Experimental Study On Vibration Effect Of Deep Hole Bench Blasting In Near Field

With the development of mining, water conservancy and hydropower, highway and railway engineering industry, deep hole bench blasting, due to its good technical and economic effect has become the most used blasting operation in rock blasting engineering. However, due to a large amount of charge and large blasting scale, the harmful effects from blasting also seriously affected the normal production and life of the surrounding residents. Among all the adverse effects, the blasting vibration is the main factors of blasting harm. In the on-site construction process, accidents and disputes caused by blasting vibration can be found everywhere. Especially near the village, monuments and other important buildings, blasting often will bring more problems. Therefore, it is very necessary to deeply study the vibration effect of deep hole bench blasting.About safety criterion of blasting vibration, Safety regulations for blasting(GB 6722) made it a rule that using vibration velocity and frequency as a comprehensive criterion for the ground buildings, in which the vibration velocity is the main, the frequency as a reference for correction, still followed the M.A Sodev’s formula of former Soviet Union. Sodev’s formula is based in the charge chamber blasting, obtained through dimensional analysis, which has clear physical meaning and usage conditions. If it is applied to the deep hole bench blasting, it is bound to be a lot of problems. However, in a large number of engineering practice, blasting workers found for the prediction of blasting vibration, Sodev’s formula can still meet the engineering requirements in middle and far field. The reason is when the distance is large enough for the distribution of the blasting area, the multiple holes in the distance can be approximately as concentrated charge, which meet the basic assumptions of Sodev’s formula. But in near field, the prediction error of PPV can reach 200%~300%, which is not ideal. It is clearly unfavorable for advance understanding and evaluating the impact of blasting vibration in blasting construction near structure and buldings.At present, there is little research on the vibration of deep hole bench blasting in near field at home and abroad. Research on the characteristics, distribution law, main influencing factors and their relationship of vibration in near field is less. How to solve the problem of prediction accuracy for vibration intensity and frequency in near field has a very important significance to the blasting design and the process improvement. In this paper, by carrying out a large number of field tests, using signal processing and analysis, probability theory and mathematical statistics, modeling calculation and so on, the vibration characteristics in near field were studied systematically and deeply.The main achievements are as follows:(1) The characteristics of vibration intensity in the near and far field are compared.Under the condition of the test site, when the scaled distance is less than 5, the horizontal radial velocity is the largest, when the scaled distance is greater than 5, vertical velocity.is the largest. In terms of the attenuation law, the values of K and α in near field are all greater than that in far field. In the goodness of fit, the residual square sum in near field is greater than that in far field, and the correlation coefficient is smaller. The correlation coefficient of the vertical component is the largest in the three components. From the point of view of random variables, the variation coefficient of the vibration velocity in near field is greater than that in far field for the same measuring point.(2) The energy distribution characteristics in the near and far field were compared. When the maximum charge weight per delayed interval or distance is increased, energy in the near and far field has gradually moved toward the low frequency band, in which it moved faster in near field. At the same time, the frequency band width of energy distribution in the near and far field also tends to be concentrated, and the frequency band width in near field is larger than that in far field.(3) The reasons for the trend of blasting signal in near field are summarized, which is nonlinear distortion and low frequency interference superposition under the condition of large amplitude pulse input. Using EEMD, the wavelet analysis and other signal analysis method, a trend elimination method is proposed, which is based on the combination of frequency band distribution of each IMF component and artificial identification. In addition, a wavelet threshold denoising method based on autocorrelation analysis to identify noise characteristics is also proposed. Examples show that the method is effective and can be realized by batch pretreatment of blasting signal.(4) By statistical analysis for the 54 sets of single hole waveform measured in near field, vertical vibration velocity and basic frequency of single hole attenuation law can be obtained under the test site condition. By changing the delay time, the 5 sets of single hole waveform which represent the site characteristics in different distance are superposed by two-stages. Results show that the vibration reduction rate of the superimposed waveform changes continuously with the delay time, but the basic frequency has a certain step characteristic. With the increase of the distance, the delay time of the maximum vibration reduction rate and the lowest basic frequency is gradually increased, the vibration reduction rate is decreased, and the change of basic frequency with time is gradually weakened.(5) Based on the assumption that the medium is uniform and continuous, constructing a 5 rows and 17 columns matrix model behind the blasting area to simulate the changes of vibration "field" of single row holes under different blasting conditions. By calculation, the influence of the number of holes, the hole spacing and the hole charge on the velocity distribution in near field is obtained:1) With the increase of the number of holes, the vibration velocity of the same position increases, but the increase decreases with the increase of the number of holes. This trend is gradually decreasing from the center to both sides. Parallel hole line direction, the decline rate of the vibration velocity from the center to both sides increases with the increase of the number of holes. The shape of the vibration velocity contour is gradually transition from the approximate circle to the envelope of the multiple holes. Vibration velocity contour in the center is approximately parallel to the hole line, which shows the direction of seismic wave energy flow is gradually unified within a certain range.2) With the increase of the hole spacing, parallel hole line direction, vibration velocity of the measuring point close to the center decrease, close to the edge increase. The two cut-off point is associated with the actual number of holes and hole spacing. The decline rate of the vibration velocity from the center to both sides gradually decreases. Because the energy density in the hole line decreased, the vibration velocity contour is more rough. The curvature decreases and the diffusion degree increased.3) With the increase of the hole charge, parallel hole line direction, the decline rate of the vibration velocity from the center to both sides gradually increases. The shape of the vibration velocity contour has no significant changes.4) With increasing of wave propagation distance, the influence of blasting conditions on the vibration velocity distribution gradually decreases, which shows that the influence of multiple holes on vibration intensity distribution in near field is larger than that in far field.(6) With a single hole test in near field as an example, analyzing the influence of detonator delay error, frequency difference, and the vibration velocity calculated error etc. on the vibration velocity prediction of deep hole bench blasting. Through the probability statistics, the maximum delay time of two detonators can reach 40.4544 ms under the 95% confidence interval. Detonator delay error is the main factor affecting the prediction effect.7) A method is proposed for determining the detonator delay time based on waveform recognition. In this case, the calculated actual delay time for each hole under the optimal confidence are-15.84 ms, 4.04 ms, 15.82 ms, 9.43 ms, 15.84 ms, 6.97 ms and 2.74 ms. After eliminating the effect of detonator delay error and others, prediction error of vibration velocity in each measuring point is from the previous 83.49%~140.66% to 0~39.13%, and prediction error of basic frequency is from the previous 1.68%~31.96% to 1.54%~17.79%. The prediction effect enhanced significantly. On this basis, try to predict vibration velocity of unknown points in a certain range. From the prediction results, the vibration velocity prediction can meet the basic requirements of the field, while the frequency prediction still needs to be improved.