Research on Optimization Design of Tunnel Blasting Scheme Adjacent to Buildings

Abstract

The section of Jialingjiang Road Station to Xiangjiang Road Station along Qingdao Metro Line 13 is located in Qingdao, China. All of them show obvious characteristics, being soft on the top and hard on the bottom, and the interval tunnel is faced with the problem of existing adjacent buildings. In order to ensure the smooth progress of construction, as well as minimize the damage to the buildings, a new mechanical excavation combined with a blasting construction scheme for the adjacent buildings is proposed. In this scheme, the step method is used for excavation. The upper step is in the weak stratum, and the mechanical method is therefore used for excavation; the lower step is in the hard stratum, and the drilling-and-blasting method is thus used for excavation. Using FLAC^3D 5.0 finite difference software and the method based on blasting an equivalent load, the vibration velocity at adjacent buildings caused by the combined mechanical excavation and blasting scheme, as well as the traditional full-section blasting scheme, is compared and analyzed. Further, the construction parameters of the combined mechanical excavation and blasting scheme are compared and selected based on building settlement, the plastic zone of surrounding rocks, building vibration velocity and other factors. The results show that under the mechanical excavation blasting scheme, the peak particle velocity of each monitoring point decreases significantly compared with that under the full-section blasting scheme, with a maximum reduction of 61.1%, which is within the allowable range of the project, demonstrating the rationality of the new scheme. Finally, the mechanical excavation advance in the upper step is determined as 0.5 m. The optimized parameter construction effect is monitored and evaluated, the problems encountered in the project are successfully solved using the combined mechanical excavation and blasting scheme, and the expected construction period is shortened by 3 months, which shows the rationality of the blasting construction scheme proposed and its parameters, as well as the validity of the calculation results. The research results can be used as a reference for the construction scheme design of similar projects.

1. Introduction

The rapid development of urbanization has brought on great pressure to the traffic of cities, which can be effectively alleviated by the urban underground rail transit system traffic pressure. As an important part of subway engineering, the design and construction of subway tunnels are restricted by many factors, such as geological conditions and the surrounding environment [1,2,3,4]. The drilling-and-blasting method, the most commonly used method for tunnel construction, is low cost and effective [5,6,7,8,9,10,11]. However, in the construction process of blasting control sections, the advantages and disadvantages of the blasting design programs will directly affect the quality of the blasting construction [12,13]. The shallowly buried strata of Qingdao, Dalian and other areas are dominated by miscellaneous fill, clay, sand, granite and tuff with different degrees of weathering, and all them show obvious characteristics, being soft on the top and hard on the bottom [14]. When subway tunnel construction is carried out on this type of strata using the shallow subway excavation method, the rock bodies in the upper and lower sections of tunnels are often found to be very different. The surrounding rocks in the upper section are weak and easily broken, sometimes even with poorly stabilized sand layers, while those in the lower section are intact and hard. In these formations, if a single drilling-and-blasting method is used for excavation, not only is the process cumbersome, making accurate blasting difficult, but it is also very easy for the softer upper strata and adjacent buildings to cause greater disturbance, which leads to an excessive deformation of the surrounding rocks or even to surface and tunnel collapse [15,16].

Recently, in terms of blasting vibration control in shallow urban subway tunnels, scholars have carried out a great deal of research through numerical modeling and on-site monitoring. Zhu [17] used numerical simulation and on-site monitoring to study the Chongqing Light Railway Daping Station, determining the shallow holes’ multi-circulation, division excavation, and additional peripheral shock absorbing holes, as well as the strict control of the number of dregs and other measures to provide an important basis for the determination of construction parameters and blasting parameters of large sections of urban tunnels under complex conditions. Huang [18] fit the coefficients in Sadoff’s formula based on field monitoring data to make a prediction of the blasting velocity of neighboring buildings, meanwhile timely adjusting the blasting parameters during construction to provide a reference for the subsequent blasting construction scheme design. Wang [19], aiming at the Yichang Expressway Tunnel, proposed the double-sidewall guide pit method for the blasting and excavation scheme of class V perimeter rocks, which was used for tunnels underneath buildings, and a good blasting effect was obtained. Yu [20] combined the Shanghai Fuxing East Road Tunnel project, blasting three sections of diaphragm walls in four sections, which ensured the stability of the structure underneath the tunnel. Song [21] addressed the issue of buildings over the Anliu High-Speed Railway Tunnel and proposed a blast control method based on wavelet analysis. Yang [22] optimized a trenching and blasting plan for the Kaiyuan Temple Tunnel in conjunction with on-site monitoring to reduce the vibration hazards of blasting. Shan [23] analyzed the methodology through numerical modeling and field monitoring, obtaining the maximum charge for the blasting construction of the Chongli Tunnel going down through the village to ensure the stability of the village.

