Effect Of Open-Pit Blasting Vibrations on a Hanging-Wall Slope: A Case Study of the Beizhan Iron Mine in China

With the increase in the depth of large-scale open-pit mining, many mines have to face problems such as what is the effect of open-pit blasting on the rock slope and how to ensure the stability of a high-steep slope. The east slope of the Beizhan iron mine in Hejing County, Xinjiang, belongs to the typical open-pit high and steep slope of mined hanging-wall ore. To study the effect of open-pit blasting vibrations on hanging-wall slope stability, the intelligent blasting vibration detector was used to monitor the open-pit blasting wave of the Beizhan iron mine and the corresponding numerical model was established. We fitted the transmission law of slope blasting vibration by Sodev’s regression formula and calculated stress, strain, and vibration velocity of the whole slope by numerical simulation. The result showed that the fitting functional relationship was correct and could be the basic rule of predicting the maximum charge amount per delay interval and minimum safe distance for this area. The estimated open-pit blasting charge weight was reasonable and blasting vibrations would have little effect on the hangingwall slope. The research method and conclusions in this article have numerous reference values for studying the hanging-wall slope’s kinetic stability.


Introduction
With the quickened pace of large-scale open-pit mining and open-pit mining depth increasing, more and more highsteep slopes are formed. Most mine slopes face problems such as the impact of open-pit blasting on hanging-wall ores and how to ensure the stability of high-steep slopes [1][2][3]. The hanging-wall ore is part of the ore body extending outside the open-pit boundary [4]. Large-scale blasting during open-pit mining caused great stress disturbance to the surrounding rock mass and stress concentration formed at the bottom of a pit and the foot of a slope. When the hangingwall ore is mined, a more complex secondary stress field will be formed, causing further deformation and destruction of the surrounding rock of the slope and even engineering disasters such as landslides.
Given the influence of open-pit blasting on high and steep slopes, the commonly used research methods are blast-ing vibration tests and numerical simulation. The effect of controlled blasting on the stability of mine slopes was studied by Singh et al. [5] through presplit blasting tests. Hu et al. [6] analyzed the correlation between blasting damage depth and peak particle velocity and proposed a new damagevibration coupling control method for high rock slopes. Yin et al. [7] and others monitored blasting vibration signals using different rock blasting scenes and studied the attenuation characteristic of blasting vibration waves transmitted in the joint rock slope. Rajmeny and Shrimali [8] monitored the sliding of the hanging wall caused by multiple blasting and fitted predictive equations for blast vibration at the Rampura Agucha Mine. Tao et al. [9] used remote monitoring and early warning system to continuously monitor a slope. The monitoring results showed that the open-pit mining technology of a "mechanical gun" instead of "blasting" had effectively reduced the impact of mining disturbance on the stability of the western slope. Bao and others [10] monitored an open-pit mine using a blasting vibration instrument and they found that the main influencing factors for slope stability were vibration velocity and duration. Xie et al. [11] monitored an open-pit mine slope using a blasting vibration system, and they found that changing the blasting order and direction could reduce blasting vibrations. Ma et al. [12] proved that presplitting blasting played a major role in absorbing blasting shock waves based on the blasting vibration testing of an open-pit mine slope. The numerical simulation analysis method is a common tool to solve mining rock mechanics problems. Hu et al. [13] proposed the relationship between peak vibration velocity and soil properties based on the improved Sodev equation. The classical Sodev empirical formula was improved by Ma et al. [14] to predict the vibration velocity of pipe explosions. A linear fitting model based on Sodev's empirical formula was devel-oped, and blast vibration data from lighthouses near the blast area were monitored by Gu et al. [15]. Researchers used numerical simulation programs such as the finite element method (FEM) [16][17][18][19][20][21][22], discrete element method (DEM) [23][24][25][26][27], and finite difference method (FDM) [28][29][30][31][32] to study the dynamic response characteristics of blasting vibration to high and steep slopes. These response characteristics mainly included vibration velocity, displacement, amplification factor, frequency, stress, and strain of slope after blasting. Hu et al. used LS-DYNA software to study the effect of smooth blasting and presplitting blasting on rock highslope damage and analyzed the spatial distribution of blasting damage [16,33,34]. Jiang et al. [35] used blasting excavation of Beijing Metro Line 16 as an example to monitor blasting vibration in the field. A three-dimensional numerical model was established to analyze the response   In this paper, vibration monitoring of Beizhan's open-pit mine slope was carried out by using NUBOX-6016 intelligent vibration monitors. According to the measured blasting vibration wave data, the propagation law of blasting seismic waves on high and steep slopes was studied. Then, the numerical model of the eastern slope of the Beizhan iron mine was established and the measured velocity timehistory load at the bottom of the slope was input. Finally, the dynamic response characteristics such as stress, strain, and vibration velocity of high and steep slopes under open-pit blasting vibration were analyzed.  3 Geofluids coming to an end. In order to maintain the normal production of the mine and ensure the smooth transition from open-pit mining to underground mining, the hanging-wall ore is currently being mined. The present situation of open-pit slope mining and the morphological characteristics of hanging-wall ore are shown in Figure 1.

