Surface Vibration of Throw-Type Blast in an Open-Pit Mine

Abstract

In the process of Open-pit mining in China, throwing blasting is an important method, which is very likely to cause serious damage to the slope of the discharge field under the action of vibration load of throwing blasting. With the background of throwing blasting process in Heidaigou open-pit mine in Ordos, vibration velocity data were collected from the discharge field near the throwing blast, and the vibration signal of throwing blast was analyzed by means of Fourier transform to obtain the characteristics of throwing blast vibration velocity wave and the attenuation law and prediction formula in the process of propagation. The results show that: 100 ~ 300 m away from the blasting area, the radial direction (X direction) of the blasting area produces the largest vibration velocity of 26.8 cm/s, but at the same time, the decay rate of the peak vibration velocity of the survey line 2 in each direction is small, and the decay percentages of 56, 75 and 70% are smaller than that of the survey line 1 and survey line 3 in the lateral direction of the blasting area, and the decay rate of the velocity is smaller as the propagation As the distance increases, the decay rate of the velocity decreases. The curve gradually tends to flatten and the vibration velocity of the three directions gradually close. The frequency band of the blast vibration is distributed within 200 Hz and the frequency and energy are mainly distributed in the low frequency stage (0–20 Hz), accounting for more than 50% of the total energy.

1 Introduction

As one of the most primary and important energy sources in China, coal plays a vital role in China's economic development and energy security. Although the proportion of coal resources in the energy consumption structure has been declining due to the country's promotion of green energy and the requirements of reducing carbon emissions, coal resources still account for 57.7% of the energy consumption structure according to the 2019 China Mineral Resources Report [1]. In the future, coal resources will still be one of the most important and largest energy sources in China.

China's mines have developed rapidly and their scale and efficiency have been continuously improved due to the importance of mineral resources and the high demand for social and economic development. According to statistics, about two-thirds of the world's solid mineral resources are mined by open-pit mining, which plays an important role in the mining industry. There are about 1200 open-pit mines in China, and the throwing blasting is widely used as an efficient mining method in open-pit mines. The vibration generated by throwing blasting has become a problem that must be concerned and studied. It should be noted that the throwing blasting in the mine will have a negative impact on the stability of the slope in the mine, so it is of great significance to study the characteristics and propagation laws of the vibration caused by the blasting load.

The paper is based on the mining project of Heidai George Open-pit Mine in Ordos. The on-site monitoring test of blasting vibration and numerical simulation are carried out in order to analysis the dynamic response of rock and soil mass in Heidai George Open-pit Mine under blasting vibration. The characteristics and the attenuation law of throwing blasting seismic wave are obtained in the process of propagation.

2 Engineering Condition

Heidai George Open-pit Coal Mine of Zhungeer Energy limited ability company is located in the Zhungeer Banner of Nei Monggol Autonomous Region, where is located on the west bank of the Yellow River (111°11′–111°25′ E, 39°25′–39°59′N).

Heidai George Open-pit Coal Mine is located in the northeast of Ordos Plateau, facing the Yellow River in the east, and the surface is covered with thick Quaternary clay. As the climate in this area is dry and rainless, vegetation is scarce, rainfall is concentrated, and the loss of water and soil is serious. Dendritic gullies and river valleys dominated by the Yellow River are developed on the surface, so that platform gullies are cut into vertical and horizontal gullies and the terrain is fragmented and extremely complex, forming a source hill landform with gentle ridge valley and Gaoliang valley terrain. Heidai George Open-pit Coal Mine in Ordos is a large-scale Open-pit coal mine designed and constructed by China, which belongs to SHENHUA GROUP ZHUNGEER ENERGY CO., LTD. The mine was commenced in 1990 and put into trial production in 1998, and then officially handed over for production in 1999 and reached production capacity the next year. In June 2006, the annual output of raw coal of Heidai George Open-pit Coal Mine after capacity expansion and transformation reached 25 million tons, becoming the largest Open-pit coal mine in China. In 2011, its annual output exceeded 31 million tons which rank first in terms of capacity and scale in Asia.

3 Materials and Methods

3.1 Experiment Instrument

TC-4850 blasting vibration meter is used for on-site blasting vibration monitoring test. The instrument can record the blasting vibration velocity, acceleration and other data, and is suitable for data acquisition of the scene of throwing blasting vibration.

The technical indicators and advantages of TC-4850 blasting vibration meter are listed as follows: the acquisition channels are X, Y and Z channels for parallel acquisition, and the sensors are three-dimensional, which is convenient for burial. The sampling frequency is 1–50 kHz, which can meet the requirements of this vibration measurement. The error is less than 0.5% and the reading accuracy can reach 1, which can meet the accurate measurement and recording of the vibration signal.

