Mine blasting vibration and control technology and earthquake reduction measures

In the mine blasting construction, due to the size of the blasting, the method of blasting, the free space of blasting and the environmental conditions of the blasting area, the vibration caused by the blasting, the air shock wave, the noise, the toxic gas and the flying stone caused by the open blasting, to the surrounding The environment, buildings (structures), facilities and personnel will have varying degrees of impact. In particular, the damage caused by blasting vibration is more serious. It not only has adverse effects on the surrounding structure (structure), but also causes civil disputes between the mine and local villagers. At present, most mining companies, in order to avoid and reduce blasting vibration, the main measure is to reduce the amount of blasting explosives. The amount of blasting explosives is reduced, and the total amount of blasted ore is also reduced, which in turn affects the mining intensity and the increase in production scale in mine development. Therefore, it is very necessary to study and analyze the control technology and earthquake-reduction measures of mine blasting vibration, and it is also an important task to ensure the normal production order in mine development.

2Blasting vibration and propagation of blasting seismic waves

2.1 Blasting Vibration and Formation of Blasting Seismic Waves

Blasting “destruction” is the process of releasing, transferring and working on the energy of explosives. This process is very short, only a few tens of microseconds. In this short period of time, the explosive charge exploded in the rock, and the stress wave formed by the detonation propagated from the center of the drug packet, that is, the explosion center to the surroundings, firstly causing crushing rings and rupture rings in the rock surrounding the drug pack. The size of the broken ring and the rupture ring, determined by the type and quantity of the explosive, and the nature of the rock, form a crush ring and a rupture ring, which is a useful work of the explosive explosion we hope to obtain. When the stress wave passes through the rupture ring, its strength can be abruptly attenuated, which can no longer cause the rock to rupture. It can only cause the rock mass to generate elastic vibration and propagate outward in the form of elastic waves. This elastic wave is also called seismic wave. . When the blasting seismic wave propagates to the surface, it will cause surface vibration, which is blasting vibration. The resulting phenomenon of bumps and shaking on the ground and objects on the ground is called a seismic effect. The occurrence and propagation of blasting vibration, although the time is very short, but without control, brings great harm. 

2.2 Characteristics of blasting seismic waves

Seismic waves have more complex waveforms, but the whole process can be roughly divided into three parts: (1) the initial phase; (2) the main phase; and (3) the aftershock. As shown in the figure, the amplitude of the main shock phase is the largest, so the destructiveness is also the largest. The peak value of the blasting seismic wave is related to the amount of charge and the distance to the source, and changes with it.

Seismic waves are composed of several kinds of waves. According to different ways of wave propagation, they are roughly divided into two types: volume waves composed mainly of longitudinal waves and transverse waves, and surface waves composed mainly of Rayleigh waves and Love waves. The volume wave, especially the longitudinal wave, can cause compression and tensile deformation of the rock, which is the main cause of rock rupture during blasting. The surface wave, especially the Rayleigh wave, is the main cause of earthquake damage due to its low frequency, slow decay and carrying more energy.

2.3 The law and characteristics of seismic wave propagation

2.3.1 The energy carried by the seismic wave is very small. When the explosive is exploded, although the energy used to break the rock only accounts for 10% to 15% of the explosive energy released by the explosive, it does not exceed 25% when loosely blasting, but the energy converted into the seismic wave is smaller. It only accounts for a few percent of the total energy released by the explosion of explosives, and varies slightly with the nature of the rock, about 2% to 3% in dry soil and about 5% to 6% in wet soil. About 2% to 6% in rocks.

2.3.2 Blasting seismic waves are different from natural seismic waves Although blasting earthquakes and natural earthquakes belong to the phenomenon of surface vibration caused by energy release, there are obvious differences between the two. First, the frequency is different, the natural seismic frequency is very low, the blasting seismic frequency is higher, from tens to hundreds of Hz; the second is the wave attenuation rate is different, the natural seismic wave decays slowly, the blasting seismic wave decays quickly; the third is the duration Unlike natural earthquakes, which often last for several minutes, the duration of a blasting earthquake does not exceed hundreds of milliseconds.

2.3.3 Frequency of blasting seismic waves related to explosive properties The experimental description shows that the blasting seismic waves generated during the explosion of explosives are affected by the nature of the explosives. The detonation pressure of the low detonation explosives rises slowly, and the blasting vibration generated is also small. On the contrary, the detonation pressure of the high detonation explosives rises rapidly, and the blasting vibration generated is also large.

