puts forward the comprehensive anti-seismic measures of rockburst control,path clipping and roadway reinforcement.The engineering application effect is good.The main research results are as follows:(1)Based on the comprehensive analysis of geological conditions,production technical conditions and rockburst monitoring results,the root causes of roadway rockburst in the mine are studied.The main coal seam of HQH coal mine has large buried depth,high mining height,large roadway section and wide coal pillars left along the goaf,resulting in high static load of roadway surrounding rock.There is thick and hard rock layer in the roof of coal seam.After mining,the fracture produces strong minning tremor impact load,which is transmitted to the surrounding rock of roadway in the form of seismic wave to form dynamic load.The roadway is impacted and damaged under the superposition of dynamic and static load.(2)Based on the measured location data of microseismic in the joint monitoring station above and below the mine,the temporal and spatial distribution law of mining tremor in the mine is expressed,and the propagation characteristics of seismic wave of high-energy mining tremor in high-level roof are compared and analyzed.The results show that:(1)The distribution characteristics of microseismic clusters on the solid coal side of the gob-side roadway have a directional arrangement consistent with the direction of the maximum principal stress,which is reflected in the fissure area of the roadway roof.(2)The formation of microseismic clusters along the front and back of the roadway is symbiotic in time,which is speculated that it is related to the collapse state of the goaf,and the location of microseismic swarms may also be the location of stress arch.The mining tremor with high energy in the microseismic swarms is the result of the “silent period” or continuous active mutations of microseismic events.(3)There is a direct mutual promotion between mining tremor and roof subsidence.(4)The comparative analysis of the received waveforms of the two seismic receivers with the same focal distance and different propagation path media shows that the energy of the received waveforms of the two measuring points is mainly concentrated in the low-frequency band,and the energy carried in the high-frequency part is much greater than that of the received seismic wave in the goaf comparing with the received seismic wave of dense rock mass.This is consistent with the comparison results of the optimization corner frequency and low-frequency amplitude under the waveform displacement spectrum fitted by Brune’s theoretical source spectrum.(3)It reveals the focal mechanism of high-energy mining tremor in HQH coal mine and its regional stress field environment,and analyzes the relationship between focal parameters.The results show that:(1)The main rupture strike of most mining tremors is almost parallel to the longwall face,indicating that the mining tremors are mainly caused by the mining of the gob-side rock stratum.Most tension ruptures are in the vertical direction,there are no strike slip faults,and mainly dip-slip normal faults and reverse faults,which shows that the local horizontal stress near vertical to the strike of the longwall is the main force source of these fractures.(2)The azimuth of the regional stress field inversed from the focal mechanism solution is highly matched with the measured maximum,minimum and intermediate principal stress,and the normal direction of the seismogenic nodal plane is almost parallel to the axis direction of the maximum principal compressive stress.The inversion value results show that the difference between the maximum principal stress and the intermediate principal stress is smaller than field measurements,which is why the non-double couple component accounts for a large proportion in most of the 11 focal mechanisms.(3)There are 3 inversion focal mechanisms,which are the internal contraction type of fracture mechanism of mining tremor dominated by vertical stress.The P-axis of the source ball is close to the vertical direction,and the medium displacement in the P-axis area points to the source.The other inversion focal mechanisms are mainly squeezed by horizontal stress,while the tension fracture mechanism in the T-axis of the source ball is close to the vertical direction,and the medium displacement in the T-axis area deviates from the source.The radiation patterns of P - wave and S - wave of seismic wave of mining tremor are different with different focal mechanisms.(4)The source parameters relationship of 90 mining tremor events is fitted: the corner frequency in the range of 0.0001 ～ 0.1 MPa stress drop is inversely proportional to the seismic moment; the stress drop is positively correlated with the seismic moment and source radius,and the seismic moment is linearly positively correlated with energy.The linear fitting slope of source parameters with stress drop greater than 0.1MPa is smaller.(4)The propagation process of stress wave energy of mining tremor is studied by spectral element method.