Abstract: As known that the blasting on the slope is very dangerous in the exploration of the mine andneeds complex analysis and calculation on the slope and dump. In the study it adopts the regression method in the analysis of important parameters of the vibration experiments of iron ore stope which influence the stability of slope and vibration velocity of mine blasting, vibration acceleration, etc., it makes the analysis of power spectrum and dynamic response. Through the model of landslide dynamic response it analyzes the rule of blasting vibration and the blasting vibration influence on the stability of slope, the research can provide a safety criterion in the exploitation of the iron ore.
INTRODUCTION
The stability of the dump has a very important meaning on regular production of mine and safety of workers and machine. The stability of slope is also a very important issue in the process of open pit mining while the vibration mining of blasting has some impact on the stability of dump and slope (Xing, 2004; Zheng et al., 2011; Li et al., 2010; Huang et al., 2011). Therefore, the study of blasting vibration’s influence on the stability of dump and slope has important significance.
The experiment is performed in the Huaheng iron ore which is designed and produced by Sino-steel Engineering Research Institute in 2009 and its production scale is about 80 million tons. In the exploitation and production process it adopts the vertical blast holes cushion blasting instead of the pre-splitting blasting which made fixed slope uneven and cause certain degree of damage (Liu et al., 2001; Ling et al., 1997; Zhu et al., 1997; Li and Tong, 2004).
There are four iron ore dumps from south to north; as known that the blasting vibration has a certain influence on the stability of slope and dump. So it is necessary to carry out blasting vibration test and make the research of blasting laws and selection of security parameters in blasting is very important which can provide the stability of slope and dump (Cai et al., 2006; Liu et al., 2010; Tan et al., 2009; Wang et al., 2008).
Through the experiments, some important security parameters can be obtained andmany meaning results of the stability of slope and dumpWill be gotten.
MATERIALS AND METHODS
Experiments of blasting vibration
Experiment conditions: Fixed slope rock in Huaheng iron ore stope can be
divided into hard rock and soft rock. Specifically, the hard rock includes amphibolite
schist itabirite and mica quartz schist; this type of rock is relatively strong
with structural integrity; As for the soft rock, they are mainly chlorite schist
British class, such rock group is easily weathered suffering from water. When
this type of rock is undergoing faults and weathering affects its slope stability
is extremely weakened.
The basic dump material is amphibolite schist crushed stone and the thickness of the accumulation is about 35 to 50 m and height is about 100-270 m, rest angle is 38-42°.
In the experiment it adopted WS-5921 blasting vibration apparatus and ancillary sensor produced by Beijing Spectrum Century Technology Development Co., Ltd. The collection frequency was set at 1001 Hz and the data acquisition, storage and analysis were obtained in multi-channel.
According to the purposes of vibration experiment and the actual situation in the field, measuring points are arranged in the four dumps and two sets of slope step in open-pit stope which are set on the platform and the stairs of the slope and transportation trail. In order to observe seismic physical quantities within a certain range it is necessary to arrange a large quantity of measuring points along the radial direction with blast center position.
Besides, the choice of measurement points should be considered and representativeness of the measuring points should be selected.
Therefore, the measuring point should be arranged in the different height and slope surface perpendicular to the same profile and the different distance on the slope surface.
Results of the experiments: In the experiment, five blasting vibration tests are conducted. Each point is monitored once and the slope of stope is monitored twice through using software and the analysis is shown as Fig. 1 and 2.
Each monitoring blasting parameters are shown in Table 1; the results of vibration velocity monitoring of blasting vibration and vibration acceleration monitoring results are shown in Table 2 and 3, respectively.
ANALYSIS OF BLASTING VIBRATION LAW
Regression analysis of blasting vibration without considering the altitude effect: According to “Blasting Safety Regulations” and research results around the world, blasting vibration propagation and attenuation law are generally adopted the Sa Rodolfo J Ki equation (Dowding, 1992):
| Table 1: | Parameters of the blasting source in the tests |
| Fig. 1: | Monitoring results obtained through WS-DAQ data acquisition software (Dump) |
| Fig. 2: | Monitoring results obtained through WS-DAQ data acquisition software (Slope) |
| Table 2: | Monitoring results of vibration velocity for blasting vibration test |
| Table 3: | Monitoring results of vibration acceleration for blasting vibration test |
| (1) |
In this equation: Vvertical is the maximum speed value of The bursting (cm sec-1); K is the Coefficient related to the geology and blasting methods; α is coefficient related to the Geological conditions and seismic wave attenuation; Q is the peak vibration velocity value correspondi ng to the maximum period of initiation dose (kg); R is a straight-line distance between the measuring point and the Ground Zero (m); ρ is Proportional dose:
During the analysis, K and α are the unknown coefficients. Similarly, the acceleration can also use Sa Rodolfo J Ki equation regression:
| (2) |
In this equation: aacceleration is the bursting of the maximum acceleration value(g).