2.3.4 Blasting seismic wave propagation is affected by topographic geological conditions. Blasting earthquakes propagate slowly in hard rock, and the attenuation is fast, while in soft rock, the propagation is fast and the attenuation is slow. When the blasting seismic wave encounters faults, fissures, cleavage planes, goafs, roadways, valleys, and ravines, the cracking degree is significantly reduced.

2.3.5 Blasting seismic wave intensity and blasting methods and parameters In blasting construction, the blasting method directly affects the intensity of blasting vibration. If the burst is triggered by the burst or the moment, the blasting vibration intensity generated is large; when the differential is used, the blasting intensity generated is small. In addition, the blasting vibration intensity is also directly related to the parameters such as the explosive charge and the blasting effect index.

2.4 Blasting seismic vibration intensity and criteria

2.4.1 Physical quantity measuring blasting vibration intensity The physical quantity of blasting vibration intensity is very complicated, but the main ones are particle vibration displacement, particle vibration velocity and vibration acceleration. Which kind of quantity can best reflect the intensity of blasting vibration? Up to now, the opinions of experts at home and abroad cannot be unified, but most people think that it is more appropriate to select the vibration speed of the particle. China GB6722-86 "Blasting Safety Regulations" is also based on the vibration velocity of the particle as the criterion for determining the blasting vibration intensity.

2.4.2 Calculation of Physical Quantity of Blasting Vibration A large number of studies and actual tests have shown that the vibration velocity of a particle is directly proportional to the amount of explosive that is detonated at one time, and inversely proportional to the distance to the source of the explosion. Because the conditions of the test are different, the calculation formulas obtained by countries are different, but the trend is the same. When the blasting effect index is 1, the commonly used particle velocity calculation formula [1] in China is:

In the formula, 安R An - blasting seismic safety distance, m.

3 Control of blasting vibration and measures for earthquake reduction

Mine blasting, whether it is medium-deep hole blasting in underground mines or large-scale open blasting, must pay attention to the hazards of blasting vibration. In particular, blasting in areas close to village houses or fixed buildings (structures), in order to ensure safety and avoid civil disputes, the hazards of blasting vibration must be controlled within the allowable range.

3.1 Select reasonable blasting parameters to reduce blasting vibration

3.1.1 Selecting the appropriate blasting index for large-scale open blasting, especially in open-air chamber blasting, the value of n value of blasting index is large, which affects the blasting vibration intensity. In a certain range, they are inversely proportional. relationship. According to the data, the throwing blast with n of 1.5 can reduce the vibration speed by 4% to 22% compared with the loose blast with n of 0.8 [2]. Therefore, in mine blasting, the maximum should be obtained as much as possible.

Loose blasting effect to reduce the intensity of blasting vibration. In the medium-deep hole blasting of underground mines, while the n value of the blasting action index is reasonably selected, a certain free space must be created to maximize the blasting.

3.1.2 The parameters of the hole network should be reasonably based on the differential principle of the blasting mechanism. In order to achieve the purpose of safety and rationality, the explosives are evenly distributed in the rock mass to prevent the energy from being concentrated and the blasting vibration intensity is reduced. This requires the selection of reasonable hole network parameters in the blasting design. First, the blasting hole density coefficient should be greater than 1; the second is to use the new technology of large hole spacing and small row blasting; the third is to reduce the blasthole ultra deep; The length of the block in the middle hole of the fan should be reasonable to prevent the concentration of the orifice from being concentrated; the fifth is to use the space in the hole.

3.1.3 Taking the appropriate unit explosive consumption unit Explosive consumption is a very important parameter for calculating the explosive quantity in the blasting design. It not only determines the effect of ensuring the blasting effect, but also affects the intensity of the blasting vibration. Excessive explosive unit consumption will increase blasting vibration and air shock waves, and cause excessive movement or throwing of rock blocks. On the contrary, the consumption of the explosive is too small, and the blasting vibration is increased by delaying and reducing the stretching wave effect reflected from the free surface. The optimal explosive unit consumption is determined by field testing and long-term practice.

3.1.4 Controlling the amount of blasting explosives The maximum amount of explosives at the time of blasting is proportional to the intensity of blasting vibration. The greater the amount of blasting, the greater the blasting vibration intensity. The amount of explosives must be strictly controlled during blasting. When the mining intensity needs to increase the amount of explosives, it must be detonated by segmentation (including inter-segment segmentation, inter-hole segmentation and intra-pore segmentation), but it does not affect the total blasting charge and the total amount of blasting ore to meet the production. need.

