Risk Analysis and Control Factors Based on Excavation of a Large Underground Subway Station under Construction

Considering the convenience of pedestrian transfer, reasonable structural stress and beautiful shape design, most subway stations adopt symmetrical design. At present, the new subway station is developing in the direction of a multidimensional space, as well as a large scale, and complex structure. Tunnel construction also presents unpredictability, coupling amplification and high risks. For example, a subway extension project involves construction, which would affect the normal use of the subway or damage its structure. Based on excavation of the largest underground subway station under construction in China, the Erligou station extension project (line 16 of Beijing Metro), and using theoretical analysis, numerical simulation, monitoring data, and other research methods, this paper quantitatively analyzes the risk of a large space station’s construction process on the adjacent existing station structure and track, as well as highlights key, high-risk sub-projects, or construction steps, combined with specific engineering measures to ensure safety during construction of a new station. The general rules concerning large space subway station construction are further summarized to provide reference for similar projects.


Introduction
Generally, subway stations are designed symmetrically. However, due to the increasing number of urban public transport trips, its construction also presents another trend: a multidimensional space, as well as a large scale, and complex structure. Its construction risk also presents unpredictability, coupling amplification and high risk [1,2]. A subway is usually built in the center of a densely populated city. Since a large number of subways, municipal bridges, roads, municipal pipelines, and high-rise buildings have been built in city centers, newly-built subways often pass through existing buildings and structures in a short distance. Table 1 lists some engineering cases at Beijing Metro crossing existing lines [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. If the stratum deformation caused by engineering construction is too large, it will cause excessive subsidence, differential subsidence, cracking, and even collapse of adjacent existing projects, and then cause irreparable economic property loss and casualties [18,19]. For example, in July 2003, an accident occurred in the tunnel between Pudong South Road and Nanpu Bridge Station of Shanghai Metro Line 4, resulting in three buildings on the ground, the fracture of surrounding roads, fracture of the Pujiang levee due to excessive deformation, as well as river water flowing back into the tunnel. In November 2008, 17 people died and more than 50 people were injured due to the collapse of the subway foundation pit at Hangzhou Fengqing Avenue. In July 2010, the collapse of Xisanqi station (line 8 of Beijing Metro) caused the rupture of an underground natural gas pipeline, and gas leakage, which affected the gas consumption of tens of thousands of nearby households [20][21][22].  [13] Chegongzhu ang pinganli section of line 6 Chegongzhu ang station of line 2 Horseshoe shape 6.6 * 6.9 Crossing below 90 2.47 Step method 12 [14] Guangdouble section of line 7 Subway extension constructions take place close to existing lines. Existing stations are built into new subway stations by means of close-up extensions, connections, and vertical increases. These kinds of newly-built subway stations are very close to existing stations, and are combined with a large scale of undercut sections and obvious spatial effects, which significantly increase construction risks. Construction risks in subway extension projects are mainly due to excessive deformation of existing station structures caused by close-distance construction, which affects normal use of the subway or causes structural failure. Some scholars have carried out research, combined with specific projects, to analyze the mechanical behaviors of subway construction close to existing lines.
Regarding Suzhou Street Station (line 16 of Beijing Metro), Jia Longfei [23] simulates the construction process of the new station by combining the stratum structure method and the load structure method, and analyzes the influence of the existing station on the stress deformation caused by the existing station. According to the deformation control standard of the existing structure of the urban rail transit, Jia Longfei [23] puts forward the deformation control value for the construction procedure of the new station, and gives engineering measures to control the deformation.
Wu Zhijian [24] used ANSYS finite element analysis software, combined with the actual monitoring data, and obtained the deformation law of the concrete structure and track geometry size of the new station, parallel, crossing the existing station, and proposed construction safety control measures.
According to the engineering characteristics of Xuanwumen Station (line 4 of Beijing Metro), passing under the existing station, Liu Lei et al. [25] put forward comprehensive construction technology of a large pipe shed and segmented backward curtain grouting for pre-reinforcement, tracking compensation grouting in the whole process of excavation and support, and block construction of secondary lining to ensure normal operation safety of existing lines.
In view of engineering problems (i.e., tunnels crossing existing lines and deformation control), Chen Mengqiao [26] and Yang Guangwu [27] proposed technology used for synchronous jacking. This involves dynamically adjusting the jack layout and jacking force distribution based on the deformation monitoring data of the existing line. The technology is based on the principle of "zoning, phased, grouping and grading". It was applied in the turn back line project of Dongzhimen Station at the Beijing Airport line after crossing line 13 of Dongzhimen Station.
Relying on the Gongzhufen station (line 10 of Beijing Metro) closely passing through the Gongzhufen station of line 1, Tao Lianjin et al. [28], through theoretical analysis and numerical calculation, concluded that the stress release of the tunnel face is the main factor causing the settlement of the existing station. However, timely closing the initial support, adding temporary jack column, and back-filling grouting can effectively control the settlement deformation of the existing structure.
Park, boo Seong et al. [29] introduced the design and construction method of the largest tunnel station in Seoul. The station is located 15 cm below the existing Metro Line 3, and an abandoned underground shopping center has been built in the highly congested area. In order to overcome the obstacles of adjacent construction, civil complaints, and traffic jams caused by site conditions, a construction method combining pipe shed method and honeycomb arch method is proposed and applied. The subway station has been successfully constructed without any damage to adjacent structures.
Through a variety of advanced monitoring instruments, including distributed optical fiber strain sensing (DFOS), photogrammetry, and strain gauge instrumented tunnel bolts, Gue, C.Y., etc. [30], studied the mechanical and deformation behavior of the existing royal post tunnel during the new passenger tunnel construction, and guided the design and construction of the tunnel according to data feedback.
Researchers mostly focus on the study of construction deformation, under a new subway crossing an existing line, while research on the construction risk of crossing existing lines, above, is relatively small, especially for subway extension projects involving large spaces and large sections. Using Erligou station (line 16 of Beijing Metro) as the engineering background, this paper analyzes the influence of construction processes of new stations (involving large spaces) and the resulting stress and deformation on adjacent structures, and track crossing existing lines, by using the research methods of theoretical analysis, numerical simulation, and monitoring data. The construction risk is quantified by force and deformation, and the key construction process sensitive to the deformation of existing structures, is discovered. Control measures are taken to ensure the normal use of existing stations in the construction process.  The newly-built Erligou station is composed of three main structures at both ends (double-layer three column four span structure) and middle (single-layer and double span structure), as well as auxiliary structures, such as transfer channel, external hanging hall, and air duct, as shown in Figure  2.  Figure 3. The middle section of the station crosses the existing Erligou Station (of line 6 above), which is constructed by the middle tunnel method. The excavation section is 19.8 m wide and 8.57-9.67 m high. The existing line 6, here, is a flat-topped, straight wall section ZB; the initial lining adopts a 350 mm thick C20 grid shotcrete structure, and the second lining adopts a C40 molded concrete, as shown in Figure 4.

