Keywords

1 Introduction

In the construction process of tunnel drilling and blasting method, the change of different blasting positions of the hole will have a certain impact on the blasting vibration and blasting effect, mainly because the blasting position will determine the direction of the explosion energy propagation of the cylindrical charge pack and the time effect of the superposition of the blasting stress field of each charge pack [1, 2]. Therefore, it is necessary to study the blasting vibration effect caused by blasting at different blasting positions.

In view of blasting at different initiation positions of the gun hole, relevant scholars have carried out corresponding research from theoretical analysis, field test and numerical simulation. Based on the Heelan short column theory, Gao Qidong et al. [3] discussed the influence law of different blasting positions of the hole on blasting vibration, and obtained the calculation formula of blasting vibration field generated by blasting at different blasting positions. Leng Zhendong et al. [4] found that double point initiation was conducive to reducing rock size and improving blasting effect by comparing the experiments of double point initiation with hole bottom initiation. Liu Liang [5] and Wang Yapeng [6] simulated blasting at different initiation positions based on LS_DYNA, and the research results showed that positive initiation was helpful to rock bottom fragmentation and reduce blasting vibration. At the same time, Gao Qidong [7] also used LS_DYNA to study the impact of blasting at different initiation positions of cut holes on the tunnel support structure. The research results showed that the forward initiation was helpful to reduce the vibration of the structure, while the reverse initiation was helpful to rock fragmentation.

The above scholars conducted a detailed study on the blasting effect of different initiation points of the blasthole from theory and simulation experiments. However, what was relatively lacking was related research on different initiation points of the full-section blasthole on site, and the relationship between blasting vibration signal frequency and energy under different initiation positions of the blasthole on site. Therefore, based on the reconstruction project of G528 National Road Suichang Xinluwan to Shilian section, this paper conducts blasting test research on the full-section blasting holes of the tunnel at different initiation positions in the guided tunnel, and studies the relationship between the frequency and energy of each signal.

2 Field Blasting Test

2.1 Project Background

This paper takes the dam tunnel in the reconstruction project of G528 National highway from Suichang Xinluwan to Shilian section as the background. The length of the tunnel is K4 + 928~K5 + 025, the total length is 97 m, and the maximum buried depth is 25 m. The surface of the tunnel is composed of residual slope silt clay containing gravel, the thickness is 1–2 m, and the underside is strongly to moderately weathered tuff, the rock mass is broken and the integrity is poor, which belongs to the IV grade surrounding rock. The whole section of the tunnel is excavated, and the area of the excavation section is 31.5 m2.

2.2 Field Experiment Scheme

In view of the factors such as broken surrounding rock and poor stability of the tunnel from the dam, and considering that the vibration generated by the blasting of the main tunnel is likely to have a greater impact on the middle partition wall, the blasting vibration reduction experiment is carried out in advance in the middle pilot tunnel of the tunnel, that is, reverse blasting, intermediate blasting and positive blasting are respectively adopted for all blasting holes in the full section. The blasting vibration effect under each blasting mode is obtained by conducting experiments on different blasting positions of the blast hole, which provides guidance for the subsequent main tunnel to select the appropriate blasting position of the blast hole. The schematic diagram of the positions of different initiation points of the blast hole is shown in Fig. 1 below. Detonation is conducted through the detonating tube, and the TC-6850 blasting vibration monitor is used to monitor the ground vibration. The measuring point is arranged 20 m from the ground to the working face.

Fig. 1
Three diagrams of reverse initiation, intermediate initiation, and forward initiation. Each diagram has 3 labeled parts namely the tunnel face on the left, the cartridge on the right, and the initiation point. The initiation points are marked on the right, center, and left, respectively.

Schematic diagram of different initiation points of the hole

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2.3 Blasting Parameter Design of Middle Guide Tunnel

Based on the site engineering geological conditions and considering the use of the full-section method in the blasting construction of the guide tunnel, the layout of the shot holes in this experiment is designed, as shown in Fig. 2 below, with a total of 6 sections. Its hole parameters are shown in Table 1 below.

