Numerical Simulation For Vibration-reduction Effect Of Presplitting Crack

Blast vibration is required to be strictly controlled as a major blasting hazard.Presplitting blasting has been widely used for its good effects of damping andcutting off the blasting crackle’s extension. However, its vibration-reduction effectare strongly affected by the depth of presplitting crack and the distance beyond theblasting ranges, as well as filling water or rock debris in the presplitting crack.Therefore, it is of practical significance to optimize presplitting crack parametersand take effective vibration-reduction measures in blasting engineering, byexploring the changes of presplitting crack parameters and filling medium onvibration-reduction effect. Based on this, the vibration reduction mechanism ofpresplitting crack was analyzed. And three-dimensional numerical models ofpresplitting crack vibration-reduction effect under different conditions weresimulated by LS-DYNA software. Then several conclusions could be made asfollows:(1) Presplitting crack was studied to possess obvious vibration-reduction effect,but the vibration reduction ratio of presplitting crack was decreased with theincreasing distance between measuring point and presplitting crack. And there was azone with notable vibration-reduction effect behind presplitting crack, once beyondthis zone, the vibration reduction ratio declined rapidly. When the presplitting crackhad definite depth, length and width, the vibration reduction ratio in horizontaldirection was larger than that in vertical direction, and the barrier effect of thepresplitting crack on P-wave was greater than that on surface wave.(2) After the depth and length of presplitting crack reached optimum values, theimprovement of the vibration-reduction effect would not be obvious. When thedepth of presplitting crack was0.75~1.5times of borehole depth, and half of thepresplitting crack length (length beyond blasting range) was0.75~1.13time ofborehole depth, the vibration-reduction effect would be notable and economical.(3) If the presplitting crack was not closed after the boreholes of main blastingarea were detonated, the width of the presplitting crack had no influence on thevibration-reduction effect. The vibration-reduction effect of presplitting crack wasdampened with the increasing distance between the borehole and presplitting crack.Therefore, the presplitting crack should be arranged next to the blasting source. Thepeak particle velocity on the palisades behind the presplitting crack firstly declined, and then rose slowly with the increase of the presplitting crack depth, and themaximum value was observed on the surface.(4) Under the conditions with no presplitting crack, and water or rock debrisfilled in presplitting crack, the attenuation law of peak particle velocity both fittedSadov’s formula. But the attenuation coefficient and attenuation index in the lattercondition were relatively lower. When the presplitting crack was deep, theattenuation law of peak particle velocity was disturbed and didn’t fit Sadov’sformula any more.(5) The vibration-reduction effect in presplitting crack filled with water or rockdebris was not as good as that in crack filled with air. When the filling height waslower than0.4times of presplitting depth, the vibration reduction ratio decreasedgently with the increase of the filling height, and the vibration-reduction effect wasnot influenced by filling material. However, when the filling height was higher than0.4times of presplitting depth, the vibration reduction ratio decreased abruptly withthe increase of the filling height. Therefore, in practice, the filling height exceeding0.4times of presplitting should be avoided.