In most scholarly studies, the problem of full-section blasting has been dealt with for specific projects, including the control of blasting vibration by means of simulation and monitoring, as mentioned above, and scholars have contributed a lot to infrastructure work. However, the problems encountered vary from project to project. The tunnel of Qingdao Metro Line 13 between Jialingjiang Road and Xiangjiang Road is in a typical stratum that is soft in the upper section and hard in the lower section; the upper and lower sections of the tunnel are in different strata, with more buildings on both sides of it. If a traditional full-section blasting program is used at this time, then the vibration caused by blasting will definitely have a great impact on the buildings; if a single mechanical method is used for excavation, not only will the cost be higher than that using the drilling-and-blasting method, but when carrying out construction in the lower section, the drivage efficiency will also be reduced due to the obstruction of hard rocks. On the basis of such special circumstances, a composite construction method combining mechanical and blasting construction is proposed. No studies have been conducted by scholars on how to optimize the construction parameters of this composite construction method. To solve these problems, in this paper, FLAC^3D 5.0 finite difference software is used to simulate construction, and the numerical calculation results are compared and analyzed with the field monitoring results. A tunnel blasting program was established for tunnels adjacent to buildings on strata that were soft in the upper part and hard in the lower part. Combined with actual projects, the vibration impact control of controlled objects and the blasting quality control of excavated tunnels are realized. By applying this option to the sections of tunnels adjacent to buildings, the impact of tunnel construction on buildings over them can be effectively reduced.

2. Project Overview

The tunnel of the Jialingjiang Road Station–Xiangjiang Road Station section starts from Jialingjiang Road Station, which is in the east along Jialingjiang West Road, then turns into Jinggangshan Road, and ends at the intersection of Jinggangshan Road and Xiangjiang Road. Two shafts are set in this section, with a total length of 1211.124 m. A single-hole double-line horseshoe tunnel occupies the first 407.9 m, and a single-hole single-line horseshoe tunnel occupies the last 803.2 m, with a buried depth of 10.56 m~21.86 m.

The tunnel between Jialingjiang Road Station and Xiangjiang Road Station is facing geological fragmentation and other problems. For example, it passes through heavy industrial plants and other dense buildings, in addition to encountering other problems. The stratigraphy along the line is complex; where the groundwater is serious, the rock fragmentation strength is low, and the fracture’s structure has great influence on this project. All the tunnel intervals are located in class V rock sections. The section in this study is from ZSK5 + 376.877 to ZSK5 + 799.997. The schematic diagram of the surrounding environment of the metro intervals is shown in Figure 1.

Since the tunnel will laterally pass through a section of dense buildings, the vibration velocity needs to be strictly controlled. In this project, more stringent blasting vibration control is adopted than what is stated in the Blasting Safety Regulations (GB6722-2014) [24]. In order to ensure the safe completion of the blasting operation while minimizing its impact on surface buildings, a combined mechanical excavation and blasting scheme is proposed.

The tunnel is excavated in several steps, and the upper guide hole is in a soft stratum. Therefore, the mechanical method is used for excavation. After the mechanical excavation process is completed, a critical surface will be formed to facilitate the blasting operation, which, at the same time, can take advantage of the roundness and smoothness of the mechanical method for the excavation contour line, effectively solving the problem of over (under)-excavation. The initial support leveling is improved, with the installation velocity of grating greatly improved and the quality of shotcrete ensured. The lower guide hole was constructed using the drilling-and-blasting method. Due to the proximity of the mechanical excavation of the upper guide hole, the vibration impacts the surface buildings. At the time of the lower guide hole blasting, because part of the energy will be dissipated into the air from the existing free surface in the upper part, it results in the reduction in the vibration velocity at the work surface directly above the measurement point, thus protecting the buildings. At the same time, the drilling-and-blasting method of excavation can be used to solve the problem of hard rock excavation in the lower section. The drilling-and-blasting method not only has characteristics such as a low cost, and high efficiency and velocity, but with the use of light surface blasting and differential control blasting, the profile of the section can also be controlled. The actual view of the adjacent buildings is shown in Figure 2.

