It is essential to control the damage to the surrounding rock and engineering structures in the process of cut blasting with a single free surface in underground mining. To reduce vibration induced by cut blasting, this paper proposes short-delay cut blasting, in which blast holes that are near each other are sequentially initiated with short-delay times. Experimental tests of cut blasting were conducted in a roadway in the Shaxi copper mine to compare the peak particle velocity (PPV) and frequency characteristics of simultaneous blasting and short-delay blasting. Numerical modelling was then developed to study the influence of short-delay times on blast vibration. The accuracy of the numerical simulation was verified by the comparison of the test and simulated data of single-hole blasting. The results show that the amplitude reduction ratio (ARR) value increases gradually with the increase in delay intervals, and the vibration reduction for delay intervals smaller than 6 ms is very limited, particularly in the near field. The principal frequencies (PFs) for short-delay blasting are similar to those for simultaneous blasting, which implies that the frequencies do not increase directly with the decrease of the delay intervals. The experimental tests also show that the mean frequencies (MFs) for the 8 ms delay are slightly higher than those for the 0 ms delay blast. In the case of ensuring the rock breaking of cut blasting, longer delay intervals of 8∼10 ms are beneficial to further reduce PPV in practical blasting.
Hard rock fragmentation by blasting is the most widely used and cost-effective means in mining and construction operations. However, only a part of the energy released during blasting is utilized directly for breaking rock within the target range, while the rest causes damage to the surrounding rock, structures, and environment owing to ground vibration, noise, flying rocks, backbreak, and air blasts [
The previous studies on precise delay blasting with electronic detonators focused more on the feasibility of fragmentation improvement with the improvement of delay precision [
There is normally only one single free surface for underground cut blasting, which increases the difficulty of the blasting construction. To improve the blasting efficiency of cut blasting, there can be a set of blast holes near each other that are initiated at the same nominal delay time to create enough compensating space and new free surfaces for subsequent blasts. Reducing the harmful vibration induced by cut blasting with large charge weight per delay is a critical challenge in underground mines [
It is well known that peak particle velocity (PPV), frequency, and duration time are the three parameters used to assess the blast-induced vibration in rock mass [
Many uncontrollable factors, such as rock property, joint fracture, and nature of the terrain, have important influence on blasting performance; thus, it is expensive to study the characteristics of blast-induced vibrations simply by field-blasting tests [
To compare the vibrations induced by the simultaneous blasting and short-delay blasting and to obtain the characteristics of vibrations induced by blasting with different delay intervals, we conducted some experimental tests in Shaxi underground copper mine using four-hole schemes with a single free surface.
The test mine, the Shaxi copper mine, which is located in the southeast of Hefei city in Anhui province, China, is a newly constructed copper mine with a designed annual ore quantity of 3300 kt. The ore grade is 0.59% copper with a total reserve of 80.9 million tons. The ore body type is porphyry copper, and the surrounding rock types are massive rock and stratified rock. The main ore body is divided by fault F7 into two ore blocks, which are called Tongquanshan ore block and Fengtaishan ore block. As shown in Figure
Tested Shaxi copper mine.
The tests were carried out in a roadway with a width of 5.0 m and height of 2.8 m. First, single-hole blasting tests with a single free surface were conducted to investigate the propagation law of seismic waves. The hole diameter and depth are 89 mm and 0.8 m, respectively. Next, blasting tests with four holes were carried out to study the vibrations induced by simultaneous blasting and short-delay blasting. The delay schemes can be seen in Figure
Delay schemes of blasts with four blast holes. (a) Simultaneous blasting. (b) Short-delay blasting.
To monitor the vibration signals, three-component seismic instruments were placed in 6 station points at distances ranging between 10 and 100 m, shown in Figure
Blasting vibration monitoring in the blasting tests.
Figure
Velocities of single-hole blasting.
PPV for single-hole blasting versus scaled distance. The largest amplitude of (a) vector summation and (b) the radial component.
The Fourier transformation is adopted in this study to obtain the frequency information of the test signals [
PF for single-hole blasting versus scaled distance.
Blasting tests with four blast holes for 0 ms and 8 ms delayed blasts were conducted, and the velocity time histories were measured to contrastively investigate the characteristics of peak particle velocity and frequency.
Figure
Velocities of simultaneous blasting and short-delay blasting. (a) Simultaneous blasting. (b) 8 ms delayed blasting.
PPV between simultaneous blasting and short-delay blasting.
As seen from Figure
It is understandable that delay blasting can reduce vibration amplitude compared with simultaneous blasting. However, a major concern regarding short-delay blasting is that the frequencies will decrease as the time delays change from 0 ms (simultaneous blasting) to small delay intervals (short-delay blasting) and thus cause damage on constructions and structures as the resonant frequencies for the structures are in the range of 4 Hz to 28 Hz [
PFs between simultaneous and 8 ms delay blasting.
It can be seen that the 0 ms and 8 ms delayed blasts produce similar principal frequencies, which is inconsistent with the popular conception. This is because the spectral banding for small delay intervals is not as obvious as that for large delay intervals, for which the dominant frequencies are mostly determined according to the delay intervals between blast holes. As noted by Blair [
The PF mostly gives the frequency characteristic in a certain range and could miss the frequency components with small amplitude. Thus, besides the PF, the mean frequency (MF) is also applied to obtain the comprehensive characteristics of the frequency composition. The MF of the amplitude-frequency spectrum is defined as [
Figure
MFs between simultaneous blasting and short-delay blasting.
