Splitting Mechanism And Influencing Factors Of The Cumulative Blasting

The geological structure of coal field in China is complex.With the continuous increase of mining depth,the coal seams show the characteristics of high gas and low permeability,and the disaster of coal and gas outburst intensifies in the process of coal mining.What is known to us,gas drainage is one of the main measures to prevent and control mine gas disasters.How to effectively improve the permeability of coal seam is an urgent problem to be solved.The technology of cumulative blasting has been applied to improve the coal seam permeability,and has achieved good results.The current theoretical research on cumulative blasting is far from satisfying its engineering application.In view of this,theoretical analysis,numerical simulation,indoor model experience and field experience are used to systematically study the mechanism and influencing factors of cumulative blasting.The main research contents are as follows:（1）The mechanism of cumulative blasting is analyzed,and the comparative experiments of concrete cracking caused by cumulative blasting and conventional blasting are designed.The macroscopic comparison showed that the crushed zone of concrete formed after comparative blasting is smaller,while the crack zone is larger.It is found that the compressive strain of the strain gauge in the energy accumulation direction is the largest,the compressive strain of the strain gauge in the vertical energy accumulation direction is the middle,and the compressive strain of the strain gauge in the diagonal line is the smallest.At the same time,it is found that the strain peak value of the strain gauge in the direction of cumulative blasting is much larger than that of the conventional blasting strain gauge,the strain peak value of the strain gauge in the vertical direction of shaped charge blasting is basically equal to that of the conventional blasting strain gauge,and the strain peak value of the strain gauge at the diagonal line is less than that of the conventional blasting strain gauge.Finally,ANSYS/LS-DYNA is used to reproduce the whole process of the formation,migration and penetration of shaped charge jet into concrete.After cumulative blasting,the concrete forms a crushing area with"fat at both ends and thin in the middle",while the fracture area is"long in the middle and short at both ends".The crushing area is smaller than that of conventional blasting,and the fracture area is denser than that of conventional blasting,which is the same as the experimental conclusion.Based on the above analysis,it is found that cumulative blasting can effectively solve the problem of serious coal crushing and insufficient fracture expansion in conventional blasting.（2）By changing the size of the air area in the blast hole,radial decoupling charge directly affects the size of the crushing zone and fracture zone of coal after cumulative blasting.If the radial decoupling coefficient is too small,the crushing area of coal will be too large,and the fracture area of coal will decrease.If the radial decoupling coefficient is too large,the crushing area of coal will decrease,but the fracture area of coal will also be small.Using ANSYS/LS-DYNA,the formation and migration process of shaped charge jets with five radial decoupling coefficients are compared and analyzed,and the propagation law of explosion stress wave with the change of decoupling coefficients is obtained.When the radial decoupling coefficient is moderate,that is,the decoupling coefficient is 1.67 and 2,the reflection improvement effect of cumulative blasting is better.At the same time,the concrete model specimens are poured to further analyze the effect of radial decoupling coefficient on the cracking of cumulative blasting.It is found that the crushing zone of concrete is the largest and the crack area is small under the coupling charge.When the radial decoupling coefficient is 3,there is no obvious macro crack on the surface of concrete specimen after blasting.When the radial coupling coefficient is 1.67,the crack area is larger and the cracking effect is better.The concrete model experiment and numerical simulation prove that too small or too large radial decoupling coefficient can’t get good cracking effect.（3）The effect of joint fillings on the dynamic fracture behavior of PMMA was studied by dynamic caustics experiment.Firstly,the effect of filling medium on crack propagation under dynamic load is analyzed.Caustics experiment shows that filling medium has obvious effect on crack propagation.The existence of filling medium not only changes the crack propagation trajectory,but also increases the overall penetration time of the specimen.On this basis,the effect of filling thickness on crack propagation was further studied.It indicates that the thicker the joint fill,the more difficult the horizontal crack initiation.The overall penetration time of the specimen increases with the increase of the joint fill thickness.At the end of the first propagation of the main crack B,the thicker the filling material of the specimen,the larger the K_II/K_Iat the crack tip of the specimen.It indicates that the greater the effect of KII during the first propagation of the crack B,that is,the fracture point is closer to the impact load point.（4）Crack propagation and coalescence mechanism of multi-hole cumulative blasting in coal seam.A model of multi-hole cumulative blasting was established by using ANSYS/LS-DYNA to study the mechanism of crack propagation and permeability increasing under the multi-hole cumulative blasting in coal seam.The model represents all of the explosion process,including the formation of shaped jet,the propagation and superposition of explosive stress waves.The variation of coal element stress with time under different holes spacing is analyzed.The results show that with the increase of the spacing between adjacent holes,the residual stress produced by the stress wave at the adjacent holes decreases,resulting in the stress intensity after superposition becoming smaller and smaller,and the larger the spacing between adjacent holes,the longer the time required for the superposition of stress waves.At the same time,it is found that there are two stress peaks in the unit stress of coal between adjacent holes,which is equivalent to that the coal here is subjected to two blasting actions.When the stress wave propagates to the coal body in a certain area,a crack will be formed.When the stress wave generated by the adjacent blast hole passes through the crack,under the reflection and refraction of the stress wave,the coal body will be further cracked,which is equivalent to the second blasting.At the same time,the field test shows that the multi-hole cumulative blasting can effectively improve the coal seam cracking effect and coal mining efficiency under the condition of reasonable adjacent blasting spacing.