Analysis and optimization of mutual influence of single channel tunnel construction blasting

1. Introduction

Nowadays, the urban rail transit project is in the stage of vigorous development, but it’s quite common to encounter the parallel operation of the tunnel and the overlapping of the tunnel in the tunnel construction project by the restrictions of the site and the line. The problem of mutual interaction between a new tunnel and existing structures has become very prominent. In the parallel small clear distance and large cross section shallow tunnel group, the construction process, the safety construction section spacing, the influence of blasting vibration on the supporting structure, the reasonable timing of support has become an urgent problem to be solved.

The construction of underground caverns has attracted the attention of scholars at home and abroad, and put a lot of effort into the research. The research on the small-scale tunnels has yielded fruitful results [1-6]. In China, there are few studies on the construction technology of small horizontal tunnels with parallel holes of three holes and above, which mainly rely on very few concrete engineering examples.

According to the design summary of the 3-hole tunnel group of Yuexiu Park Station of Guangzhou Metro Line 2, Zhang Hongwei [7] has carried on the preliminary discussion on the construction of 3-hole tunnel scheme. Based on the combination of model test and finite element method, Wang Mingnian [8-9] studied the construction mechanics characteristics of 3-hole parallel tunnel with shallow buried and small spacing in weak surrounding rock. Guangzhou Metro Design and Research Institute [10] summarized the successful design and construction experience of Guangzhou Metro project, and provided technical guidance for similar projects in the future. Yang Hong she [11] also carried out in-depth analysis of the Guangzhou subway project, summed up the subway shallow buried hole construction process and key technology. Sun Zhongcheng [12] also combined with the project summed up the construction of the tunnel hole tunnel technology. In the study of the caverns, the study of blasting vibration and seismic effect is one of the focuses.

In summary, for more than 3 tunnels of single-channel tunnel group construction, reference experience needs to be studied and accumulated. Based on the engineering examples and the industry standard, this paper studies the blasting vibration of the parallel tunnel group in the five tunnels by combining the measured data of the field and the numerical simulation results. In order to obtain reasonable tunneling distance and support time, the mechanism of blasting vibration propagation in tunnel group is explored, which can provide reference for the safety and design of similar construction

2. Propagation effect analysis model of shock wave in group chamber

The main two main models of the shock wave propagation effect model of the shock wave generated by the blasting in the construction of the cavern group are the model of the dampness wave propagation attenuation model (Fig. 1) and the surrounding rock mass response model (Fig. 2). The former mainly studies the propagation vibration of the shock wave along the roadway in the surrounding rock medium during the tunneling process, and explores the construction of the single tunnel. The latter mainly studies the influence law of the vibration of the blasting vibration wave in the adjacent tunnel group, and qualitatively reveals the response characteristics of the surrounding rock of the different distance tunnel to the shock wave in order to arrange the longitudinal space of the construction.

Fig. 1Attenuation law of blasting seismic wave propagation models

Fig. 2Shock response characteristics of surrounding rock of tunnel and Chambers model

3. Introduction to engineering

Yuelai station to Wangjiazhuang station interval and Wangjiazhuang parking lot entry and exit project is Chongqing Rail Transit Line 10 part of the project, the starting point for both Yuelai station, terminal for Wangjiazhuang station. The tunnel passes through the strata mainly sand and mudstone interbed, the surrounding rock level IV-VI level, the groundwater is bedrock fissure water. Wangjiazhuang parking lot entry and exit tunnel dome depth of 8.8-43.5 m, interval line tunnel dome depth of 4.8-40.1 m. The distance between the right and left of the line is 1.5 m, which is a small clear distance tunnel. The distance between the left line and the out-of-section structure is 3-6.5 m, which is a small-distance tunnel. The interval line set two contact channel, a channel construction. Due to the tight construction period, the construction land is limited, a single channel is used as the auxiliary channel construction, the left and right of the five independent channels, forming a unique single channel tunnel construction characteristics. Among them, the construction traffic organization is difficult, the ventilation pressure is big, the interaction crosses the big influence, especially the blast has the influence to the construction of the small clear distance tunnel.

4. Field test scheme design

For the sake of generality, according to the blasting theory model and the existing experimental research results, arranged vibration equipment for testing in the 5, 6, 7, 9 tunnel. In the process of testing, considered the purpose and requirements of the test project, combined with the economic and effectiveness testing. The specific test results are shown in Fig. 3.