3.2 Using differential differential technology to reduce the intensity of blasting vibration

3.2.1 The differential detonation differential detonation is to sequentially blast the total amount of blasting in groups of milliseconds. This fully conforms to the differential principle of the blasting mechanism and has a great effect on reducing the blasting seismic effect. A large number of experimental studies have shown that the vibration intensity of the differential detonation is reduced by 1/3 to 2/3 compared to the uniform blasting when the total charge and other conditions are the same. The formula for calculating the vibration reduction rate [2] is:

δ = (V-V1) / V = ​​1 - η2/3(3)

δ in the formula - vibration reduction rate, %;

V ——The vibration velocity of the bursting point of the blasting, cm/s;

V1——the vibration velocity of the fine blasting particle, cm/s;

 η —— The ratio of the total charge of the blasting and the maximum amount of charge for the blasting.

3.2.2 Determining the differential time according to the principle of minimum seismic effect A large number of experimental studies have shown that the minimum time difference for generating the seismic effect is consistent with the time required to supplement the free surface and to supplement the fracture by collision with the blasting block. That is to say, choosing the minimum time difference of the seismic effect will not affect the blasting effect. There are three principles for determining the time of the differential: one is that the main shock phases of the seismic waves generated by the amount of explosives that are detonated before and after do not overlap; the second is that the time of the differential should be such that the seismic waves generated by the amount of explosives that are detonated before and after are mutually interfered; The sending time is greater than the delay time in the row. Long-term research and practice have proved that this time is generally 30 ~ 50ms. This is also determined by testing and long-term observation according to different geological conditions and environments.

3.3 Improve blasting conditions and reduce blasting vibration

3.3.1 Select low explosive speed, low power explosives in the blasting construction, use low explosive speed, low power explosives, have a certain positive effect on reducing the blasting vibration intensity. The experimental research shows that the wave impedance ρD of the explosive is different, and the blasting vibration intensity is also different. The larger the ρD, the greater the blasting vibration intensity. When the wave impedance ρD of the explosive is closer to the wave impedance ρC of the rock, the vibration intensity is greater. If the detonation velocity of 2# rock explosive is reduced from 3200m/s to 1800m/s, the seismic effect can be reduced by 40%~60% [2].

3.3.2 Creating a good free space blasting test study shows that the blasting blasting with good loosening conditions, that is, the blasting vibration generated when the blasting of the blasthole near the free surface is small, therefore, there must be sufficient free space in the blasting construction. The micro-difference technology enables all the blastholes to have a good free space, so that after the blasting of the blasthole, especially the compression wave generated after the blasting of the rear blasthole can be reflected from these free surfaces, the maximum looseness can be obtained to reduce the blasting. The effect of vibration.

3.3.3 Adjusting the direction of blasting and blasting During blasting construction, especially in open-pit blasting construction, the relative orientation of the explosion source and the protected object is different, and the vibration impact is also different. Practice has shown that when throwing blasting, the vibration in the direction of minimum resistance is the smallest, the reverse is maximum, and the sides are centered. When the rows of group packets are blasted, the connection speed in the center of the package is reduced by 25% to 45% compared to the vibration speed in the direction perpendicular to the connection.

3.3.4 In the construction of natural conditions, the natural conditions such as natural rivers, deep trenches, channels and faults can be fully investigated and utilized to reduce the propagation of seismic velocity. If no natural conditions are available, if necessary, the trenches are excavated, or pre-split blasting is used to artificially form a fracture surface perpendicular to the surface to cause reflection when the seismic wave arrives. The use of shock absorbing measures can generally absorb 30% to 50%, which is an effective measure for damping.

3.3.5 Comprehensive analysis of protected objects During mine blasting, the protected objects and seismic propagation media are generally fixed. Therefore, it is necessary to take long-term observation, analysis, understanding and master its properties, structure and seismic performance, take corresponding measures, conduct scientific tests when necessary, from the fluctuation law and propagation performance of blasting seismic waves, and the response characteristics of buildings. In terms of the spectral coherence characteristics and the influence characteristics of the building, the blasting parameters and the coefficients related to the geological conditions are reasonably determined to determine a reasonable safety distance.

4 Conclusion

The blasting seismic effect of mines is a very complicated problem with many influencing factors. However, after careful analysis and research, grasping several key factors and taking appropriate measures can completely reduce the impact of mine blasting vibration. Even if the protected object is not damaged, and the civil dispute between the mines caused by the blasting is avoided, the normal mine production order is ensured.

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