3.2 The Blasting Site

The blasting area is located at the elevation of 1095–1130 m in the south of Heidai George Open-pit Mine with a length of 400 m, a width of 85 m and a bench height of 43– 49 m whose average height is 46.6 m. The on-site explosive holes with a depth of 45–54 m, an average hole depth of 50 m and a blasting amount of 1,511,580 m³ and blasting conditions are shown in Fig. 1 and Fig. 2, respectively. The hole spacing of throwing blasting is 12 m and the row spacing is 6–7 m. The total rows of holes and the total number of throwing blasting holes are 12 and 388, respectively. The average unit consumption is 0.753 kg/m³. The presplitting holes were blasted first with a hole spacing of 3.5 m. There are 114 and 23 presplitting holes in the back row and the south end, respectively. The holes are 137 in total with a unit consumption of 1.3 kg/m². The total number of holes is 525 with a charge of 1138 tons, including 551 tons of heavy ammonium oil explosives and 587 tons of ammonium nitrate oil explosives.

Fig. 1

The throwing blasting holes

Fig. 2

The blasting site

3.3 The Arrangement of Measured Points

The whole history of blasting vibration is recorded by the site survey in order to study the attenuation law of blasting seismic wave and achieve the detection purpose. In addition, the test scheme of blasting seismic is determined according to the layout principle of vibration measured points, the characteristics of throwing blasting and the actual geological and topographic conditions of the blasting area. The relative position of measured points is shown in Fig. 3.

Fig. 3

The schematic diagram of relative position of measuring points

• (1) The survey Line 1 is arranged in order to monitor the blasting seismic intensity and developed law on the left side of the blasting area. The measured points are arranged on the same steps of the blasting area, which are located in the south of the blasting area. The measured points of Line 1 are numbered 1 #, 2 #, 3 # and 4 # respectively.

• (2) The survey Line 2 is arranged in order to monitor the blasting seismic intensity and its developed law behind the blasting area. The measured points are arranged on the same steps in the blasting area. The measured points of survey Line 2 are numbered 5 #, 6 #, 7 # and 8 # respectively.

• (3) The survey Line 3 is arranged in order to monitor the propagation law of blasting seismic wave on the right side of the blasting area. The number of measured points is 9 #, 10 #, 11 # and 12 # of survey Line 3 respectively. The measured points are arranged on the same steps.

Each measured point can simultaneously monitor the vibration velocity in X, Y and Z directions. The horizontal radial vibration velocity of vibration propagation is recorded in X direction, the tangential vibration velocity in Y direction is perpendicular to the vibration propagation direction, and the vertical vibration velocity in Z direction is perpendicular to the platform ground.

The embedding method of the sensor is crucial in order to accurately record the whole process vibration of the throwing blasting and the main characteristics of the vibration waveform. The vibration speed sensor needs to form an integral part with the platform soil. Therefore, it is necessary to dig a trench on the test platform and bury the sensor according to the layout plan of the measured points.

On the site, a trench with a width of about 40 cm and a depth of about 30 cm is opened on the designated platform by the slotting machine in the direction of the preset measuring line. The sensors are placed at the corresponding positions according to the design distance, as shown in Fig. 4. The surrounding soil is used to fill the trench and compact it in order to reduce the impact of other interference vibration and the sensor's own movement on the data collection of blasting vibration.

Fig. 4

The arrangement of on-site measuring points

4 Result and Discussion

Vibration amplitude, main vibration frequency and duration are three parameters to describe blasting vibration. Peak vibration velocity refers to the maximum vibration amplitude of medium particle, and main vibration frequency refers to the frequency of wave The amplitude of the medium particle reaches the maximum. In China's Code for Blasting Safety GB 6722–2014 [3], vibration velocity and main vibration frequency of slope particle are taken as the basis for safety discrimination. Li et al. [4] thinks that the influence of frequency should also be fully considered in the safety evaluation system of blasting vibration except for vibration amplitude, Wang et al. [5] introduced the necessity and feasibility of using particle vibration velocity and frequency as safety criteria for blasting vibration, and the calculation formula of blasting vibration frequency. Yang et al. [6] Zhuang et al. [8] proposed to incorporate the amplitude, frequency spectrum and duration of blasting earthquake into the blasting earthquake safety criterion on the basis of on-site blasting monitoring, so as to establish a multi parameter safety criterion. Previous studies have shown that particle vibration velocity, that is, the magnitude of vibration amplitude and main vibration frequency, will have an impact on structures. The two should be combined to analyze the slope in blasting vibration. At the same time, obtaining the vibration amplitude and main vibration frequency can help us simulate the vibration wave generated in the blasting process. Therefore, the paper will analyze the peak vibration velocity in the blasting test.