3. Numerical Simulation to Verify the Feasibility of the Solution

3.1. Simulation Solutions

In this study, the two construction schemes are compared using numerical simulation. Among them, the first scheme is a traditional full-section blasting scheme. As the name implies, the upper and lower section are set up with artillery holes for full-section blasting using the drilling-and-blasting method.

The second scheme is the combined blasting scheme. In the upper step, excavation and digging are completed via a cantilever machine, and in the lower step, blasting is completed using the drilling-and-blasting method. The upper and lower steps are staggered by 15 m for a parallel construction, with the upper step in the front.

3.2. Numerical Model and Material Parameters

In this paper, we apply FLAC^3D finite difference software for numerical calculation. The size of the rock body in the model is 80 m × 60 m × 100 m, which contains 257,759 model elements and 265,381 nodes. Due to the similar locations of buildings on both sides of the tunnel, there is no special situation involving the tunnel passing under buildings. Therefore, the building closest to the tunnel was selected for modeling and analysis, which was 9.3 m from the edge of the tunnel, with a height of 72 m, a width of 15 m and a length of 55 m. The numerical model is shown in Figure 3. The representative geological section at the construction site was selected, and the compositions of the stratum from top to bottom were plain fill, powdered clay, strongly weathered granite, mediumly weathered granite and slightly weathered granite, in order. The Mohr–Coulomb elastoplastic ontology is adopted on the rocks, linear elastic ontology is adopted on the building, foundation and lining, and the parameters of stratigraphy and of the building are shown in

Table 1. Formation and building parameters.

Material	E/MPa	μ	γ/(kN·m3)	c/kPa	φ/(°)
Vegetative fill	6	0.4	17	10	15
Powdery clay	15	0.37	19	21	18
Strongly weathered granite	65	0.29	25	50	40
Mediumly weathered granite	450	0.25	27	200	50
Slightly weathered granite	2500	0.21	29	310	55
Building	15	0.16	22	/	/

.

3.3. Blast Pressure Induced by Rock Blasting

The blasting equivalent load method and the EOS-JWL method [25,26] have been adopted by many scholars as the main means of rock blasting simulation, of which the blasting equivalent load method has higher computational efficiency compared with the EOS-JWL method. In this study, the equivalent dynamic load of triangular blasting was used for calculation, and the load time curve is shown in Figure 4. The boost time is taken as 100 μs, the positive pressure action time is 600 μs, and the duration of the blast seismic waves is taken as 0.6 s. The average burst pressure of the explosive is as follows:

$$P_D={\rho }_e{D}^2/2\left(1+\gamma \right)$$

(1)

where $P_D$ is the average initial pressure of the explosive blast; ${\rho }_e$ is explosive density; $D$ is explosive blast speed; and $\gamma$ is the isentropic index of the explosive.

The initial average pressure $P_0$ of the hole is calculated using the following formula:

$$P_0=\left[{\rho }_e{D}^2/2\left(1+\gamma \right)\right]{\left(d_c/d_b\right)}^{2\gamma }$$

(2)

Equivalent calculations of the blasting load on the hole wall were carried out based on Saint-Venant’s principle, which has also been adopted by many scholars [6,27,28,29]. The equivalent form of the blasting load is shown in Figure 5. Explosive properties in this project are shown in

Table 2. Explosive properties in this project.

ρ (kg·m−3)	D (m/s)	dc (mm)	db (mm)	a/(m)	ta	T (μs)	PD (Gpa)	P0 (Mpa)	Pe (Mpa)
1000	3200	32	40	0.6	100	600	2.127	380.56	53.28

.

$$P_e=\left(2r_0/a\right)P_0$$

(3)

where $P_e$ is the equivalent load, $a$ is the distance between adjacent holes, and $r_0$ is their diameter.

