Splitting Mechanism Of Deep-Hole Cumulative Blasting With Linear-Shaped Charge In Coal Seam

Coal-seam-gas drainage is one of the key measures to prevent coal and gas outburst,gas explosions and other coal mine gas disaster accidents.As the depth of coal mining gradually extends deeper,the coal seam with high gas content and low gas permeability is increasing,the difficulty of gas drainage is increased,and the tendency of coal and gas outburst is increased.The deep-hole cumulative blasting technology has unique advantages in coal seam modification and improvement of coal seam permeability.It is of great scientific significance and application value to carry out research on cracking mechanism of deep-hole cumulative blasting with linear-shaped charge in coal seams.Hence,based on the study of the failure characteristics of coal samples or rock samples under impact loads,this dissertation systematically studies the mechanism of deep-hole cumulative blasting and the laws of coal seam crack propagation by theoretical analysis,engineering experiments,laboratory experiments and numerical simulation experiments.The finite element numerical(FEM)analysis software LS-DYNA was used in numerical simulation experiments.The study were mainly carried out in No.10 coal mine of Pingdingshan Tianan Coal Mine Co.Ltd.,by means of explosion mechanics,explosive detonation dynamics,impact dynamics,fracture mechanics,damage mechanics and coal mine gas prevention and utilization.The main research work carried out by the project is as follows:(1)Dynamic mechanical efects of deep-hole cumulative blasting in coal seam.This section is concerned with the splitting mechanism of cumulative blasting using a linear-shaped charge in a coal seam.A model of cumulative blasting was established to study the mechanism of crack propagation and permeability increasing under cumulative blasting in a coal seam.The forming mechanism of shaped charge jet,the propagation characteristics of stress wave and detonation wave,the stress distribution characteristics of coal elements,and the mechanism of crack extension under cumulative blasting were investigated by theoretical analysis and numerical simulation.It is found that the propagation characteristics of detonation wave promote the forming of cumulative jet,and change the propagation characteristics of stress wave and the stress distribution characteristics of coal elements significantly.The stress peak values of coal in the major and minor cumulative directions are 1.10～1.29 times that of coal in the non-cumulative direction,and the stress peak value of coal in the major cumulative direction increases from 0.85 times that of coal in the minor cumulative direction to 1.06 times in the range of blasting effect,slowing down the attenuation rate of the stress peak value of coal and increasing the crack propagation velocity.Besides,field experimental results of cumulative blasting in a coal seam indicate that the average concentration of extracted gas after blasting in two different areas are 1.58 and 2.25 times that before blasting,respectively,showing that cumulative blasting can promote crack propagation effectively.(2)The partition characteristics and effective fracture zone under deep-hole cumulative blasting in coal seam.This section is concerned with the range of the effective fracture zone under cumulative blasting using a linear-shaped charge in a coal seam.Based on the analysis of the mechanism of the directional cumulative-blasting in coal seams,the response characteristics of the coal under the coupled effects of the blasting-induced shock wave,stress wave,detonation gas and the energy-cumulative effect,and the partition characteristics of the crack in the cumulative-blasting-affected area were studied by theoretical analysis;moreover,a numerical analysis model of cumulative blasting was established,and the propagation distribution characteristics and range of coal seam fracture under cumulative blasting were investigated through numerical simulation.The results of the theoretical analysis and simulation show that the cumulative-blasting-affected area can be divided into crushed,crack,and elastic-deformation zones;further,the crack zone can be divided into crack-intensive and main crack-propagation zones according to the type and number of cracks.Additionally,a partition model for the influence of deep-hole cumulative blasting with linear-shaped charge in coal seams was constructed.Meanwhile,under the influence of the shaped-charge structure,the crushed zone has an oval-like shape with a small radius in the direction of the shaped-charge jet,while the crack-intensive and main crack-propagation zones have oval-like shapes with a larger radius in the direction of the shaped-charge jet.In addition,field experiments of deep-hole cumulative blasting with linear-shaped charge in coal seams were carried out and the experimental results show that the influence of the cumulative blasting on the increase of the coal-seam-gas volume fraction in each observation hole weakened in a step-wise manner(strong-medium-weak)with increasing distance from the blasting borehole;this is consistent with the partition model of the constructed cumulative blasting,i.e.,the cumulative-blasting-affected area has certain zoning characteristics,and the crushed,crack-intensive,and main crack-propagation zones are the main components of the effective fracture zone.(3)Mechanism of control hole under cumulative blasting load.This section is concerned with the effect of control hole on the crack initiation and propagation under cumulative blasting using a linear-shaped charge in a coal seam.The propagation characteristics of explosive stress wave,the expansion characteristics of the main coal seam cracks caused by explosion,and the stress state and displacement characteristics of coal seam in the area around the control hole were investigated theoretically and numerically.The results demonstrated that the superposition of the incident compressive stress wave and the reflected tensile stress wave changed the stress field around the control hole and the main crack tip significantly and caused the directional expansion of main cracks and the formation of annular cracks around the control hole.In addition,the displacement compensation space provided by the control hole and the surface properties of the hole wall enhanced the tangential tensile stress between the coal particles on and around the control hole wall,which promoted the development and expansion of the radial cracks around the control hole.Furthermore,during the process of cumulative blasting and breaking,the combination of main cracks created by the explosion,the radial cracks and circumferential cracks around the control hole constitutes a large-scale fracture network under the effect of control hole.Besides,the deep-hole cumulative blasting tests were carried out to study the influence of presence and absence of control hole in coal seam on the degree of permeability increasing.The test results showed that the presence of control hole in coal seam under cumulative blasting improved significantly the permeability of coal seams.The increasing of the mean concentration of extracted gas in the area influenced by the control hole after blasting in the tests carried out in two different time periods are 1.78 and 2.58 times respectively that in the area without influenced by control hole.(4)Crack propagation and coalescence mechanism of double-hole cumulative blasting in coal seam.This section is concerned with the crack propagation and coalescence mechanism under double-hole cumulative blasting using linear-shaped charge in a coal seam.Based on the analysis of the mutual superposition effect of the explosion stress waves during the simultaneous detonation of two blastholes,the numerical analysis model of the double-hole cumulative blasting with linear shaped charges was established,and the propagation characteristics of the stress wave during simultaneous detonation of two blasting holes,the stress state of coal body,the mechanism of coal crack propagation and coalescence and the influence of the stress wave superposition effect on crack propagation were studied.The results show that the stress wave superposition effect causes the pressure equalization zone to be formed in the partial region of the middle section of the two blasting holes and its adjacent regions,forcing the radial cracks(except the directional cracks)of the two blasting holes to turn and cannot connect with each other,which leads to the formation of the gap blank zone between the two blasting holes.After the directional cracks generated under the cumulative blasting load coalesce,the collision of the explosive gases from the two blastholes further promotes the expansion of the cracks in the directional crack coalescence zone and eventually penetrates the gap blank zone.The field test results of deep-hole cumulative blasting in coal seams show that the explosion stress wave from the blasthole in the opposite side will promote the propagation of the blasting-induced crack on the left or right side of the two blastholes,and the promotion will increase first and then decrease as it moves away from the blasthole.Between the two blasting holes,the superposition effect of the stress waves from the two blasting holes will inhibit the propagation of the cracks in some areas,resulting in a W-like fluctuation in the degree of improvement of the gas drainage effect at different positions in the area between the two blasting holes.