Regression analysis results of blasting vibration in the dump: Through Eq. 1 and 2, regression analysis of the measured data in Table 2 and 3 can be made, the blasting vibration velocity and acceleration propagation law in vertical and horizontal direction can be obtained without considering the elevation amplification effect. Velocity attenuation equation are represented as:
| (3) |
| (4) |
Acceleration attenuation equation are represented as Eq. 5 and 6:
| (5) |
| (6) |
Regression analysis of slope blasting vibration: By using Eq. 1 and 2 and making the regression analysis of the measured data in Table 2 and 3, blasting vibration velocity and acceleration propagation law in vertical and horizontal direction are obtained without considering the elevation amplification effect.
Velocity attenuation equation are Eq. 7 and 8:
| (7) |
| (8) |
Acceleration attenuation equation are as Eq. 9 and 10.
| (9) |
| (10) |
Regression analysis of blasting vibration with the elevation amplification effect: Elevation amplification effect is a phenomenon that particle vibration velocity increases with the increase of the elevation difference between measuring points and blasting center.
The elevation amplification effect of blasting vibration velocity is not only related to the factor of integrity, lithology, slope, blasting scale but also the slope of the mountain thickness. When the rockmass has a better integrity, a steep slope and the blasting is relatively in a large scale, the velocity elevation amplification effect does exist in some slope. But this amplification is only limited to a certain height range. In most cases, the vibration speed of blasting is lessened with the increase of blasting center distance, elevation and excavation depth.
In the high steep slope of different regions, the slope face and its angles affect blasting seismic wave with different degree. When the slope is greater than 1:2, the amplification effect of the slope will come up; on the contrary, the amplification effect will disappears. In addition, the slope takes an effect on intensity and frequency of wave of the blasting vibration; therefore it is related to the slope stability.
When the slope is large, the concrete vibration velocity of measuring point presents the attenuation distribution with the increasing of propagation distance and it is increased with the increase of slope gradient, slope height. The characteristic is determined by the slope elevation, slope ratio, the geological condition and explosive source energy.
Therefore, as to the slope, amplification effect needs to be considered. But as for the waste dump, because of its loose structure, slope angle is less than slope, there is no need to consider the amplification effect.
As for the slope, Eq. 1 can be expressed as follows:
| (11) |
Equation 2 can be amended as Eq. 3:
| (12) |
In this equation: S is the horizontal distance between Ground Zero and points (m); ε is proportional elevation, ε = R/S; β is the elevation influence coefficient. In the analysis, K, α, β are regression coefficients. Through regression analysis of measured data in Table 2 and 3 using Eq. 7 and 8, the vertical, horizontal direction blasting vibration velocity, acceleration propagation laws are obtained when the elevation amplification effect is taken into consideration:
Velocity attenuation equation are as follows:
| (13) |
| (14) |
Acceleration attenuation equation:
| (15) |
| (16) |
Analysis of seismic wave frequency: The power spectrum analysis is a method combining Fourier analysis and statistical analysis. It shows that the frequency distribution of the seismic wave signal and give out the main frequency of blasting vibration frequency spectrum. The t represents a function of time, such as displacement, particle velocity and acceleration, therefore, the frequency of the seismic wave signal X (t) is proportional to the amplitude. The following equation represents the power spectrum:
| (17) |
Where, GX(t) is Power spectrum; X(t) is Seismic waveform amplitude; F(w) is Seismic wave spectrum.
There are many factors affect the frequency and the factors that affect the seismic amplitude also can affect the size of the frequency, such as the explosive performance of equivalent size, charging structure, initiation, site geological conditions and explosive center distance.
In the power spectrum analysis, vibration wave produced by blasting frequency is in the range of 10-35 Hz and average value is 17.35 Hz; The main frequency in vertical direction was greater than that in horizontal direction. Compared with the natural seismic waves, seismic wave frequency of blasting is much greater and the strength is decreased more rapidly, the duration time is shorter, so its destructive was much smaller than natural earthquakes.
It can be founded that the seismic wave with frequency range of 20-50 Hz, has greater influence on the slope.