3.4. Monitoring Point Arrangement

The calculation conditions of the model are set according to the actual construction process. Twelve monitoring points at different locations of the building were arranged at each floor and foundation through calculations. A total of 216 different monitoring points were arranged, with their numbers corresponding to that of floors, plus different A-L measurement point numbers, which were used to record changes in the peak particle velocity and vertical displacement at different locations of the building. The relationship between the tunnel structure and building measurement points in the numerical calculation model is shown in Figure 6.

3.5. Comparison Analysis of the Vibration Velocity of Different Building Story Heights

The vibration velocity at different building layers under the full-section blasting scheme is analyzed and plotted on a graph, as shown in Figure 7. It can be seen from the figure that the vertical direction of vibration velocity with the rise of floors first decays after the phenomenon increases. Taking seven floors as the demarcation, floors 1 to 7 can be taken as the vibration velocity decay area, and floors 7 to 18 can be taken as the vibration velocity amplification area. According to the variation curves of vibration velocity at different floors in Figure 7, it can be seen that the vibration rate of high-rise buildings under the influence of blasting vibration is divided into two processes: decay and amplification. Regarding floors 1 to 7, there is no high-rise amplification effect. The peak value of vibration velocity attenuates with the increase in detonation distance, which is consistent with the relationship between blasting seismic wave strength, as the increase in burst center distance and attenuation increase, with a peak attenuation of 42.3%. The ppv attenuation rate is calculated by dividing the attenuation value by the vibration velocity value at floor 1. The main reason for the appearance of the attenuation zone is that with the increase in burst distance, when the vibration waves come from a small area of the column section to a relatively large cross-sectional area of the floor section, there will be a reflection of the wave transmission; then, the wave energy’s loss of part of the peak particle velocity will become smaller. Elevation amplification occurs from floor 7 to 18. The main reason for the appearance of the amplification zone is that the top layer is less constrained and that the amplification of vibration velocity occurs relative to the bottom layer, which is more constrained. With the increase in floor, the decaying trend of blast vibration velocity is gradually less obvious than the amplifying trend, which happens when the high-level amplification effect dominates. Therefore, the elevation amplification effect should become a non-negligible consideration during the construction of tunnels passing under high-rise buildings.

3.6. Comparative Analysis of the Building Vibration Velocity for Different Construction Schemes

The equivalent load calculated in Section 3.3 is applied to the edge of the tunnel excavation face, where an equivalent load is applied in the common direction of the outer contour of the full section of the tunnel during full-section blasting. In the combined mechanical excavation and blasting scheme, the equivalent load is applied to the normal outside direction of the tunnel section profile. The data on the building monitoring points at this blast location were compared between the two blast scenarios. The peak particle velocity of full-section blasting scheme is 1.627 cm/s, which exceeds the allowable blasting vibration safety standard of this engineering section. In the combined blasting scheme, the peak particle velocity is 0.636 cm/s. From a comparison of the calculation results, it can be seen that using the combined mechanical excavation and blasting scheme, the peak particle velocity at each monitoring point is significantly reduced compared with that using full-section blasting, with a maximum reduction of 61.1%.

From an analysis of the results, it can be seen that the partial vibration velocity exceeds the blasting vibration velocity control standard according to the project blasting design when using the full-section blasting program for simulation, while the blasting vibration velocity after the simulation of the mechanical excavation joint blasting construction program is in line with the standard according to the blasting organization design of the Jiaxiang interval. A comparison between the vertical vibration velocity simulation results using the two blasting schemes at the monitoring point 1A-1L is shown in Figure 8, along with that between the radial and tangential vibration velocity in such cases. The results show that the vertical vibration velocity of the building under the influence of tunnel blasting is greater than the radial and tangential vibration velocity. The vibration velocity on the blast side is slightly greater than that on the back side, and the results of a three-way vibration velocity comparison at the measurement points in the tunnel blasting case of the building are shown in Figure 9.

4. Optimal Design of Construction Parameters

The numerical simulation results show that through the construction of the combined mechanical excavation and blasting scheme, the vibration velocity of the building can be effectively reduced. As for the combined mechanical excavation and blasting scheme proposed, the problem of further optimization of the excavation feed problem for the upper section and the blasting parameters in the lower section is put forward.