4.1 Monitoring Results of Throwing Blasting Vibration

Figure 5 is the measured vertical and horizontal vibration velocity waveform diagram of each measurement point on each survey line of 5# under the blasting vibration in the same time period. (Note: v is ordinate vibration velocity in cm/s).

Fig. 5

Waveform Chart of Vibration Velocity of Measuring 5#. a Channel X; b Channel Y; c Channel Y

Table 1 and Table 2 are summary tables of blasting vibration data of each measured line. The comparison of peak vibration velocities of measured points on each measured line in X, Y and Z directions is shown in Fig. 6. It can be seen from the figure that, in general, the vibration velocities of survey line 2 are greater than those of the other two measuring lines in X, Y and Z directions. The closer to the blasting area, the faster the maximum vibration velocity of throwing blasting decays with the increase of distance. In addition, the variation of vibration speed tends to be gentle with the increase of the propagation distance and the vibration speed of different measuring lines tends to be consistent.

Table 1 Summary of blasting data of survey line 2

Table 2 Summary of blasting data of survey Line 1 and survey Line 3

Fig. 6

Comparison of vibration velocity of each survey line. a X direction; b Y direction; c Z direction

The safe range of blasting vibration can be controlled by exploring the attenuation law of blasting vibration wave in the process of propagation in geotechnical media. Many scholars have proposed some empirical formulas with good correlation by using mathematical methods based on field blasting tests [7, 8]. Sadovsky formula is the most widely used and traditional formula for predicting blasting vibration.

$$V = K \cdot (\frac{{\sqrt[3]{Q}}}{R})^{\alpha }$$

(1)

where V is the vibration speed of particle in cm/s; Q is the maximum initiation quantity of a single section in turn in kg, R is the horizontal distance from the test point to the explosion source in m; K is the coefficient related to factors such as geology and blasting methods and α is the attenuation coefficient of vibration wave related to geological conditions.

Figure 7 shows the relationship between the vibration velocity of each survey line in the X direction and the proportional distance ($\frac{{\sqrt[3]{Q}}}{R}$). Points of Figures are fitted by Sadowski's formula and then K and α are obtained. The attenuation prediction formula of the peak vibration velocity in each direction is obtained by taking values of K and α so that the peak vibration velocity in any distance around the blasting perimeter can be predicted. The two survey lines are analyzed together because survey lines 1 and 3 have symmetrical similarity. It can be seen from the figure that the formula fits well with the point. Therefore, the formula can be used to predict the attenuation of vibration velocity at different positions.

Fig. 7

Fitting regression of vibration velocity in X direction. a survey line 1 and line 3. b survey line 2

The vibration attenuation formula in X direction of survey line 1 and 3 can be explained as:

$$v = 467.9 \cdot (\frac{{\sqrt[3]{Q}}}{R})^{1.816}$$

(2)

The vibration attenuation formula in Y direction of survey line 1 and 3 can be explained as:

$$v = 99.6 \cdot (\frac{{\sqrt[3]{Q}}}{R})^{1.204}$$

(3)

The vibration attenuation formula in Z direction of survey line 1 and survey line 3 can be explained as:

$$v = 447.6 \cdot (\frac{{\sqrt[3]{Q}}}{R})^{1.891}$$

(4)

The vibration attenuation formula in X direction of survey line 2 can be explained as:

$$v = 87.8 \cdot (\frac{{\sqrt[3]{Q}}}{R})^{0.752}$$

(5)

The vibration attenuation formula in Y direction of survey line 2 can be explained as:

$$v = 156.2 \cdot (\frac{{\sqrt[3]{Q}}}{R})^{1.222}$$

(6)

The vibration attenuation formula in Z direction of survey line 2 can be explained as:

$$v = 130.4 \cdot (\frac{{\sqrt[3]{Q}}}{R})^{1.103}$$

(7)