The experimental study described above preliminarily compares the characteristics of blast-induced vibrations of simultaneous blasting and short-delay blasting. Limited by experimental conditions, the systemic influence of short-delay times on blast-induced vibration is not revealed above. This is studied in this section by numerically modelling the blasting test using the LS-DYNA program, whose accuracy in simulating delay blasting in a rock mass has been proven in previous studies [
According to the blasting conditions shown in Figure
Numerical model used for modelling of blast-induced vibration.
Rock masses in the immediate vicinity of the charge will undergo high pressure and large strain instantaneously. In the present study, the MAT_BRITTLE_DAMAGE (MAT_96) material model is adopted to simulate the response of rock mass. It is an anisotropic brittle damage model designed for a wide variety of brittle materials, allowing progressive degradation of tensile and shear strengths across smeared cracks that are initiated under tensile loadings. The details of the brittle damage model can be seen in [
In the LS-DYNA software, the explosives are modelled by the John–Wilkins–Lee (JWL) equation, which is the most commonly used equation for modelling high explosives due to its simple form, experimental basis, and easy calculations of hydromechanics [
Material Type 5 of the LS-DYNA (
Parameters of stemming.
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2700 | 0.19 | 16 | 0.018 | 0.7 | 35 |
Numerical simulation of blast-induced vibration for a single-hole case is firstly carried out to verify the accuracy of the model. The comparisons of the tested radial velocity time history and simulated waveform in the
Comparisons of the velocities for test and numerical results.
Attenuation of PPV between test and numerical results.
Using the above numerical model with LS-DYNA, numerical simulations of the influence of short-delay intervals on blast-induced vibration are carried out. To form a common blasting crater, the delay intervals between blast holes in cut blasting should be less than the formation time of a new free surface [
Along the monitoring line shown in Figure
Velocities of (a) 0 ms and (b) 8 ms cases.
Figure
Simulated PPV at recording points along the monitoring line for different delay intervals. (a)
To further assess the influence of short-delay intervals on the reduction of PPV, the amplitude reduction ratio (ARR) proposed by Chen et al. [
Figure
ARR of simultaneous blasting and delayed blasting. (a)
The simulated waves are calculated by the Fourier transform, and based on equation (
PFs versus scaled distances between delay intervals.
The test and numerical results show that the PFs for short-delay blasting are similar with simultaneous blasting. It indicates that a change in delay time from 0 ms to small delay intervals would not increase the possibility of structural resonance. In addition, the above test and simulation also show that short-delay blasting could effectively reduce PPV compared with simultaneous blasting. Therefore, it is of great value to reduce PPV with similar principal frequencies via short-delay cut blasting.
This paper proposes short-delay cut blasting to reduce blast-induced vibration in underground mining. Single-hole blasting tests with a single free surface were firstly carried out in a roadway in the Shaxi copper mine to obtain the site-specific attenuation relations of PPV and PF of vibration waves. Cutting blasts with four blast holes for delay intervals of 0 ms and 8 ms between blast holes were then conducted to compare the characteristics of PPV and frequencies. A numerical model was developed to investigate the blast-induced vibration of short-delay blasting with a single free surface. The measured data of single-hole blasting were used to verify the accuracy of the numerical simulations, and a very good agreement between the measured and numerically simulated data was obtained. The numerical model with four blast holes was used to systematically investigate the influence of short-delay intervals on PPVs and PFs of vibration waves.
The experimental tests and numerical results show that the ARR value increases gradually with the increase of delay intervals. The vibration reduction for delay intervals smaller than 6 ms is very limited, particularly in the near field. The principal frequencies for short-delay blasting are similar to those for simultaneous blasting, which implies that the frequencies do not increase directly with the decrease of the delay intervals. The experimental tests also show that the mean frequencies for the 8 ms delay are slightly higher than those for the 0 ms case, which may be caused by the reflection of the compressive stress waves caused by a later detonated blast hole on the initial cracks caused by a former detonated blast hole.
The experimental tests and numerical simulation in the present study show that it is of great value to reduce PPV with similar frequencies via short-delay cut blasting. In practical blasting, longer delay intervals such as 8∼10 ms are preferentially selected for further reducing PPV in the case of ensuring the rock breaking of cut blasting.
The influence of the reflection effect (the reflection of the compressive stress waves caused by a later detonated blast hole on the initial cracks caused by a former detonated blast hole) on the frequency characteristics of short-delay blasting cannot be ignored. However, owing to limited current computing resources, it is impossible to quantify the influence of the reflection effect. The precise frequency characteristics of short-delay blasting may be determined in the future with the help of supercomputers.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This work was supported by the National Natural Science Foundation Project of China (51874350) and the National Key R&D Program of China (2017YFC0602902). The support provided by the China Scholarship Council (CSC) during the visit of Xianyang Qiu to Curtin University is acknowledged. The authors also thank Hong Hao, Yifei Hao, and Jian Cui for their excellent advice.