(1)Four schemes such as no roadway,with roadway,with roadway – “strong – weak – strong” support structure,with roadway – 6 m small coal pillar – goaf are implemented.During the propagation of seismic wave,compared with no roadway,the latter three roadway schemes have different degrees of PPV value amplification effect at different positions of the roadway surface.The maximum value appears in the upper left part of the roadway surface on the far-source side,which is the super-position result of surface and body waves.The large amplification area of PPV value is still on the near-source side of the roadway.Since the weak structure of the“strong-weak-strong” support structure plays the role of “filtering” the locked energy,the PPV value of the roadway surface in scheme 3 is lower than that in scheme 2,but it is not obvious.The function of the weak structure is to form a seismic wave shielding belt below the near-source side,so that the PPV value of the surrounding rock at the upper right of the near-source side of the roadway is significantly lower than that in scheme 2.It can be seen that the weak shielding layer of seismic wave plays a certain role in reducing the energy transmission of mining tremor.In scheme 4,since the seismic seismic wave passes through the goaf with low wave impedance,the energy transmitted to the roadway surface is lower than that in scheme 3,and the propagation speed of seismic wave is significantly slower.(2)The site effect of seismic wave propagation in three-dimensional roadway under specific focal mechanism is further numerically simulated.During the diffusion of seismic wave,there are main source energy radiation directions.Due to the influence of roadway site effect,the PPV values of the roadway surface and shallow surrounding rock of different sections have amplification and attenuation effects.When there is no roadway,the maximum PPV value on the section x= 145 m is transferred to the section x = 106 m.When there is roadway,the amplification and reduction times are 1.9 times and 0.255 times,respectively,and the enlarged or reduced area of PPV is only obvious in the depth of 1 m of the surrounding rock of the roadway.The influence range of site effect is about 10 m around the roadway.PPV in the depth of 1 ～ 10 m is mainly characterized by an amplification effect,and the larger PPV accumulation area is enlarged by 1.2 times.This fully shows that the distance from the source does not fully represent the magnitude of the source energy radiation,and the attenuation curves of the source energy in different directions are also different.During the seismic wave diffusion process,the magnitude and main direction of PPV value are affected by the source radiation effect and site effect.(5)The dynamic and static load numerical calculation software and similar material simulation are used to study the mechanical failure response characteristics of roadway superposition of dynamic and static load.(1)The numerical simulation shows that:During the goaf compaction process,the maximum vertical concentrated stress and energy concentrated area are transferred from the top-side of the coal pillar to the top-side of the roadway exploitation block,and the axial force of the bolt is distributed asymmetrically.In the process of dynamic load,the surrounding rock of the roadway is repeatedly loaded and unloaded under the uneven principal stress difference.The maximum principal stress difference of the diagonal of the roadway on the side of the goaf under the static load is transferred to the diagonal of the roadway on the side of the longwall under the dynamic load.After the dynamic load,the vertical stress forms a pressurization area and a depressurization area on the left and right sides of the roadway.The PPV value of seismic wave on the wave-upstream side is greater than that on the wave-shade side.Moreover,the seismic wave of shallow surrounding rock is reflected and superimposed.The PPV value is basically consistent with the maximum displacement of the shallow surrounding rock after dynamic load.During the dynamic and static load process,the differences of stress field between the mining side and pillar side of the roadway and the difference of dynamic load stress field are the key factors leading to rockburst of the gob-side roadway.(2)Physical similarity simulation test shows that: The acoustic emission of the surrounding rock of the roadway is relatively concentrated when the oil cylinder above the model is 2.2 MPa.At this moment,the released dynamic load from the explosion source kick off timely.The seismic wave attenuates at least 85.1% when the seismic wave propagates 22.5 cm to the measuring point on the top of the roadway.The blasting source is a pure external blasting source only with P-wave,which is compressed in the radial direction of propagation and pulled in the tangential direction of propagation that is comfirmed by the waveform monitoring.Bolt with large preload compared with bolt with small preload in the dynamic load response,the amplitude of the end stress is smaller and the holding time is longer.Bolt with extended anchor compared with end anchor,the amplitude and holding time of the end stress of the extended anchor are smaller.The peak strain of roadway surrounding rock with strong preload support or extended anchor support is smaller under dynamic load.The final failure mode of the roadway is similar to that of the site.(6)Based on the mechanism of roadway rockburst in HQH coal mine,the comprehensive rockburst countermeasures of mining tremor treatment,path clipping and roadway reinforcement are formulated.The prevention and control technologies such as hydraulic fracturing of hard roof,blasting pressure relief of roof and floor,pressure relief of large-diameter borehole in roadway side,controlling the advancing speed of mining face and strengthening roadway surrounding rock are put forward,which have achieved good results in field application. |
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