Effect of Excavation Blasting in an Under-Cross Tunnel on Airport Runway

Abstract

Blasting excavation in a tunnel passing beneath an airport is quite rare at home and abroad. In this paper, the blasting excavation for the construction of a four-arc double-track tunnel passing beneath Xujiaping Airport in Enshi is introduced and in-site tests are carried out to monitor the vibration in airport runway. Field monitoring data indicated that, throughout the tunneling operation, frequencies between runway’s self-vibration and blasting vibration differed appreciably, and no resonance would be caused. In terms of the influence mechanism, the dynamic finite element program LS-DYNA was adopted, whose reliability was verified by field monitoring data. The numerical results showed that the airport runway was affected little and an existed cave could amplify the amplitude of blasting vibration to an extent. Meanwhile, based on the stress wave theory and the failure law of maximum tensile stress, the safety threshold for the runway induced by blasting vibration was obtained. Then the safety threshold of peak particle velocity (PPV) was also decided through the statistic relationship between the peak value of tensile stresses and the PPV on the runway. As the safety thresholds derived from both fore mentioned methods was compared, with Safety Regulations for Blasting in China (GB 6722-2014) taken into account, a final safety threshold was decided.

1 Introduction

In recent years, due to the rapid development of society and economy, a lot of public facilities such as traffic tunnels and underground sewerage systems have been constructed. Because of the limited space, more and more tunnels must be constructed under existing transport lines. In these constructions, interaction effects between existing transport lines and tunnel constructions exist objectively. How to ensure the safety of both of them is a critical subject in engineering constructions.

It has been extensively studied at home and abroad to excavate tunnels under existing structures, roads and railways. Fang and He analyzed the displacement, deformation and internal forces of the existing tunnel affected by an undercrossing tunnel by means of 3D finite element method (3D-FEM) (Fang and He 2007). Wang et al analyzed the settlement of the express way induced by an under-crossed tunnel by employing the 3D-FEM (Wang et al. 2009). Wu et al studied the effect of blasting excavation on existing structures by using the software UDEC (Wu et al. 2010). Gongsun et al. (2011) analyzed the ground settlement law during shield tunneling under an airfield runway by employing the finite element software ABAQUS. Fang et al. stated that the advantages for a DOT (double-O-tube) shield tunnel under the river are resulted by short tunneling duration and low risk (Fang et al. 2012). Zhang et al. (2012) studied the influence of a under-cross tunnel on the existing metro lines and the surrounding environment. Gui and Chen estimated the DOT shield induced ground surface settlement in combination with the Taoyuan International Airport Access Mass Rapid Transit system in Taipei (Gui and Chen 2013). Lin et al. (2013) introduced the key techniques and important issues in the construction of Hangzhou Qiantang River Tunnel, which is the first cross-river tunnel in China.

As described above, many tunnels are constructed under existing structures, roads and railways. However, blasting excavation of tunnels under an airport is still rare at home and abroad up to now. And correlated researches are insufficient. In this paper, in combination with the construction of Xujiaping Tunnel crossing under Xujiaping Airport, the effect of tunneling under the airport runway with explosive method was analyzed based on field monitoring data and numerical simulations, and the safety criterion was also given.

2 Construction Conditions

Xujiaping Tunnel is located in Enshi Tujia and Miao Autonomous Prefecture, Hubei province, PR China. As illustrated in Fig. 1, the tunneling was between Shizhou Avenue and Jingui Avenue. This project started from Hongmiao, which is near the Shizhou Avenue, passed beneath Xujiaping Airport, and terminated at Jingui Avenue. The total route length is 1490 m and the tunnel gradient is 3 %.

Fig. 1

Plan of the double-track tunnel under Xujiaping Airport and arrangement of monitoring points

The tunneling under Xujiaping Airport was carried out in Enshi basin, which is an area of Cretaceous clastic rocks. As reported in the reconnaissance report, in descending order, the clastic rocks consist of a highly weathered layer (0.8–12 m) and a weakly weathered layer, which both belong to the Zhengyang Formation. The tunnel was constructed larger than 30.0 m below ground level; hence the construction was carried out in the weakly weathered layer.