Fig. 3Schematic diagram of the original layout of horizontal tunnel test. Dotted lines represent constraints; digital number for the channel signal; X for test instrument

5. Test results

6. Transfer law analysis of numerical simulation and field test result

6.3. The establishment and calculation result analysis of group hole model

5 hole and 7 hole dislocation 40 meters, 5 hole and 9 hole dislocation 30 meters, 7 holes and 9 holes dislocation 70 meters of the situation:

Fig. 4Excavation surface of each tunnel dislocation distance sketch

Under this condition, 5 tunnel face vault $Z$ direction vibration velocity 33.12 cm/s, combined vibration velocity is 33.17 cm/s. No. 7 tunnel face vault $Z$ direction vibration velocity 1.24 cm/s, combined vibration velocity is1.50 cm/s. No. 9 tunnel face vault $Z$ direction vibration velocity 1.16 cm/s, combined vibration velocity is 1.21 cm/s. This can be seen when the dislocation of No. 5 tunnel and No. 7 tunnel face 40 m, No. 5 and No. 9 tunnel face dislocation 30 m. The 5 hole is the explosion source, No. 7 tunnel face vault $Z$ direction of vibration speed and combined vibration speed meets the current national standard of newly poured concrete age 3 days allowed by the 3.0 cm/s velocity requirements, and No. 7 tunnel face vault $Z$ direction of vibration speed and combined vibration speed meets the current national standard of newly poured concrete age 3 days allowed by the 3.0 cm/s velocity requirements.

6.4. Comparison chart of single hole measured trend chart and simulation under the condition of group tunnel simulation

As can be seen from Table 3, when the No. 5 tunnel and the blasting source have different longitudinal distance, the combined vibration velocity of the vault is different, when the distance of explosion source distance of 12 m, 28 m, 40 m, 48 m, 52 m, 68 m, 80 m, 92 m, 140 m, vault combined vibration velocity were 12.8 cm/s, 9.3 cm/s, 6.9 cm/s, 3.2 cm/s, 2.4 cm/s, 2.5 cm/s, 2.3 cm/s, 1.6 cm/s, 1.8 cm/s. It can be seen that the combination of the No. 5 number of the tunnel vault at the distance of the explosion source 48 m is 3.2 cm/s, more than the current national standard, but when the No. 5 tunnel vault distance explosion source 52 m, the combination of vibration velocity for the 2.4 cm/s to meet the engineering requirements.

From Table 3 can be drawn out of the vault combination speed trend chart 5. From the figure it is obvious that the trend of distance blasting source closer, vibration velocity increasing; blasting source distance farther, vibration velocity is small, there is a sharp drop in the middle process, after falling slowly. Taking into account the factors of blasting damage and other factors, the best distance of the two lining of the No. 5 tunnel should be about the distance from the explosion source 65 m, at this time, it can not only ensure that the impact of blasting on the two lining is in line with the requirements of the specification, but also can effectively make the two secondary lining construction operation, at this time of the second lining of the construction work distance is more reasonable.

Table 3No. 5 tunnel dome of the measured vibration value

Vibration velocity (cm/s)	12.8	9.3	6.9	3.2	2.4	2.5	2.3	1.6	1.8
explosion source distance (m)	12	28	40	48	52	68	80	92	140

Fig. 5Vault combination speed trend chart

Fig. 6The trend chart of the combined vibration velocity of single tunnel vault

Fig. 7July 9, Field monitoring data and simulation data contrast trend chart

Can be seen from the Figs. 6-9, blasting seismic waves in the transmission process from the blasting source near the blasting seismic wave velocity sharp attenuation, the more distant regions tend to be gentle. In a certain total charge, charge structure, blasting pitch cases, the trend of simulated vault combination velocity is basically the same as the trend of measured vault, there is a certain error in the specific value. This is mainly to simulate the parameter selection and the actual situation there is a certain discrepancy, which is mainly the difference between the charge, simulation of the charge density and the actual blasting construction of the charge is different.

Fig. 8July 16, Field monitoring data and simulation data contrast trend chart

Fig. 9July 18, Field monitoring data and simulation data contrast trend chart

7. Conclusions

When blasting, the two lining is generally the new cast concrete, by the current national standard “blasting safety procedures GB6722-2014” article 13.2.2 of the provisions can be safely allowed to allow the value of 3 cm/s. The comparison of the simulation with the actual monitoring as well as the accumulation of the damage considered can be drawn: the secondary lining of the tunnel is about 65 meters away from the blast source and is generally safe, and basically meet the requirements of the specification. If the distance of the second lining is less than 65 meters away from the blast source, the combined vibration velocity of the tunnel dome is generally more than the specification requirements. This is disadvantageous for the second lining, so the second lining of the tunnel group is somewhat lagging behind. Should be a timely lining of the second operation, to meet the requirements of the second lining immediately.