Fig. 2
A diagram of the arrangement of gun holes. It comprises 3 semicircles marked M s-11, 9, and 7, a pair of 3 vertical lines on each side marked M s 5, 3, and 1, and a horizontal line marked 5.5. A line stretched out on the right is marked as 2.96 and 2.29 on the bottom and top respectively.

Layout of gun holes (Unit:m)

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Table 1 Charge amount of each section for each hole
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3 Analysis of Blasting Vibration Signal at Different Positions of Blasting Holes

3.1 Time History Curve Analysis of Blasting Vibration Signal

In the process of blasting vibration monitoring, the peak value of blasting vibration velocity collected in the X-axis direction (tunnel longitudinal direction) is the largest, so only wavelet packet decomposition of signals in this direction is carried out later in this paper to study the relationship between frequency and energy. The blasting vibration signals of three kinds of holes at different initiation positions measured in the field are shown in Fig. 3 below. It can also be seen from Fig. 3. that the three initiation modes all reach the maximum vibration velocity during the first cut hole blasting, in which the peak velocity of reverse initiation is 7.15 cm.s−1 and the occurrence time is 0.033 s. The peak velocity of intermediate initiation is 5.84 cm.s−1, and the occurrence time is 0.031 s. The peak velocity of forward initiation is 5.58 cm.s−1, and the occurrence time is 0.029 s, the peak vibration velocity: reverse initiation > intermediate initiation > forward initiation, and peak velocity time point: forward initiation < intermediate initiation < reverse initiation. Investigate its reason, mainly when columnar cartridge blasting, energy spread along the hole, priority of reverse initiation energy spread along the hole in the direction of constraints, initiation energy transmission along the hole to both sides, among which are initiating explosive energy along the gun Kong Chao spread of surrounding rock, which causes reverse initiation at the ground vibration caused by tunnel is the largest. Further observation of the waveforms in Fig. 3 shows that, with the increase of the detonator stage, the waveforms of reverse initiation and forward initiation are wide in MS9 and MS11 sections, while the waveforms of intermediate initiation are narrow in MS9 and MS11 sections, indicating that when the intermediate initiation of the detonator is in the middle of the hole, all the holes in the two sections of the detonator can easily achieve simultaneous initiation. However, the reverse and forward initiations are easy to cause separate initiation of each hole in the high stage detonators.

Fig. 3
Three graphs plot vibration velocity versus time for reverse, intermediate, and forward initiation blasting vibration signals. The large speed and maximum vibration marked are 7.15 and 0.033 seconds, 5.84 and 0.031 seconds, and 5.58 and 0.29 seconds, respectively.

Blasting vibration signals at different initiation positions of the hole

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3.2 Frequency Analysis of Blasting Vibration Signal

In order to further analyze the frequency of blasting vibration signals under different initiation positions of the three holes, Fourier transform is carried out for the blasting vibration signals in Sect. 3.1 above and normalization is carried out to obtain the spectrum diagrams of different initiation positions of the three holes, as shown in Fig. 4 below. It can be found from Fig. 4 that the frequency distribution of forward initiation is wider than that of reverse initiation and intermediate initiation. The frequency of forward initiation is mainly distributed in the range of 0–400 Hz, the frequency of reverse initiation is mainly distributed in the range of 0–300 Hz, and the frequency of intermediate initiation is mainly distributed in the range of 0–200 Hz. At the same time, the main frequency range of reverse initiation is 100–200 Hz, the main frequency of intermediate initiation is 50–150 Hz, and the main frequency of forward initiation has two peaks, one is 100–200 Hz, and the other is 300–400 Hz. This indicates that compared with reverse initiation and intermediate initiation, forward initiation is more conducive to energy transfer and high frequency dispersion.

Fig. 4
Three graphs plot the relative amplitude versus frequency for reverse, intermediate, and forward initiation. The highest value is estimated. For reverse initiation, (150, 0.20). For intermediate initiation, (110, 0.165). For forward initiation, (325, 0.17).

