Mechanism of millisecond-delay detonation on coal cracking under deep-hole cumulative blasting in soft and low permeability coal seam

This paper is concerned with the problem of cracking and permeability increasing in soft and low permeable coal seam.Based on the analysis of the process and influence factors of millisecond-delay cumulative blasting,the numerical models were established by using fluid-solid coupling algorithm to study the stress and crack development characteristics of coal.Combined with the field experiment in coal seam,the mechanism of millisecond-delay cumulative blasting and its influence on coal seam permeability enhancement were discussed.The results show that 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 first blasting hole provides new crack surfaces for the later blasting hole.And the explosion stress wave diffracts and reflects through the crack surfaces and superimposes with the incident wave,which makes the coal mass near the cracks in a tensile state and promotes the formation of derived cracks.The residual stress field formed by the first blasting hole can increase the density of blasting cracks and expand the scope of coal cracks.In addition,the millisecond-delay time and the hole spacing are the important factors to affecting the propagation characteristics of blasting cracks,which determine the development form of blasting cracks jointly.When the propagation distance of the stress wave generated by the first blasting hole in millisecond-delay time is less than the hole spacing,the explosion stress waves will be superimposed between the blasting holes,and then reflect between the crack surfaces to form tensile stress waves,which have complex destructive effects on the coal mass and forms new cracks between the original blasting cracks.Moreover,the coal seam permeability is improved significantly after the application of deep-hole millisecond-delay cumulative blasting.The coal cracks near the first blasting hole are further developed under the action of the second blasting hole,and the coal cracks between the blasting holes are more easily connected with the decrease of hole spacing.