To understand differences of smooth and presplit blasting for the excavation of rock wells, two field experiments using these two techniques are implemented at the same test site, respectively. The ground vibrations induced by them have been monitored with the different distances through the corresponding devices. The vibration results illustrate that at the same monitoring distance and direction, peak particle velocities and dominant frequencies of vibration signals based on presplit blasting are both apparently lower than that based on smooth blasting. Meanwhile, with the increase of distance, the principle and mean frequencies based on smooth blasting always decrease, but these two frequencies based on presplit blasting might firstly decrease and then rise. In addition, frequency bands of energy distributions based on smooth blasting are more dispersive than that based on the presplit blasting at the same distance and direction. Lastly, the excavation qualities of rock wells with two techniques are also measured. The excavation results demonstrate that the contour quality and flatness of well bottom based on smooth blasting are better than that based on presplit blasting. Nevertheless, well depth based on presplit blasting is larger than that based on smooth blasting.
Drilling and blasting methods have been extensively applied to rock excavation in mining and civil engineering due to its low cost, high efficiency, and easy operation. [
Hole arrangement and sequence of two control methods. (a) Excavation of rock well. (b) A1-A1 section.
Over the past decades, some significant surveys on smooth and presplit blasting have been conducted by many researchers. Zare and Bruland [
According to the previous investigations, workers come to realize that although the two control techniques could both effectively restrain blast-related problems in most cases, their excavation effects are quite distinct due to different firing sequences. In order to explore differences of presplit and smooth blasting, the comparisons of the two control techniques have been conducted by some scholars from several perspectives. Lu et al. [
Through the above analysis, it can be found that those comparison factors like crack, flatness, and damage are all related to excavation quality, whose influences are limited to the vicinity of the explosive source. Nevertheless, in practice, residents in the neighborhood are more concerned about the adverse impacts of blast-induced vibrations because they can propagate over long distances [
In this study, to synthetically reveal the differences of presplit and smooth blasting, blast-induced vibrations and excavation qualities of rock wells based on two control techniques are investigated. Firstly, two field experiments for the excavation of rock wells using presplit and smooth techniques are carried out at the same test site, respectively. Subsequently, vibration characteristics induced by two field experiments are analyzed depending on the PPV, frequency, and energy. Lastly, the well-forming results based on two control techniques are compared from well flatness and depth.
The site of two experiments is located at Zixing city of Hunan Province in China, as shown in Figure
The field experiment site.
Physical and mechanical parameters of rock.
Parameter | Value |
---|---|
Mass density |
2518 |
Elastic modulus |
25.63 |
Shear modulus |
13.27 |
Compressive strength |
93 |
Tensile strength |
12.5 |
Poisson ratio |
0.28 |
In the two experiments, the expected diameters of rock wells both are 2.5 m. As shown in Figure
Schemes of blast implementation and vibration measurement for two field experiments.
After the abovementioned boreholes and empty holes are drilled, the charge and stemming of boreholes need to be conducted. Here the 2# rock emulsion explosive is used for main and peripheral holes. The peripheral holes use decoupling charges, and its decoupling coefficient is 1.5625. The coupling charges are applied to main holes. The charge weights of each peripheral and main boreholes are 4.5 kg and 18 kg, respectively. Consequently, the linear charge densities of main and peripheral holes are 2.40 kg/m and 0.69 kg/m, respectively. The explosive consumption is 4.56 kg/m3. To guarantee the reliability of detonation, two millisecond delay detonators with same delay times and one detonating cord as long as hole charge are arranged on each borehole. These two detonators in each hole are fixed at the top and bottom of charge, respectively.
When presplit blasting is applied to the excavation of rock well, the delay times for peripheral and main boreholes are 0 ms and 110 ms, respectively. When smooth blasting is applied to excavation of rock well, the delay times for peripheral and main boreholes are 110 ms and 0 ms, respectively. To monitor the ground vibrations, three vibration instruments (TC-4580) are also installed with 30 m, 50 m, and 150 m source distances for every field experiment.
After the two experiments are conducted, the vibration signals generated by them have been monitored and are shown in Figure
The vibration signals induced by the excavation of rock wells with (a) presplit blasting and (b) smooth blasting.