Xujiaping Tunnel is a double-track tunnel with a separation of 28 m, as shown in Fig. 2. The permanent support is 40 cm thick, and the eastbound tunnel is excavated with the drilling and blasting method in advance. Delay blasting was adopted for tunneling and the blasting design was shown in Fig. 3. 163 boreholes were divided into 7 groups according to the initiation sequence (different color circles meant different groups), which were encoded as $\overline{1}$, $\overline{3}$, $\overline{5}$, $\overline{7}$, $\overline{9}$, $\overline{11}$ and $\overline{13}$. In the 7 groups, Group $\overline{1}$ was a group of cut holes, Group $\overline{11}$ and Group $\overline{13}$ were groups of perimeter holes, and all holes between the cut and perimeter holes are referred to as relief holes. The delay time between groups is 100 ms, which could eliminate the superimposition effect of blast-induced seismic waves (Stagg 1987). Continuous charging structure was adopted for cut holes and relief holes, and spaced charging structure was adopted for perimeter holes. The cut holes were charged about 14 kg explosives, which is the maximum charge per delay in the blasting design. In the following numerical simulations, only cut holes were considered.

Fig. 2

Section of the double-track tunnel

Fig. 3

Blasting design for excavation of the westbound tunnel

3 Analysis on Filed Monitoring Data

Blasting vibration is monitored by using TC-4850 self-recording instrument [developed and manufactured by Zhongke (Chendu) Instruments Company Limited] during the tunneling in this project. And No. 2 emulsion explosive was used for the excavation. The field monitoring data are listed in Table 1.

Table 1 Blasting vibration velocities

The maximum velocities in Horizontal and vertical directions are less than 0.25 cm/s, and the main frequencies are larger than 40 Hz. The natural frequency of airport runways is within the range of 10–20 Hz (Li and Xu 2005; Xu 2011). The small velocities indicate that the airport runway is affected little. Meanwhile, the main frequency of blasting vibration is much larger than that of airport runways, and no resonance would be caused. As a result, the airport runway is safe.

4 Numerical Analysis and Parameters Selection

For deeper understanding of the effect mechanism, some numerical models for exploring the attenuation in N–S (North–South) direction is established, as shown in Fig. 4. The model is 200 m wide, 100 m high, and the thickness of the model is twice the advance per round in each case. The side surfaces except tunnel face and the bottom surface are applied for non-reflection boundary. The top surface, tunnel face and surfaces around tunnels are free.

Fig. 4

Dynamic finite element model for N-S analysis

The JWL (Jones–Wilkens–Lee) state equation can simulate the relationship between pressure and specific volume in the explosion process (Yang et al. 1996). The equation is as follows:

$$p_{eso} = A\left(1 - \frac{\omega }{{R_{1} V}}\right)e^{{ - R_{1} V}} + B\left(1 - \frac{\omega }{{R_{2} V}}\right)e^{{ - R_{2} V}} + \frac{{\omega E_{0s} }}{V}$$

(1)

where A, B, R ₁, R ₂, and W are material constants, p _eso is pressure, V is relative volume and E _0s is initial specific internal energy. The physical and mechanical parameters of the dynamite are the same with that of the field test and are listed in Table 2.

Table 2 Mechanical parameters of explosive

The physical and mechanical parameters of surrounding rock are obtained from the geological exploration report (GIFCC 2011). The primary support and permanent support are made of C25 concrete and C35 concrete, respectively. The airport runway is made of C50 concrete. Their physical and mechanical parameters are obtained from the Specifications for Design of Highway Tunnels in China (JTGD70/2-2014) (MOT and CMCT 2014). The physical and mechanical parameters of surrounding rock, airport runway, primary support and permanent support are listed in Table 3. The liner and airport runway are regarded as elastomer and fitted with the MAT_ELASTIC material model. MAT_MOHR_COULOMB is chosen to simulate the constitutive relationship of surrounding rock (Hallquist 2007).

Table 3 Mechanical parameters of different part

4.1 Attenuation in N–S Direction

With different advance per round of 1.0, 1.5, 2.0, 2.5, 3.0 m taken into consideration, the peak resultant velocities on runway with different horizontal distance from the axis of the westbound tunnel are shown in Fig. 5.

Fig. 5

Peak resultant velocities with different horizontal distances at each advance per round

Figure 5 shows that the resultant velocity with an increasing distance in N–S direction distributes nearly symmetrically, and the largest velocity is just above the blast source. This indicates that the adjacent tunnel influence little on the distribution of peak resultant velocity. The velocity increases with an increasing advance per round, but the rate of increase decreases, and the turning point is 2 m advance per round. The maximum peak resultant velocities are 2.03 cm/s at 1.0 m advance per round and 3.57 cm/s at 3.0 m advance per round. The maximum peak resultant velocity means even the blast source is just below the runway, the blast excavation under airport affects the runway little, and the airport runway would not be damaged by blasting vibration.