RESULTS AND DISCUSSION
DYNAMIC RESPONSE ANALYSIS OF THE SLID SLOPE
The landslide dynamic response analysis model: Landslide dynamic response analysis model is as shown in Fig. 3, rock mass vibration caused by the blasting vibration can be looked as simple harmonic motion. Taking a cell body on a step sliding surface as research unit, the cell body is affected by the blasting vibration acceleration αL effect in the horizontal direction. When the inertial force F caused by αL reaches the max values, F cosα will increase the downward force of unit with maximum extent, F sin α can minimize the adhesion of the unit and the bedrock to the maximum extent. At this time, the inertial force caused by the blasting vibration effecting on the decline of the unit to achieve the maximum which is the most dangerous state of landslides caused by blasting.
Relationship between blasting vibration acceleration and slope safety factor: The basic principle of the movement unit is through a strict automatically search process to find plastic sliding boundary conditions and the most dangerous sliding surface satisfying with the constraint equations and related district conditions and then safety factors of the slope can be obtained. This method consists of three parts, namely, unit motion analysis, unit static analysis and demand multivariate extreme value of the objective function. The unit motion is determined by the relative motion direction and shear direction posed by cell boundary between units; through the static analysis, the normal force on the cell boundaries can be obtained; Multi-variable optimization is to search the most dangerous slip surface of the slope which has the minimum safety factor i.e., the actual safety factor.
Considering the most adverse circumstances, the effect of horizontal acceleration will increase the sliding force of the slope and reduce the potential slip surface positive pressure (N). Because rock cannot move along the normal direction of the slip surface, the force of the normal direction is satisfied with the below balance equation.
Fsinα+N = Gcosα |
(18) |
Then:
N = Gcosα-Fsinα |
(19) |
Stabilizing force FB and FA sliding force are are given by the following equation:
FB = Nxtgφ+CA = (G cosα-
F sinα)xtgφ+CA |
(20) |
FA = Gsinα+Fcosα |
(21) |
Blasting power equivalent static value F can be determined by the following method, According to blasting acceleration measured values, acceleration αL can be regressed which is showed in a linear regression equation and thus equivalent static force value of blasting power can be obtained through Eq. 22:
F = β0Kc m
(Kc=αL/g) |
(22) |
Where F is blasting vibration force; N is slip surface of positive pressure; Gis rock weight; [αL] = 0.0644x980/0.13 = 485.48 cm sec-2 is the unit body with horizontal tangent angle; n is the friction angle; φ is bond of rock force, A is slippery surface area; β0 is blasting power conversion coefficient, the value is about 0.1- 0.3; Kc is the seismic coefficient; m is the slope of potential landslide mass;αL of vibration acceleration; g is acceleration due to gravity.
Slope stability depends on the ratio of stabilizing force FB and down force FA. Therefore, when take the role of blasting power into consideration, slope safety factor can be calculated through Eq. 23:
Kd = FB/ FA = [(Gcosα-Fsinα)xtgφ+CA]/(Gsinα+Fcosα) |
(23) |
The above equation indicates that when considering the impact of blasting vibration horizontal force, stabilizing forced and down force are decreased and slope stability also is decreased.
Determination of critical slope safety factor and horizontal direction critical vibration acceleration: The determination of critical slope safety factor should ensure it leave some margin of safety for the slope under static and blasting dynamic loads and that the safety factor must be greater than 1 to ensure non-occurrence of the sliding slope.
If consider safety need of on-site production, the safety factor cannot be too large to avoid restrictions on blasting segment dose which brings too much difficulty to the adjacent slope blasting. For the specific circumstances in open-air Huaheng iron ore stope and the experience of similar projects, when the critical slope safety factor landslide [Kd] is 1.0419, corresponding to the license seismic coefficient is 5.82, according to the following Eq. 24.
| (24) |
It can be calculated that [αL] = 0.0644x980/0.13 = 485.48 cm sec-2.
Critical vertical vibration acceleration: single level of vibration conditions are follows:
As can be seen from Fig. 4 unit body on the sliding surface is loaded by vibration horizontal force F from an alternating radial direction of source. When the horizontal force F is pointed to the stepped surface, the limit equilibrium equation of the unit body is:
| Fig. 3: | Landslide dynamic response analysis model |
| Fig. 4: | Analysis model for landslide by the vertical vibration impact |
FA+Fcosα = FB-Fsinαtanφ |
(25) |
Where, FA is Stabilizing force N; FB is Stabilizing force N; α is The unit body at the angle between the tangent line and the horizontal direction, φ is Friction angle.