According to the vibration signals of three monitoring sites, PPVs from main and peripheral boreholes in radial and tangential directions based on smooth and presplit blasting are depicted in Figure
The variations of PPVs from (a) main and (b) peripheral boreholes versus distances in radius and tangential directions based on smooth and presplit blasting.
In addition to PPV, the frequency is another evaluation indicator of the vibration signals [
The frequency spectra of vibration signals from (a) main and (b) peripheral boreholes in the radial and tangential directions based on presplit and smooth blasting.
Besides the dominant frequency, principal frequency (PF) and the mean frequency (MF) are another two analysis indicators of frequency characteristics. PF is generally determined by half of the maximum spectral peak in the Fourier spectrum (see Figure
Definitions of principal frequency (PF).
By equations (
The principal frequencies of vibration signals from (a) main and (b) peripheral boreholes versus monitoring distances based on presplit and smooth blasting.
The mean frequencies of vibration signals from (a) main and (b) peripheral boreholes versus monitoring distances based on presplit and smooth blasting.
Although the PPV and frequency can expose the transient characteristics of vibration signals, the energy effects of vibration signals are still neglected. In order to comprehensively assess the blast-induced vibrations, the wavelet packet method [
The ratio
According to equation (
The percentages of energy distribution from main boreholes in different frequency bands based on smooth and presplit blasting at different monitoring distances. (a) 30 m. (b) 50 m. (c) 150 m.
The percentages of energy distribution from peripheral boreholes in different frequency bands based on smooth and presplit blasting at different monitoring distances. (a) 30 m. (b) 50 m. (c) 150 m.
Except for the blast-induced vibration characteristics, excavation qualities of rock wells also need to be detected to reveal the differences of presplit and smooth blasting. After the broken rocks are cleared up, the excavation results of two rock wells are obtained and depicted in Figure
Excavation results of rock wells based on (a) presplit blasting and (b) smooth blasting.
To further explore differences of excavation results based on two techniques, the visible depths of five main boreholes and twelve peripheral boreholes are measured. Their results are shown in Figure
The depths of main and peripheral boreholes generated by two experiments. (a) The depths of five main boreholes. (b) The depths of peripheral holes.
In this study, the two field experiments for the excavation of rock wells are implemented to reveal the differences of smooth and presplit control techniques. Rock vibration characteristics induced by the two experiments are analyzed by the PPV, frequency, and energy. In addition, The excavation results based on two contour techniques are compared from excavation depth and well quality. By this work, the main conclusions can be drawn as follows: From the analysis results of PPV and frequency, at the same monitoring sites and directions, the PPVs from main boreholes based on smooth blasting are larger than that based on presplit blasting. The ranges of frequency bands based on the smooth blasting are wider than those based on the presplit blasting. The dominant frequencies of vibration signals based on presplit blasting are always smaller than those based on smooth blasting. Moreover, the principle and mean frequencies based on the smooth technique decrease with the increases of distances. However, the principle and mean frequencies based on the presplit technique might firstly decrease and then rise. Through using the wavelet packet method, distribution bands of signal energies based on smooth blasting are more dispersive than those based on presplit blasting. Meanwhile, the frequency bands of energy distributions based on presplit blasting are lower than that based on smooth blasting. In addition, when the monitoring distances increase to 150 m, the energy ratios of low frequency components for main boreholes based on the presplit blasting decrease obviously. However, those based on the smooth blasting are still large. Although the geological conditions and rock properties are different for the two experiments, the excavation results still show to some extent that the smooth blasting can obtain the better contour quality and the flatness than the presplit blasting. At the same time, the presplit blasting can obtain the larger well depth than the smooth blasting by the same excavation parameters. Hence, the contour blasting techniques need to be selected flexibly by engineering requirements.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The authors are very grateful to the financial contribution and convey their appreciation to the organization for supporting this basic research. The authors also thank the operators and leaders of the test site for their help. This work was supported by the National Basic Research Program of China (2015CB060200), the National Natural Science Foundation of China (41772313 and 51804339), and the Key Research and Development Program of Hunan (2016SK2003).