4.2 Attenuation in E–W Direction

Based on the analysis in Sect. 4.1, the adjacent tunnel influences little on the distribution of peak resultant velocity. In order to improve the computational efficiency, only the westbound tunnel was considered to analyze the runway vibration attenuation in E–W direction. A 20 m × 200 m × 60 m model for exploring the attenuation in E–W direction is established, as shown in Fig. 6a. The model is discretized into 516516 elements and the element type is 3D-solid-64. In the model, all side faces and the bottom face are applied for non-reflection boundary. The top surface, tunnel face and surfaces around tunnels are free. Figure 6b shows a half the model shown in Fig. 6a, which illustrates the position of tunnel face.

Fig. 6

Dynamic finite element model for E-W analysis a Whole model; b Half model

Based on numerical analysis results, the attenuation of resultant velocity on airport runway in E–W (East–West) direction is obtained and as shown in Fig. 7.

Fig. 7

Peak resultant velocities with an increasing horizontal distance from the tunnel face

Figure 7 shows the resultant velocity decreases nonlinearly with an increasing horizontal distance from the tunnel face. When the distance is less than 36 m, the resultant velocity on ground surface behind the tunnel face is larger than that in front of the tunnel face at a same distance. This indicates that an existing cave can relatively amplify the surface vibration, which maybe because stress wave reflects at the surface of existing caves and multiple superposition of stress wave takes place at the ground surface. With an increasing distance, the difference of resultant velocities behind and in front of tunnel face degenerates. When the distance is equal to 36 m, the difference eliminates. This means that the amplification to surface vibration decreases with an increasing distance, and only takes place within a limited region of <36 m away from the tunnel face.

4.3 Verification

Numerical simulations must be verified by field monitoring data to ensure its reliability. Because the in-site advance per round is 1.5 m, simulation results with 1.5 m advance per round were selected to be verified by field monitoring data. In numerical simulation, vibration velocities at the same positions as the field monitoring points are selected to verify the reliability of numerical simulations. The selected nodes’ IDs in LS-DYNA are No. 238916, No. 237706 and No. 238070 with respect to No. 1, No. 2 and No. 3 field monitoring point respectively. Figure 8 shows vibration curves at Node No. 238916 measured in numerical model and Fig. 9 shows vibration curves at No. 1 field monitoring point.

Fig. 8

x-, y- and z- velocity at Node No. 238070 in numerical model

Fig. 9

x-, y- and z- velocity at No.1 field monitoring point

Field monitoring data and simulation results are listed in Table 4. Both of their velocities approximately share the same magnitude. Field monitoring velocities are in the interval between 0.048 and 0.23 cm/s, and simulating velocities are in the interval between 0.054 and 0.254 cm/s, which are slightly greater than the field monitoring data. Meanwhile, both of their main frequencies approximately share the same magnitude.

Table 4 Field monitoring data and simulation results

5 Safety Criterion of Runway

5.1 Safety Criterion of Stress Wave Theory

In stress wave theory, no transmission would be excited when a plane stress wave is incident on a free surface (Chen and Lu 2008; Wang 1985). In order to study the reflection of a blasting stress wave on a free surface, a coordinate system $Ox_{1} x_{2}$ is set up, and the $x_{1} = 0$ is defined to be the free surface, as shown in Fig. 10. The airport runway is located in the $x_{1} > 0$ region, and the $x_{1} < 0$ region represents the air.

Fig. 10

A plane P-wave incident on a free surface

Suppose a blasting stress wave is a plane harmonic P-wave, and α ₁, α ₂ and β ₂ are defined in terms of the incident angle of the incident P-wave, the reflection angle of the reflection P-wave and the reflection angle of the reflection SV-wave. The displacement Φ₁ orthogonal to the wave front is given as follows (Li et al. 2007; Yi et al. 2008):

$$\varPhi_{1} = A_{1} \,\text{sin}\,(pt + f_{1} x_{1} + g_{1} x_{2} )$$

(2)

where A ₁ is wave amplitude, $\frac{p}{2\pi }$ is the frequency of the incident wave, $f_{1} = \frac{{p\,\text{cos}\,\alpha_{1} }}{{C_{p} }}$, $g_{1} = \frac{{p\,\text{sin}\,\alpha_{1} }}{{C_{p} }}$, C _p is longitudinal wave velocity and $C_{p} = \sqrt {\frac{{E_{d} (1 - \mu )}}{\rho (1 + \mu )(1 - 2\mu )}}$, E _d is the dynamic young’s modulus, ρ is the density of the medium for stress waves, μ is the Poisson’s ratio of the medium for stress waves.