Geology
The maximum depth of the stratum revealed in this survey is 51 m. According to the drilling data and indoor geotechnical test results, the stratum is mainly composed of artificial fill, silty fine sand, silty clay, and sand pebble. The Erligou station, of the new line 16, is mainly located in pebble and silty clay layers. The geological profile is shown in Figure 5.

Description of Calculation Model
(1) Calculation Assumption In this paper, MIDAS/GTS finite element software is used to establish the stratum structure model, to analyze the influence of the construction stages of the newly-built M16 Erligou station on the station structure and track of line 6. The purpose of using the finite element software is, not to pay attention to the accuracy of the absolute value of the calculation results, but to find out the rules and trends by analyzing the relative values of deformation or stress.
The calculation assumes the following: (1) During the construction of the new structure, the Erligou station of the existing metro line 6 only considers normal operational conditions, and does not consider earthquakes and civil air defense conditions.
(2) It is assumed that the soil layer is homogeneous and horizontally distributed.
(2) Mechanical Model and Calculation Parameters The upper boundary of the numerical calculation model is the surface, which is 50 m in Z direction, 215 m in X direction, and 200 m in Y direction, as shown in Figure 6. Different constitutive models are used to simulate different materials, a linear elastic model is used for concrete, and the Mohr Coulomb (M-C) model is used for soil. The model is divided into 139,811 elements with 21,134 nodes. The ground overload is considered as 20 kPa. The values of physical and mechanical parameters of materials in the model are shown in Table 2.