4.1. Excavation Feed Comparison

Due to the presence of buildings adjacent to the tunnel, there will be a certain degree of damage to those above the tunnel as its excavation continues. The main problem is the differential settlement of the buildings caused by an uneven ground settlement. The building in this study is of a frame concrete structure with a strip foundation. In Zhang Dingli’s study [30], it was shown that the building settlement caused by the excavation of the upper half tunnel section accounted for more than 80% of the total settlement. In order to simplify the calculation, the upper section was excavated separately based on the numerical simulation to compare the effect of different working conditions on building settlement. The results recorded at the monitoring points during excavation with different feeds were compiled by considering the self-weight load of the building, and the settlement of its foundation under the two working conditions was plotted, as shown in Figure 10.

Comparing the circular feeds of 0.5 m and 0.75 m, the maximum settlement value of the building foundation when excavating using a circular feed of 0.5 m was 18.53 mm at the upper section of circular feed excavation of 0.5 m, and 22.71 mm using that of 0.75 m. In the actual construction, the excavation of the lower section of the tunnel is also taken into account; therefore, the foundation settlement after the excavation of the upper section divided by 80% is used for the calculations, and the foundation settlement with a circular footage of 0.75 m reaches 28.38 mm. An uneven settlement occurred on the sides of the building near and facing away from the tunnel; thus, the tilt degree of the building should be considered. The building tilt rate was 0.099% at a cycle feed of 0.5 m, which reached 0.122% at a cycle feed of 0.75 m.

Although the settlement and tilt degree of the building met the regulations in the Code for the Design of Building Foundations [31], the foundation settlement of the building reached 28.38 mm at the circular feed of 0.75 m, which was close to the limit value of the foundation settlement of the building in the Code. However, the construction tunnel is in a class V fenestration section [32] with poor geological conditions. Mohammed Sazid [33] showed in his research that the damage area reduced with the increase in rock class. The stability of the fenestration rock needs to be compared. Figure 11 shows the surrounding rock plastic zone at a circular footage of 0.5 m and 0.75 m. By comparing the plastic zone of the surrounding rocks, it can be seen that the plastic zone at the shoulder of the tunnel arch increases significantly in extent at a cyclic feed of 0.75 m, which is not conducive to construction safety. Taking into account the settlement of the building foundation, the tilt degree and stability of surrounding rocks, the final determination on the cantilever boring machine excavation in the upper section of the tunnel is 0.5 m in a cycle.

4.2. Optimized Design of Blasting Parameters

In the project on which this paper is based, mechanical excavation combined with a blasting construction program is used. There are two drilling methods for blasting: one is the conventional palm face drilling method, and the other is similar to the surface mining bench blasting method [34,35]. After mechanical excavation of the upper bench, the lower bench has a free surface condition; meanwhile, due to the large tunnel section, the resulting hole is very deep, which will increase the difficulty in drilling and charging, thus increasing the construction cost, and there is no way to ensure the shaping of the tunnel contour line if the surface mining bench blasting method is used. In contrast, using the method of tunnel step blasting, it is less difficult to drill holes or load charges. The blasting method was used to control the quality of the tunnel profile. The blasting parameters of the perimeter hole, including the charge and the minimum resistance length, are the main determinants of the quality of light face blasting.

Since a new proscenium is created through the mechanical excavation of the upper step, there is no need to arrange trenching holes, as auxiliary and peripheral holes are enough.

(1) The determination of the charge amount.

The charge length per meter of the perimeter hole is as follows:

$$L\le \left(8K_b{S}_c/n{\rho }_0{D}^2\right){\left(d_b/d_c\right)}^2$$

(4)

where $L$ is the length of charge per meter of the hole, m/m; $S_c$ is the uniaxial compressive strength of rocks, Pa; $K_b$ is the coefficient of increase in the compressive strength of rocks, generally taken as $K_b$ = 10; $n$ is the collision pressure increase factor of the hole wall, generally taken as 8 to 11; ${\rho }_0$ is the explosive density, kg/m³;$D$ is the explosive blast speed, m/s; $d_b$ is the diameter of the hole, m; and $d_c$ is the diameter of the explosive roll, m. Then, the charge per meter of the blast hole is as follows:

$$q_L\le \left(\pi {d_c}^2/4\right)L{\rho }_0$$

(5)

where $q_L$ is the charge per meter of the cannon hole, kg/m;

(2) Determination of the minimum resistance line.