Consider the airport runway as a free surface, the reflection of P-wave is illustrated (Jiang 2013; Jiang et al. 2012). The maximum velocity due to the dynamic tensile strength $\left[ {\sigma_{t} } \right]$ is given as follows:

$$V_{P} (\sigma ) = \frac{{[\sigma_{t} ] \cdot (2\,\text{cos}\,\alpha_{1} \,\text{sin}^{2} \beta_{2} \,\sin 2\beta_{2} - \,\text{cos}^{4} \beta_{2} \,\sin \alpha_{1} )}}{{2\,\text{cos}^{2} \alpha_{1} \,\sin^{2} \beta_{2} \,\sin 2\beta_{2} - \,\text{cos}^{4} \beta_{2} \,\sin \alpha_{1} \,\text{cos}\,\alpha_{1} }} \cdot \sqrt {\frac{(1 + \mu )(1 - 2\mu )}{{E_{d} (1 - \mu ) \cdot \rho }}}$$

(3)

where $\left[ {\sigma_{t} } \right]$—dynamic tensile strength, $\left[ {\sigma_{t} } \right] = 1.2\sigma_{t0}$, $\sigma_{t0}$ is the static tensile strength; α ₁—incident angle for incident P-wave; β ₂—reflection angle for reflected SV-wave.

In case of unexpected damage, the maximum velocity must be modified according its importance. 2.0 is chosen as the modifying coefficient in this paper, and the safety criterion is 8.83 cm/s.

5.2 Safety Criterion of PPV

The statistical relationship model of the peak effective tensile stresses and the peak particle velocity (PPVs) in airport runway is established and shown in Fig. 11. The statistical relationship model established is as follows:

$$\sigma_{t} = 0.0751PPV - 0.047$$

(4)

where σ _t —peak effective tensile stress, MPa; PPV—peak particle velocity, cm/s.

Fig. 11

The statistical relationship of the peak effective tensile stresses and PPV

Equation (4) indicates that a linear relationship exists between the PPVs and the peak effective tensile stresses (Chen et al. 2007). Based on the maximum tensile strength theory, when the PPV reaches 25.66 cm/s, the peak effective tensile stress will approach the maximum tensile strength. 2.0 is also chosen as the modifying coefficient in this paper, and the safety criterion should be 12.83 cm/s.

5.3 Comparison Analysis

According to the Safety Regulations for Blasting in China (GB 6722-2014) (AQSIQ and SAC 2014), the safety criterion of blasting vibration velocity for 28-day fresh concrete is 10–12 cm/s. 12.83 and 8.83 cm/s are derived as safety criterions for airport runway based on numerical simulation and stress wave theory respectively. The results are consistent with that in the Safety Regulations for Blasting in China (GB6722-2014). However, numerical result is more accurate and reliable. In order to be more practical, the safety criterion for the runway is determined to be 8 cm/s.

6 Conclusions

By analysis of numerical simulation and theory analysis, the following conclusions can be drawn:

• 1. Field monitoring data shows that, throughout the tunneling operation, peak velocities in every direction are quite small, frequencies between runway’s self-vibration and blasting vibration differed appreciably. So no resonance would be caused and the airport runway would not be damaged.

• 2. The existence of the adjacent tunnel does not influence much on the traveling of stress wave, but in the same tunnel, the excavated part can amplify the amplitude of blasting vibration to an extent in a limited region of less than 36 m away from the tunnel face.

• 3. Based on the maximum tensile strength theory and stress wave theory, the safety threshold is calculated in the elastic range.

• 4. Combined with numerical simulations, the relationship between dynamic stresses and PPVs is established for airport runway. Based on the maximum tensile strength theory, the safety thresholds of stress wave theory and PPV in the project are determined.

• 5. With Safety Regulations for Blasting in China (GB 6722-2014) taken into account, a final safety threshold of 8.0 cm/s was decided.