Study On Influencing Factors And Mechanism Of Deep Hole Cumulative Blasting In Coal Seam

Most coal seams in China belong to high gas and low permeability coal seams.Gas drainage is the core method to prevent coal mine gas disasters.Coal seam permeability enhancement is the key technology to improve coal seam gas drainage rate.Coal seam deep hole cumulative blasting has achieved good results in coal seam cracking by relying on its special charging structure.However,the coal seam is located in a complex environment,and there are many factors affecting the cracking and permeability enhancement effect of coal seam by cumulative blasting.In order to improve the penetration enhancement effect of cumulative blasting and further improve the gas extraction rate,it is of great significance and value to carry out the research on the influencing factors and mechanism of cracking and permeability enhancement of coal seam deep hole cumulative blasting.Therefore,this dissertation comprehensively uses the relevant theories and methods of explosion mechanics,rock dynamics,fracture mechanics,damage mechanics and coal mine gas prevention.Based on the coal seam gas geological conditions of No.10 and No.12 coal mine of Pingdingshan,supplemented by similar simulation test and numerical simulation,combined with the technical characteristics of coal seam deep hole cumulative blasting,this dissertation studies the four factors affecting the cracking and permeability enhancement effect of coal seam deep hole cumulative blasting,including charge structure,initiation mode,coal seam fracture and in-situ stress,The specific research contents are as follows:(1)This chapter focuses on the influence of the charge structure on improving coal seam permeability.Experimental results of deep-hole cumulative blasting in coal seam indicate that the charge structure has an influence on coal seam permeability.In the horizontal blasting area,the average increase in the volume fraction of gas is 52.78 %after blasting.In the inspection holes which located above and below the blasting hole respectively,at a vertical distance of 1 m,after blasting,no blasting smoke escapes from the hole above the blast hole,but the blasting smoke escapes from the hole below the blast hole,which proves that the influence of the eccentric decoupled charge structure on coal seam below the blast hole is greater than that on coal seam above it.Moreover,the range of the fracture above the blast hole is smaller than that below the blast hole.(2)This chapter is concerned with the effect of detonating mode of deep-hole cumulative blasting on coal seam cracking and permeability enhancement.The results indicate that the initiation position has an influence on the development of coal rock blasting cracks,by controlling the propagation mode of detonation wave and stress wave in cumulative charge.Detonation wave collision and stress wave superposition effect are important reasons for the uneven distribution of explosive cracks along the axis of charge.The effect of deep-hole cumulative blasting initiated at different positions makes the fracture development show different characteristics.With the increase of the initiation point,the charge length between the initiation points is shortened accordingly,which weakens the superposition effect between conical stress waves,reduces the gap between the development range of explosive cracks at the impact point of detonation wave and that at other positions,and makes the explosive cracks distributed more evenly along the axis of charge.As for the millisecond delay cumulative blasting in coal seam.The new free surface effect and the superposition effect of stress wave are the key factors to promoting the expansion of blasting cracks and the formation of derived cracks.Millisecond-delay cumulative blasting promotes the further development and expansion of coal seam cracks between holes.The millisecond-delay time and the hole spacing are the important factors to affect the propagation characteristics of blasting cracks,which determines the developed form of blasting cracks jointly.(3)This chapter discusses the influence of coal seam fractures on the cumulative blasting cracking effect.The results show that the spatial relationship between the fracture and the blasting hole is an important factor affecting the fracture propagation of cumulative blasting.The existence of the fracture hinders the propagation of the explosion stress wave,greatly reduces the peak value of the stress wave passing through the fracture,makes the outer area of the fracture unable to reach the failure strength,and then forms the fracture empty white belt.The size and distribution of fracture blank zone is controlled by the center distance between artificial fractures and blasting hole and the angle of artificial fractures.At the same time,the explosion stress wave is reflected on the fracture surface and superimposed with the part of the subsequent stress wave compression phase,which weakens the compression effect of the explosion stress wave on the fracture surface and hinders the formation of the fracture near the inner side of the artificial fracture.The compressed phase of explosion stress wave is completely reflected on the fracture surface to form tensile wave.The superposition of tensile wave and subsequent stress wave can promote the formation of cracks in the area between artificial cracks and blasting holes.Stress concentration occurs at the tip of artificial fracture under the action of explosion stress wave,and due to the blocking effect of fracture on explosion stress wave,the stress wave action on both sides of the end of artificial fracture is greatly different,and the materials at the fracture tip are easier to be damaged,resulting in the formation of derived fractures on both sides of artificial fracture.(4)This chapter aims at the problem of increasing permeability of coal seam by deep-hole cumulative blasting under in-situ stress.The results show that the role of in-situ stress in different stages of radial crack expansion of coal seam deep-hole cumulative blasting is quite different.Before blasting,the stress state and deformation characteristics of borehole surrounding rock are determined by borehole shape and in-situ stress.In the initial stage of cumulative blasting,the impact of cumulative blasting on surrounding rock is obviously stronger than in-situ stress.Therefore,the expanding direction of blasting cracks in the initial stage is mainly determined by the cumulative structure,and directional cracks are formed along the opening direction of cumulative charge groove.With the crack extending around,the blasting effect is gradually weakened,and the in-situ stress is dominant.The surrounding rock of the borehole produces tangential compressive stress under the in-situ stress,which limits the radial crack expansion of blasting.Meanwhile,the coal cracks which are not collinear with the principal stress gradually deflect towards the direction of the maximum principal stress under the action of strong shear stress.When the equivalent dynamic stress produced by blasting can not continue to further compress the coal,the elastic strain energy accumulated in the surrounding rock of the borehole begins to release towards the blasting center,causing the coal to crack and produce new cracks.In addition,the crack expansion range in different directions is controlled by the lateral pressure coefficient.When the vertical principal stress is constant,the crack range in the direction of minimum principal stress further decreases with the increase of lateral pressure coefficient.