Blasting Vibration
Test. The blasting area was located at the 3380 elevation mining area on the eastern slope of the pit. This blasting was for progressive mining down to a 3368 elevation. The blasting area was surrounded by buildings of 3404 flat caverns, pumping, and pumping pipes, which were close to the blasting area. The blasting area was all ore. The 3380 m steps were constructed from south to north undercut as required. The main borehole spacing was 2.5 m, the row spacing was 2.0 m, and the inclination angle was 80°. Millisecond delay blasting between rows was adopted in this blasting scheme. Blasting was launched from the west to east row by row in order to achieve the design purpose. The blasting sequence was rowed to row. The way of blasting was reverse initiation. That was, the detonating charge was located at the bottom of the hole and the direction of the energy accumulation hole of the detonator was towards the opening of the hole. The detonating network adopted an electronic-delay detonator. The delay time was set by the electronic detonator and the blasting was carried out from west to east in sections in order to achieve the purpose of blasting in sections and rows.
This test used the NUBOX-6016 intelligent vibration monitor produced by Sichuan Tuopu Measurement and Control Technology Company Ltd. The instrument is connected to a TP3V-4.5 3D velocity-type sensor through a special input signal cable. The first channel of the device is connected to the horizontal x-direction vibration signal,   The horizontal x-direction and vertical z-direction vibration waves caused by blasting vibration have the greatest influence on the slope stability. Therefore, in the numerical simulation, the xand z-direction vibration wave signals collected from the test were loaded into the finite element model of the slope at the same time for analysis. The current slope profile of the hanging-wall ore in the Beizhan iron mine was selected for the study. A numerical model was established according to the shape of the slope and stratigraphic distribution (Figure 3). The model size was 587 m × 716 m, and the maximum step height was 36 m. In order to simulate the near slope blasting, the loading area of the blast load was located on the left boundary of the model, 14 m from the foot of the bottom step. The lithology of the magnetite ore body (layer) in the top and bottom parts is epidote skarn, diopside epidote skarn, tremolite skarn, and tremolite wollastonite skarn, which has great difference in rock integrity. The magnetite massive rock group is mainly composed of brecciated magnetite, disseminated magnetite, and massive magnetite. Its structure is tight, and the joints and fissures are not developed. The ore body has good structural solidity. The statistical values of the physical and mechanical properties of the slope rock test results are shown in Table 1.
This study adopted the low reflection boundary method. The slope surface adopted a free boundary and the rest adopted a low reflection boundary. By default, low reflection boundaries fetch data from adjacent domain materials. The nodes of low reflection boundary let the nonreflection waves flow out with the model to establish a perfect impedance matching for the P wave and S wave. Therefore, n and t were the unit normal vector and the tangent vector to the boundary, respectively. c p and c s were each represented longitudinal wave speed and transverse speed of the material, respectively. This method was most effective when the wave approaches the normal direction of the boundary.
In order to calculate the mechanical characteristics of the slope under different working conditions, three research steps were set up in this study. In the first step, the state of ground stress of the slope before the hanging gang mine was mined was calculated. In the second step, the stress state of the slope after mining was calculated based on the calculation results of the first step. In the third step, the impact of blasting on the slope of the hanging gang mine was calculated based on the first two steps and the response characteristics of the slope to blasting vibration were analyzed.
In general, the vertical and horizontal radial vibration rates have a greater impact on slope stability, while the horizontal tangential rate has a smaller impact on that. In order to avoid large errors in the calculation, this study choses the velocity time history curve that is near the measuring point of the blasting source in the production blasting when calculated as the blasting dynamic load. Only the horizontal radial and vertical vibration rates of this measurement point were selected. The selected time curves of vertical and horizontal radial vibration rates were shown in Figure 4.

