Research On Dynamic Fracture Process And Rockburst Mechanism Of Cracked Coal Rock

Rockburst is a complex coal and rock dynamic disaster,which has a serious threat to the safe and efficient mining activities.With the increase of mining depth and strength,rockburst becomes more and more serious.Coal rock contains different types and sizes of structural surface,and its bucking failure is the result of crack propagation and coalescence.Based on this,the paper focuses on the dynamic rupture behavior and rockburst mechanism of cracked coal rock,through experimental test,theoretical analysis,numerical verification and field test to study the dynamic buckling failure characteristics and criterion of cracked coal rock.In the paper,we established the rockburst model of coal rock with structural surface,revealed the effect of resonance effect and microseisms(MS)wave disturbance to the rockburst and layered crack structure of coal rock masses,and obtained the dynamic characteristics of energy field and MS waves in the rockburst evolution process.The main research achievements are as follows:We carried out the uniaxial compression test to study the mechanical characteristics of the cracked coal rock.Results show that with the increase of the prefabricated crack angle,the stress peak,elastic modulus and strain energy decrease first and then increase.With the increase of the prefabricated hole diameter,the stress peak,elastic modulus and strain energy decrease gradually,while the decreasing rate decreases.The crack initiation starts at the hole periphery while the prefabricated crack angle is small,and it starts at the crack tip while the angle is large.The macroscopic failure occurs at the crack coalescence of the crack secondary initiation.In the paper,based on elastoplastic mechanics and damage mechanics,a statistical constitutive model,which takes into account the closure effect of the fissure compaction is established.The model reveals the crack initiation criterion,and the time-varying evolutionary characteristics of the normal and shear stress.The cumulative acoustic emission(AE)energy and strain energy released by the cracked coal rock in the uniaxial loading process conform to the by(28)ax power function relationship.While the coal rock closes to buckling failure,the long-range correlation and multifractal parameters of the AE enhance.And the frequency of the AE waves decreases,the amplitude increases,the wavelet packet energy increases in the low frequency bands,the low frequency IMF components energy and Hilbert instantaneous energy increase too.Based on the dynamic rupture behavior of cracked coal rock and the nonlinear characteristics of the AE parameters,the self-organized disturbance fracture mechanism of the coal rock solid-body was put forward,and the hypothesis is verified by the dynamic fracture characteristics and AE parameters of the cracked coal rock.In the paper,we established the elastic-plastic-brittle rock burst model of coal rock with structural surface,introduced the concept of volume energy potential function,researched on the relationship between energy accumulation and dissipation during the coal rock dynamic deformation and rupture process,so that obtained the rockburst energy condition.Besides,the “volume element-region-system” model of rockburst was established,which could explain the rockburst evolution process very well.In the external disturbance or high stress state,the coal rock system volume element is destroyed and it releases energy to the adjacent volume element quickly,while the adjacent volume element could not absorb this part energy,it is destroyed and the energy released quickly.So that forms the “chain” reaction and the local coal rock is unstable.Eventually,the entire coal rock system is unstable and rockburst happens.Based on the structural dynamics theory,we analyzed the influence of MS wave on the instability of coal rock with structural surface in the resonance effect aspect.It is found that the magnification is symmetrical distribution with the frequency ratio,and increases in power function with the damping ratio decreases.The failure model of MS wave disturbance to the coal rock was established.It is found that the bigger the disturbance energy,the more coal rock masses involved in the rockburst,the larger the damage range;the smaller the disturbance energy,the less coal rock masses involved in the rockburst,the smaller the damage range.We studied on the dynamic evolution characteristics of energy field and MS waves during the rockburst evolution process.Results show that t energy field is in the dynamic adjustment state,and moves to the MS source area.Coal rock rupture changes from the small cascade to large cascade while closes to the rockburst.Statistics fluctuate and the released energy become bigger,the amplitude and low frequency components of the MS waves increase,and the main frequency reduces.Besides,the similarity and correlation of the waves become bigger,and the wave energy accumulates in the low frequency.The research results have important theoretical significance and application value for deep understanding the mechanism and precursory law of rockburst,so as to carry out more effective monitoring methods to prevent and cure the coal rock dynamic disasters like rockburst.