Spectrum of blasting vibration signal at different initiation positions

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4 Wavelet Packet Analysis of Blasting Vibration Signal

According to the blasting vibration signals at different initiation positions of the hole in Sect. 3.1 above, wavelet packet decomposition is carried out to study the energy distribution of each signal in different frequency bands. The wavelet basis function and the number of corresponding decomposition layers should be chosen reasonably when wavelet packet decomposition is carried out. The reasonable wavelet basis function can effectively remove the noise generated in the tunnel construction process and extract the real blasting vibration signal. However, the number of decomposition layers will directly affect the denoising effect of the signal. Too low decomposition layers will easily lead to mixed useful information in low frequency band, while too high decomposition layers will easily lead to deviation of the results. Therefore, this paper adopts db8 basis function in db series, which is similar to the tunnel blasting vibration signal waveform, to decompose the blasting vibration signals at different initiation positions of the three holes in eight layers [8].

TC-6850 blasting vibration vibrometer produced by Chinese Academy of Sciences was used in this tunnel blasting test. The sampling frequency was set as 6400 Hz, and the Nyquist sampling frequency was 3200 Hz. A total of 256 frequency bands were generated after 8-layer decomposition of wavelet packet. The size of each frequency band is 12.5 Hz, that is, the minimum frequency band after decomposition is 0–12.5 Hz. The energy of each frequency band is obtained through the above wavelet packet decomposition, and the frequency band interval of the main frequency band is defined in this paper for each sub-band whose energy ratio exceeds 10% [8]. The energy proportion of each main frequency band of blasting vibration signals at different initiation positions of the three holes is statistically shown in Table 2 below.

Table 2 Main frequency bands of blasting vibration signals at different initiation points of the hole
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As can be seen from Table 2, the blasting vibration energy of intermediate initiation is mainly concentrated at 100–125 Hz, accounting for 67.28% of the total energy, and the energy is relatively concentrated. Compared with intermediate initiation, the main frequency band of reverse initiation is below 100 Hz and above 125 Hz, and the energy ratio of the three main frequency bands is 38.6%, indicating that the energy is relatively scattered. However, the frequency range of the two main frequency bands for forward initiation is quite different, one of which is in 87.5–100 Hz and the other is in 287.5 Hz–300 Hz. The energy ratio of the two main frequency bands is not very different, occupying 20.9% of the total energy, indicating that forward initiation can better disperse energy compared with reverse initiation and intermediate initiation. And make the energy transfer from low frequency to high frequency, so that the blasting vibration is relatively small, more conducive to the safety of tunnel lining structure.

5 Conclusions

To study tunnel hole blasting vibration effect different detonation point, with national highway G528 local practice period of reconstruction engineering of the new road bay to stone dam to tunnel as the background, elaborated to carry out the whole section in tunnel hole, experimental study on different detonation position of hole respectively take the reverse initiation, intermediate initiation, forward initiation, the results show that the:

  1. (1)

    The three initiation positions all reach the maximum vibration velocity during the first cut hole blasting, but there are differences in the occurrence time of the peak velocity. The time points of the peak velocity are: forward initiation > Intermediate initiation > Reverse initiation; The peak velocity is: reverse initiation > intermediate initiation > Forward initiation; At the same time in the high stage detonator, the intermediate initiation is more conducive to the simultaneous initiation of the hole.

  2. (2)

    The frequency ranges of blasting vibration signals at different initiation positions of the three holes are different, and the frequency distribution ranges are as follows: forward initiation > Reverse initiation > Intermediate initiation; At the same time, compared with reverse initiation and intermediate initiation, forward initiation is more conducive to the movement of blasting vibration signal frequency to high frequency.

  3. (3)

    Compared with reverse initiation and intermediate initiation, forward initiation can better disperse energy and transfer energy from low frequency to high frequency, so as to make blasting vibration relatively small, which is more conducive to the safety of tunnel lining structure.