When Horizontal force F is pointed to the original rock unit, body force limit equilibrium equation:
FA-Fcosα<FB-Fsinαtanφ |
(26) |
Equation 26 indicates that the slope side is under the conditions of safety and single vertical vibration conditions.
As shown in Fig. 4, the unit body in the slide potential surface is loaded with the vertical force f from the vertical blasting vibration up and down directions. When f is upward, the force of limit equilibrium analysis unit body can be obtained through Eq. 27.
FA-f sinα = FB-f
cosαtanφ |
(27) |
When the direction of f is downwards, Limit equilibrium analysis unit body force is:
FA+f sinα = FB-f
cosαtanφ |
(28) |
Combined with equation 24, 27 and 28, the Eq. 29 and 30 can be obtained.
| (29) |
| (30) |
Because α>φ, Eq. 29 can be only to calculate the relationship between horizontal and vertical load on calculated slope. Apart from that it can also get the following formula from the relationship between the load and acceleration:
| (31) |
Taken α = 42°, φ = 38.62°,into the Eq. 31 Vertical vibration acceleration value can be obtained as:
| (32) |
Calculation of critical vertical vibration velocity: When the particle is on simple harmonic motion, the relationship between particle acceleration and velocity V can be written as Eq. 33:
| (33) |
Where,
| (34) |
ACTIVE CONTROL OF SLOPE STABILITY
The prime factor of affecting the intensity of blasting seismic is the particle velocity. It can be found from the equation of Sa Rodolfo J Ki (Dowding, 1992; Wang et al., 2008), the variable α is changed with the geological conditions, that is, the different geological conditions have different capacities for blasting seismic wave attenuation absorption. Besides, human-controlled factors are Q and K Particularly, if reduce the period of initiation dose Q, the blasting seismic intensity can be directly reduced; in addition, blasting seismic intensity can also be reduced by reducing the blasting control means , thus it can reduce the value of K.
The charge of proportional dose could control the blasting vibration from source. Particularly, the allowed proportion drug dose should be determined by the blasting site and slope distance and critical vertical vibration velocity substituted into the regression equation during each blast process.
| (35) |
Before the mining blasting, according to the distance from detonating center to the slope, from Eq. 35, it can be used to calculate the maximum dose for the same paragraph which can avoid the threat posed on slope stability by blasting vibration.
PARAMETER CONTROL OF SEGMENT DOSE OF SLOPE IN BLASTING PRODUCTION
Rock particle vibration velocity caused by blasting vibration is an important parameter to impact slope stability. According to the current research and engineering conditions, under normal circumstances it is the simple and effective method to determine whether the slope is dangerous by using slope rock mass particle vibration velocity as the main indicators. It also should consider the particle vibration acceleration through the data obtained from the blasting test and comprehensive analysis of parameters of slope rock mass. The allowed vibration velocity is 17.56 cm sec-1, horizontal critical acceleration is 438.74 cm sec-2, the vertical critical acceleration is 1913.05 cm sec-2. According to equation (35) and then put the largest blasting quantity of blasting at different distances to guide the mine safety production into consideration. Quantity reference value in the largest blasting slope at different distances in Huaheng iron ore, as shown in Table 4.
STABILITY ANALYSIS OF BLASTING IMPACT ON THE DUMP
Dump is mainly formed by a row of loose soil material and there is a big gap between the particles.
| Table 4: | Dose reference values in the maximum segment for the different distances in production blasting |
On one hand, the blasting vibration scattered in the boundary the impact of vibration to the dump; On the other hand, surface of the dump material and slope rock can weaken due to the composition of the material dump, the loose accumulation of dump material can make the vibration ransmit to the internal of dump it can be absorbed by the gap and thus dissipate further the effects of vibration. At the same time, there does not exist elevation amplification effect in the dump, therefore, when considering the impact of blasting vibration it should be accuracy to the slope.
Through the data obtained from the blasting test and comprehensive analysis of parameters of slope rock mass, the allowable value of the vibration velocity is 17.56 cm sec-1, horizontal critical acceleration is 438.74 cm sec-2, the vertical critical acceleration is 1913.05 cm sec-2. While, the measured data are less than these three data; On the other hand, from the control of the particle velocity it is basically within the scope of the specification, therefore, the blasting has a small impact on slope stability and has a far less impact on dump than the slope.
CONCLUSION
In the study, the maximum segment dose are presented according to the most adverse impact on slope stability which can play some significance in mine safety production. And it is recommended that follow-up production control the charge amount should be strictly in accordance with the blasting safety regulations. And the study provides many meaningful results andthe study will provide the guidance in the safety production of the mine.