Study On Weakening Mechanism And Engineering Application Of Deep-Hole Blasting In Hard Coal Seam In Fully Mechanized Top Coal Caving Mining

This paper aims at solving the problem of hard top coal caving with shallow buried of 3#coal seam in YuYang area.Damage mechanism,optimal blasting parameters and pre-evaluation technology of weakening effictiveness by deep hole blasting were studied.The process of damage weakening under the action of multiple blasting holes was studied by using theoretical analysis method.The weakening effect of top coal in the form of single hole arrangement,double hole arrangement and cross hole arrangement was studied by numerical simulation.The relationship between CT value,wave velocity and damage degree of coal sample under different damage states was studied by laboratory experiment.Based on aboratory test,the characteristics of longitudinal wave velocity variation throughout the top coal weankening area were tested and the weakening effect of deep hole blasting was pre-evaluated quantitatively by seismic CT detection technology.The main achievements of this paper are as follows:?1?Analysis showed the key parameters that influence the weakening effect of blasting,and the dimensionless relationship between various physical quantities.Based on fracture energy scale effect and estimation method of blasting damage volume with probability distribution of blasting fragmentation.Four indexes for quantitative evaluation of blasting effect were proposed.that is,coal seam complete fracture volume?V_t?,coal seam equivalent fracture volume?V_e?,coal seam strong damage volume(V_Ds)and coal seam weak damage volume(V_Dw).The dynamic mechanical constitutions of coal damage-fracture considering strain rate effect and strain softening effect are proposed.To verify that,the tensile and compressive experiments at different strain rates were carried out.?2?In the case of single-hole blasting,the ultimate failure radius and the breaking degree of the final state system are smaller when the strain rate effect is taken into account than not.With the increase of radial decouple coefficient,the radius of smash district and crack area present a decrease trend of exponential decay model.When the radial decouple coefficient is more than 3,the variation of statistical indexes are not obvious.In the circumstances of double-hole blasting,with the increase of hole spacing,the damage area evolved gradually from quasi-circular to oval,dumbbell shape,and two independent circular formation in the end.The breaking degree of the final state system increases at first then decreases.?3?Weak damage volume>equivalent fracture volume>strong damage volume>complete fracture volume of top coal under different blasthole arrangement and blasting parameters.In the same situation,the diameter of explosive has a greater influence on the damage volume than that of the stemming length.With the increase of blasthole spacing,the coal seam damage volume increased gradually,but the increase rate slow at the same time.When the blasthole spacing reaches a certain extent,the complete fracture area of adjacent blasthole can not connect together,which are being cutted by several stripes of weak damage or undamage area.Clamp undamage phenomenon is formed in the top coal,caused incomplete weakening.The damage and fracture volume of unit blasthole caused by single-hole arrangement and cross-hole arrangement is basically the same,and both of them are higher than the damage and fracture volume of unit blasthole in the form of double-hole arrangement.Compared with the single-hole layout,the damage field caused by the cross-hole is more uniform,and the inclusion of weak damage areas in the intensive crushing zone is relatively less.The results show that the stemming bodies with low elastic modulus,high poisson ratio,low strength and high friction coefficient of hole wall can play a better stemming effect.?4?under uniaxial loading,the change rule of longitudinal wave velocity and shear wave velocity is almost the same.In the stage of compaction and linear elasticity,the wave velocity increases with the increase of stress and strain.In the plastic deformation stage,the wave velocity decreases with the increase of stress and strain.It can be seen from the fitting curve that the longitudinal and shear wave velocity changes with strain in an"S"shape.The former is always greater than the latter,but the variation of whose ratio is not obvious.At the initial stage of the experiment,the ratio is at a relatively high value.As the load continues to increase,the change of the ratio fluctuates and generally decreases.Under the action of triaxial load,when the confining pressure is less than or equal to 10MPa,the longitudinal and shear wave velocities show the variation of first increasing,then decreasing and then increasing with the increase of deviatoric stress.When the confining pressure is greater than or equal to 13MPa,the longitudinal and shear wave velocities show the variation of first increasing,then decreasing.?5?Before and after coal sample failure,CT value and variance presented serrated shape with different initial damage degree in the axial direction,and CT value presented normal distribution basically.When the damage degree of coal sample specimen is higher,the damage degree calculated by CT value is close to that calculated by longitudinal and shear wave velocity,and both of them are relatively larger.When the degree of local damage is larger and there are some complete block parallel to the axial direction,the damage caused by the conversion of CT number is quite different from that obtained by the conversion of the longitudinal and shear wave velocity.The calculation of damage value is more accurate based on CT number,but the difference between each specimen is smaller than that calculated by wave velocity.To make up its precision,multiple specimen and multiple measurements are needed.?6?Field application shows that it is highly feasible and reliable to use seismic CT tomography to evaluate the weakening effect of deep hole blasting in advance.