Simulation of Construction Process
According to the construction process of Erligou station, on the new M16 line, the calculation is divided into 15 construction stages for simulation, as shown in Table 3. Integrated interface of construction southeast corner 6 Ground reinforcement by grouting in the overpassing and concealed excavation section 7 Construction of overpassing and concealed excavation section 8 Four connecting channels under the construction undercut section 9 Construction of no.3 air duct 10 Construction of no.2 and no.5 external hanging halls 11 Construction of no.1 and no.4 access roads 12 Southwest entrance and exit of construction 13 Construction of external hanging hall 1 14 Northwest entrance and exit of construction 15 Construction of northwest exhaust and fresh air shaft

Stress and Deformation Analysis of Existing Station Structure
(1) Deformation Analysis The newly-built station is located above the existing station of line 6; therefore, the deformation effect of the new station construction on the existing station structure is mainly (Z direction) vertical deformation, and only the vertical deformation can be analyzed. Since there are many simulated construction stages, only partial deformation diagrams are listed, as shown in Figure 7. The deformation results of each construction stage are shown in Table 4. 2 Construction cross passage and PBA section.    The existing station is one of the risk sources in the construction environment of the new station. The construction of the new station will cause disturbance and deformation in the soil. The soil, as the transmission medium, will transfer the deformation to the existing station, causing the deformation of the existing station's concrete structure. If the deformation is too large, it will cause the concrete structure to crack. According to the cooperative deformation principle, the track will also produce excessive deformation, which will affect the normal operation of the subway. This is consistent with the principle of the tuning fork resonance test. Air, as a medium, transmits acoustic energy, which makes the non-contact tuning fork vibrate.
If the deformation is regarded as the direct factor causing the construction risk, the risk of adjacent construction can be quantitatively analyzed. It can be seen from Table 4 that the construction of the new Erligou station will cause the overall uplift of the existing station. The maximum uplift of the existing structure is 4.952 mm, which occurs in the construction of the southwest entrance and exit in phase 12. The construction of the PBA section of main structure and the overpassing and concealed excavation section has obvious influence on structural deformation of the existing railway. In the seventh construction stage, the distance between the overpassing and concealed excavation section and the existing line is the closest, which has the greatest impact on the structural deformation of the existing line 6 tunnel, with upward floating of 2.561 mm. Therefore, the overpassing and concealed excavation section is a key part of the project to control the structural deformation of the existing line. Secondly, the PBA section of the main structure causes the existing line to float 0.987 mm. Due to the close distance between the PBA construction section, the existing line, and the large scale of underground excavation space, the construction disturbance has great influence on the deformation of the existing line tunnel structure. <The technical code for monitoring of Urban Rail Transit Engineering> (GB 500911-2013) stipulates that the floating deformation of the existing line shall be controlled within 5 mm. The safety factor of 5 mm is considered here. Even if the floating deformation of the existing line exceeds 5 mm, the structure may not be damaged, but the probability of structural failure risk will increase. From the perspective of safety, we assume that 5 mm is the value limit of deformation, and failure of the existing station structure, and the contribution degree of each construction stage to the structural failure risk of the existing line 6 tunnel can be given; P = deformation value D/limit value D0 of existing station, as shown in Figure 8. It can be seen, from the figure, that the seventh stage of the construction of the overpassing and concealed excavation section has the largest contribution to the structural failure risk of the existing station, 50.38%, which is a key part of the project to control the risk. (2) Stress Analysis The position with the largest structural deformation of the existing line tunnel is selected for stress analysis, as shown in Figure 9 and Table 5. Due to the large number of stress nephogram and little color difference between stress diagrams, Figure 9 only shows the partial stress Fxx before and after construction, at the place with the largest deformation of the existing structure, and the complete stress difference is shown in Table 5.