Results and Analysis of Blast Vibration Monitoring.
Three blast vibrations in the open pit were monitored using NUBOX-6016 blast vibration detectors. Three monitoring layouts were similar, all along the direction away from the blast source of the 4 monitoring points arranged in turn ( Figure 2). The sensor on the monitoring point has the function of monitoring the horizontal radial, horizontal tangential, and vertical direction vibration speed simultaneously. Blast vibration monitoring results are shown in Table 2.
Considering the influence of the maximum charge and the distance from the explosive center on the vibration speed according to the blasting safety regulations, the mathematical method of mathematical statistics was used to cope with the measured data by regression analysis. Sodev's regression formula is In the formula, V stands for the maximum 3D synthesis velocity rate of the vibrating particle, cm/s; k and α are coefficients related to the topography and geological conditions between the blasting point and the protected object, respectively; Q is for the maximum single-section charge, kg; R is for the linear distance from the measuring point to the blasting source, m.
A least-mean-square linear regression operation was performed using Sadovsky's formula to fit a straight line ( Figure 5). With this, to determine the regression equation, k = 216:156, α = 1:739, and the correlation coefficient R value was 0.908. This equation can be used as the basis for predicting the maximum single-section charge and minimum safe distance at this site. So, the transmitted law formula of blasting vibration for east slope hanging-wall ore was V = 216:156

Geofluids
The previous experiments could tell that the maximum 3D synthetic velocity of blasting vibration in the toe of the slope was 5.986 cm/s. When the master frequency was greater than 10 Hz and less than 50 Hz according to the blasting safety regulations, the safety allowing the velocity range of particle vibration for a permanent rock high steep slope was 8~12 cm/s. The present blasting vibration of the open-pit mine had little impact on slope stability. The slope of the Beizhan iron mine had hanging-wall ore. However, there were no rules for safety allowing the velocity of particle vibration of such slope in the blasting safety regulations. Therefore, in order to study the effect of blast-ing vibration on this slope, a numerical simulation of this slope was established.