It can be analyzed that the propagation attenuation law of the vibration velocity in all directions of the throwing blasting based on the prediction formula and the speed comparison curve. The maximum vibration velocity in the X direction on the survey line 2 is about 26.8 cm/s within the range of 100–300 m from the throwing blasting area The vibration velocity decreases to about 11.7 cm/s within 200 m and the attenuation reaches about 56%. However, the maximum vibration velocity of survey line 1 and line 3 is about 26.8 cm/s and attenuates to about 3.7 cm/s respectively at the same distance, which attenuates about 86%. The maximum vibration velocity of survey line 2 in Y direction is about 22.8 cm/s and attenuates to 6 cm/s within 200 m, which attenuates to about 75%; while the maximum vibration velocity of survey line 1 and line 3 is about 15 cm/s and attenuates to about 3 cm/s respectively at the same distance, which attenuates about 80%. The maximum vibration speed on survey line 2 in the Z direction is about 23 cm/s and attenuates to 6.8 cm/s within 200 m, which attenuates about 70%, while the maximum vibration speed of line 1 and survey line 3 is about 23 cm/s and attenuates to 2.9 cm/s within the same distance, which attenuates about 87%. In conclusion, the vibration velocity in X direction is greater than that of the other two directions during the propagation of throwing blasting vibration, which indicates that the vibration generated by throwing blasting has a greater impact on X direction. At the same time, Vibration generated at the front of blasting area is close to the vibration generated laterally to the throwing blasting area when the distance to the throwing blasting area is relatively close. However, with the increase of the distance, the vibration attenuation rate generated laterally of the throwing blasting area is significantly higher than that generated directly to the blasting area. This is because there exist free faces between the area of survey line 2 and throwing blasting area. Survey line 1 and line 3 are first affected by splitting blasting vibration, and then affected by throwing vibration of block stone. Survey line 2 is affected by splitting blasting and throwing vibration at the same time, which lasts for a long time. Therefore, it can be judged that the vibration generated by blasting in the direction opposite to the blasting area is maximum and the attenuation rate is small.

4.2 Fourier Transformation

Fourier transformation is one of the widely used methods in the analysis of blasting vibration signals. It can transform the time domain of blasting vibration signals into the frequency domain for analysis. Song et al. [9] analyzed the blasting experiment of large caverns with Fourier spectrum. The result indicates that there is a significant difference between the Fourier spectrum of the waveform in the source area and that in the far area of the explosion in the frequency domain.

The collected velocity signal of blasting vibration is subject to fast Fourier transformation in combination with the blasting monitoring data in the mining area. Figure 8 is the Fourier transformations of 1#.

Fig. 8

Fourier transformation of vibration velocity of 1#. (a) channel X; (b) channel Y; (c) channel Z

It can be seen intuitively from the FFT spectrum of the vibration velocity that the frequency band of the vibration generated by the throwing blasting is wide, the frequency is mainly distributed within 200 Hz, and the energy is mainly concentrated in the low frequency band. Compared with the general natural seismic wave, the vibration frequency of the natural seismic wave is lower, generally within 10 Hz, while the vibration frequency of the throwing blasting is significantly higher than the natural earthquake. Furthermore, the energy distribution in different frequencies in three directions of each measuring point can be obtained by integrating the spectrum diagram curve. Taking measured point 5 (5#) as an example, Table 3 shows the energy proportion distribution in each frequency range in all directions of measuring point 5. It can be seen that the vibration generated by throwing blasting is mainly concentrated in 0-20 Hz, and its energy accounts for more than 50% of the total energy, and from the total integration in all directions, the energy proportion of channel X is the highest, it conforms to the above analysis of vibration velocity in three directions.

Table 3 Distribution table of energy proportion of each frequency

5 Conclusion

• (1) Among the three measuring lines, the vibration speed of survey line 2 facing the blasting area in each direction at approximately the same distance is higher than that of the other two measuring lines. At the initial stage, the vibration speed of the measuring point attenuates rapidly. The attenuation rate of the speed decreases with the increase of the propagation distance. The vibration speed distance curve gradually tends to be flat and the vibration speed in the three directions gradually approaches.

• (2) In the process of equal charge blasting, within the range of 100 –300 m from the throwing blasting area, the radial (X direction) vibration velocity produced by the survey line 2 directly opposite the blasting area is the largest, which is 26.8 cm/s. The attenuation rate of peak vibration velocity of the measuring line 2 in each direction is small. The attenuation percentages within 200 m are 56, 75 and 70%, respectively, which is less than those of the measured lines 1 and 3 on the lateral side of the blasting area.

• (3) In the frequency spectrum after Fourier transformation, the frequency band of throwing blasting vibration is relatively wide and distributed within 200 Hz, while the frequency and energy are mainly distributed in the low frequency stage (0–20 Hz), accounting for more than 50% of the total energy. It can be judged that throwing blasting is a low frequency vibration. In the later protection work, the protective measures to reduce low frequency vibration shall be taken as the main measure.