Study On Attenuation Of Dominant Frequency Of Blast-induced Vibration

Blasting is an essential rock mass excavation method,and it is frequently used in the construction of hydropower,mining,and municipal infrastructures.Blasting is highly economical and efficient;however,with some significant drawbacks,for example,blast-induced vibration is a major concern to the stability of excavated area.With the increasing demand on monitoring and constraining the blast-induced vibration related hazard,the conventional blasting safety criterion based solely on peak particle velocity(PPV)is insufficient,and the importance of dominant vibration frequency has been emphasized in the newly developed blasting safety standards.Therefore,understanding the attenuation mechanism of dominant frequency of blast-induced vibration is of great significance to mitigate the damage caused by blast-induced vibration.This dissertation focuses on the attenuation law of dominant frequency of blast-induced vibration.For this purpose,a systematical research was conducted with theoretical analysis,numerical simulation,and filed test verification.As a result,a frequency domain solution of blasting vibration in viscoelastic media was derived.Influencing factors of the attenuation of the dominant frequency were investigated,revealing the attenuation law of the dominant frequency.A prediction formula for the attenuation of the dominant frequency was proposed.The main research results are break down into the following points.According to basic theories of the propagation and attenuation of blast-induced seismic waves in viscoelastic media,an attenuation term of amplitude was proposed to quantify the attenuation of vibration amplitude in frequency domain.On this basis,a relationship between the attenuation term of amplitude and its influencing factors was obtained.The research shows that the attenuation term of amplitude increases with the decrease of rock quality factor and wave velocities.The attenuation term of amplitude is proportional to the vibration frequency and the travel distance of seismic waves.The amplitude of seismic waves corresponding to a higher attenuation term attenuates more quickly.Based on the theory of elastic wave propagation,amplitude spectrum expressions with attenuation term induced by various blasting sources in viscoelastic media were derived.With the methods of theoretical analysis and numerical simulation,amplitude spectrum analysis of blasting vibration was conducted and the attenuation law of dominant frequency of blast-induced vibration was revealed.Results illustrate that,regardless of the blasting source type,both the amplitude spectrum and the dominant frequency remain unchanged in the process of blasting seismic wave propagating in elastic media.While in viscoelastic media,the dominant frequency attenuates versus travel distance and a rapid drop of dominant frequency occurs during propagation of seismic waves.The discrepancy of decay rate of amplitudes corresponding to different frequency components and the multimodal structure of vibration spectrum lead to the unsmooth attenuation of dominant frequency.The centroid frequency,on the other hand,attenuates smoothly without any drops.Based on theoretical analysis on the vibration spectrum expression and numerical simulation,a series of study was conducted to analyze the influencing factors of frequency spectral characteristics.The results show that,the frequency spectrum and the dominant frequency of blast-induced vibration depend on a number of parameters including blast load,geometry of blast source,rock properties,and the initiation mode.The dominant frequency is inversely proportional to the rise time and duration time of blasting load.Moreover,the effect of the rise time is more significant than the duration time;the effects of blasting loads on the vibration frequency are limited to a certain degree.With increase of the rock quality factor and wave velocities,the dominant vibration frequency increases.For drill blasting,the dominant frequency is in inverse proportion to the charge length and charge radius,and the effect of charge length is more pronounced than the charge radius.Furthermore,by changing the location or the number of detonation point to shorten the charge length initiated by individual detonation point,the dominant frequency will be increased.According to the theoretical solution of blasting vibration induced by spherical seismic wave propagation,a prediction formula for the attenuation of dominant frequency was derived with the help of dimensional analysis.The prediction accuracy of proposed formula was verified by a field experiment.Moreover,attenuation laws of dominant frequency,centroid frequency,and zero-crossing frequency(ZCF)were compared by the proposed formula.No clear trend was observed in the distribution of dominant frequency versus travel distance.While the attenuation of centroid frequency and ZCF demonstrate systematic patterns that they can be predicted by the new formula with high accuracy.In the end,the new formula was modified to a prediction formula based on the theory of cylindrical wave propagation;however,the modified formula did not improve the fitting degrees of regression between the predicted and measured values in this field experiment.