In order to make the light explosion layer from the original rock, the minimum resistance line of the surrounding hole must be determined.

The determination of the minimum resistance line lies in a reasonable choice of the proximity factor, m. The value of m should be chosen according to the nature of the rock, which is taken as 0.8 in this paper, and the minimum resistance line is calculated according to the following formula:

$$W=a/m$$

(6)

where $W$ is the minimum resistance line length; and a is peripheral hole shell hole spacing, which is taken as 0.6 m according to experience.

Based on several field experiments and calculations performed in the above manner, the lower step blasting parameters were finally determined. With rock emulsion explosives used as explosives, the specifications of the volume were 32 mm × 300 mm (diameter × length), the weight of the volume was 300 g, and the minimum resistance line length was 500 mm. Digital electronic detonators are used in the holes, and then the terminal equipment is used for field delay. According to the blast hole layout and blasting parameters, the detonator of the corresponding holes is selected, and the delay time is set by sweeping the code with a handheld device, with its interval in each section set as 50 ms. The blasting parameters of the lower step section are shown in

Table 3. Blasting parameters of the lower section.

Name of Holes	Hole Depth/m	Number of Holes	Quantity Capacity of Single Holes/kg	Maximum Number of Single Sections/kg
Chipped holes	1.1	33	0.2	1.0
Peripheral holes	1.1	21	0.3	1.0

, the hole layout is shown in Figure 12, and the schematic diagram of the perimeter hole charging structure is shown in Figure 13.

5. Practical Application Effect on Construction Sites

The interval tunnel from Jialingjiang Road Station to Xiangjiang Road Station of Qingdao Metro Line 13 was constructed via a combined mechanical excavation and blasting scheme, and the vibration velocity of the adjacent buildings was controlled within 0.6 cm/s. The photos of the construction sites are shown in Figure 14. Taking into account the blasting safety regulations and safety regulations on tunnel vibration velocity between Jialingjiang Road Station and Xiangjiang Road Station, the safety regulations of the project are met. With the use of the combined mechanical excavation and blasting scheme, the construction period reduced from 13 months to 10 months, shortening the construction period and reducing cost, which illustrates the feasibility of mechanical excavation combined with the blasting construction method.

The vibration velocity monitoring of the project was completed using a TC-4850 blasting vibration meter produced by Chengdu Zhongke Measurement and Control Company (Chengdu, China). A geophone was arranged in the protected structures near the side of the blast area, and mixed lime powder was used for a rigid connection with the building foundation. The extraction of monitoring data and numerical modeling data at measurement point 1A are shown in Figure 15 together with a comparison schematic. From this figure, it can be visualized that there is not much difference among the peak vibration velocity during monitoring, the peak vibration velocity during numerical simulation and the basic variation trend, which also shows the reliability of the numerical simulation results.

6. Conclusions

In this paper, the combination of numerical and in situ monitoring was applied. A study was carried out on the different geological conditions in the upper and lower sections of the tunnel between Jialingjiang Road Station and Xiangjiang Road Station of Qingdao Metro, in addition to exploring the problem of tunnels passing under buildings. A new composite construction method was proposed for the strata that were soft in the upper part and hard in the lower part. The construction parameters were also optimized, and the following conclusions were obtained:

(1)

With the use of mechanical excavation combined with blasting construction methods for tunnel construction, the vibration velocity of buildings caused by blasting can be effectively reduced. This method can be used for projects with more stringent vibration velocity requirements on adjacent buildings, which is also used in conjunction with light-surface and micro-differential blasting.

(2)

For the construction section of tunnels adjacent to buildings where the geology is poor, the settlement of the buildings and the stability of their surrounding rocks should be considered, and the excavation progress should be reasonably controlled. The section where this project is located contains class V surrounding rocks, and it is suggested that the construction feed of cantilever boring machines in the upper section be 0.5 m.

(3)

Through mechanical excavation combined with blasting construction methods in the upper section of excavation, a free surface has been created, while also reducing the entrapment of rocks. It is possible to achieve the same good blasting effect without the creation of a trenching hole, which has been verified in this project.