Result and Discussion of Numerical Simulation
3.2.1. Analysis of Stress and Strain. In order to analyze the effect of open-pit blasting on the slope, von Mises stress clouds were obtained from the simulation results before hanging-wall ore mining, after hanging-wall ore mining, and after open-pit blasting ( Figure 6). In Figure 6(a), it can be seen that the stresses on the slope before the hanging-wall ore mining were caused by gravity. The 7 Geofluids maximum value was 9.60 MPa in A. There was no plastic strain in the slope. In Figure 6(b), it can be seen that the hanging-wall ore mining caused the redistribution of internal stresses in the slope. The maximum value appears in region B with 23.95 MPa. The contour of the figure shows the plastic strain with a maximum value of 608 με. The greatest stresses were experienced at the location of the ore pillar in area B, and plastic deformation occurred. In Figure 6(c), it can be seen that blast vibration again causes the redistribution of internal stresses in the slope. The von Mises stress maximum occurred in the B region and varied with blasting time, as shown in Figure 6(d). Taking the calculated stress response into account in the second step, the maximum value was 23.95 MPa at 0 s. When the blasting finished at 0.76 s, the maximum value was 23.41 MPa. The peak maximum value was 24.09 MPa which happened at 0.095 s. The contour lines in Figure 6 (c) are plastic strain, located in the pillar at point B, and the maximum value is 47.6 με. Compared with the stresses after hanging-wall ore mining, the stress distribution pattern of the slope after open-pit blasting is not much different.
In order to analyze the effect of open blasting on the slope surface, the variation of von Mises stress on the slope surface was calculated. The red line in Figure 7(a) shows the slope surface, where points A, B, C, and D are the locations of the foot of the slope immediately adjacent to the hanging-wall ore. We assume that the horizontal coordinate of the location of the lowest foot of the slope is 0. Figure 7 Figure 7 (c), it can be seen that among the four points, the stress at point C is the largest, 18.5 MPa at 0 s and 12.9 MPa at 0.76 s. The remaining points also showed different degrees of reduction, with point A decreasing from 10.8 MPa to 7.6 MPa, point B decreasing from 13.1 MPa to 9.3 MPa, and point D decreasing from 7.6 MPa to 5.5 MPa. The pillar of the slope adjacent to the hanging-wall ore is the security pillar, which is the weak link in the whole hanging-wall ore slope system. The surface of the security pillar has the highest force among the whole slope surface after hanging-wall ore mining. As shown in Figure 7(c), the stress distribution pattern of the slope surface after open-pit blasting is not significantly different compared with the stress after hanging gang mining. This indicates that open-pit blasting did not increase the stresses on the slope surface of the security pillar.

Comparative
Analysis of Vibration Speed. The combined vibration velocity of each moment could be collected after open-pit blasting by numerical simulation. Results show that the combined vibration velocity of the slope reached its maximum value when t = 0:265 s. The contour plot of the combined vibration velocity is shown in Figure 8(a). The combined vibration velocity of the slope surface reaches the maximum at t = 0:325 s. The contour plot of its combined vibration velocity is shown in Figure 8 (b).
As can be seen in Figure 8(a), the peak vibration velocity is located in the first row of the hanging-wall ore column, 7  8 Geofluids with a maximum value of about 7.40 cm/s. In Figure 8(b), it can be seen that the peak vibration velocity is located near position D and the maximum value is about 6.97 cm/s. According to the blasting safety regulations, when the main vibration frequency is greater than 10 Hz and less than 50 Hz, the safe allowable mass vibration velocity range for the mine tunnel is 18-25 cm/s. The safe allowable mass vibration velocity range for permanent rocky high slopes is 8 to 12 cm/s. It can be seen that the maximum peak value of particle vibration velocity of the slope under open blasting vibration is less than the safe allowable value of particle vibration velocity of the slope. It indicates that the current blasting vibration has little effect on this slope, which is stable under the joint action of dynamic and static loads.

Conclusions
According to the test data of blasting vibration wave monitoring on site, Sodev's formula was used to fit the blasting vibration propagation law of the east slope. The results of numerical simulation analysis showed that the current blasting vibration had little effect on this slope and the slope was stable under the combined action of dynamic and static loads. This can be used as the basis for predicting the maximum single-segment dose and minimum safe distance at this site.
It should be noted that in this paper, the focus was on the effect of open-pit blasting on the east slope of the Beizhan iron ore mine, ignoring the freeze-thaw action on the slope surface, anisotropy, and nonuniformity of the actual rock mass. We will take these factors into account for the next step of our research. The research methods and conclusions of this paper are good references for studying the dynamic stability of slopes with hanging-wall ore mines.

Data Availability
This study was approved and assisted by Hejing County Beizhan Mining Co. Ltd. The data used to support the findings of this study are included in the article.

Conflicts of Interest
The authors declare that they have no conflicts of interest regarding the publication of this paper.