Initial stage Fxx
Maximum stage of structural deformation Fxx  It can be seen from Table 5 that during the whole construction process of Erligou station, of metro line 16, the maximum structural internal force of the Erligou station of the existing metro line 6, and the section with the largest structural deformation, is 4043.69 kPa; thus, it is necessary to check the bearing capacity of reinforced concrete structure, and focus on real-time monitoring of this part during the construction process to ensure the safety of the existing structure.

Simulated Construction Sequence
According to the actual construction method of the sub-project, the simulation is divided into 16 construction stages, as shown in Figure 11 and Table 6.  Grouting reinforcement of soil mass of tunnels 1 and 2 5 Excavation of soil mass of tunnels 1 and 2 6 Initial support for tunnels 1 and 2 7 Construction of secondary lining of tunnels 1 and 2 8 Grouting reinforcement of soil mass of tunnels 3 and 4 9 Excavation of soil mass of tunnels 3 and 4 10 Initial support for tunnels 3 and 4 11 Grouting reinforcement of soil mass of tunnels 5 and 6 12 Excavation of soil mass of tunnels 5 and 6 13 Initial support for tunnels 5 and 6 14 Construction of base plate 15 Construction of secondary lining 16 Excavation of rectangular passage

Analysis of Stress and Deformation Characteristics of Track Bed Structure
(1) Deformation Analysis According to the calculation assumption, the existing track and track bed structure deform harmoniously, so the track bed structure deformation is used to represent the track deformation, and the stress and deformation characteristics of the existing line in the construction process of the overpassing and concealed excavation section, using the middle hole method, are analyzed. The corresponding monitoring points are set on the existing station structure, as shown in Figure 12, to monitor the vertical displacement of the station in each construction stage. The vertical displacement of the main structure of the existing station is shown in Figures 13 and  14. It can be seen from the deformation curve that due to the excavation and unloading of the soil mass of the new station, the uplift deformation of the lower station occurs, and the uplift amount is positively correlated with the excavation progress. The overall uplift deformation presents a reverse peck curve. The construction of primary support and secondary lining forms the main structure of the new station, which is equivalent to loading above the existing station, which helps to reduce the uplift deformation of the existing station. Because the soil between the two stations is reinforced by grouting, the uplift of the middle part of the curve will be reduced, and the trend curve of the reverse double peck settlement tank will be formed finally. The influence of grouting reinforcement on the bottom of the existing station structure is small, and its general trend is consistent with that of the top, but there is only a single reverse peck settlement tank.  The vertical deformation of existing station structure in each construction sequence is shown in Table 7. It can be seen from the table that the accumulated maximum floating point of the existing station was 5.473 mm, which occurred in the 12th construction stage. The maximum floating value of the existing station in each construction stage was 3.3 mm, which also occurred in the 12th construction stage, accounting for 63.4% of the cumulative deformation of the existing station. Therefore, the soil excavation of the side tunnels, 5 and 6, is the stage with the greatest risk of causing excessive deformation of the existing station structure. In the actual construction process, the deformation monitoring during the construction stage should be strengthened and reinforcement measures should be taken in time. (2) Stress Analysis During the construction of the new station, the maximum and minimum principal stresses of the existing station structure are shown in Figure 15. The maximum principal stress occurs at the top plate of the left line, with the maximum value of 12.848 MPa, which is greater than the design value of C40 concrete axial tensile strength of 1.71 MPa, and there is a risk of concrete cracking. In addition, the maximum compressive strength of the reinforced concrete is 288.1 MPa, which is smaller than the design value of reinforced concrete.

Maximum principal stress diagram
Minimum principal stress diagram

Deformation Control Measures for Construction Close to Existing Line
Deformation is the direct factor leading to the failure of the existing station concrete structure. In the construction process of the new station, it is necessary to control the deformation to ensure the normal use of the existing station. The deformation includes the structural deformation of the new station itself, the surrounding soil deformation, and the concrete structure deformation of the existing station. The construction of the new station is the source of deformation, and soil is the medium to transfer deformation/risk. Therefore, deformation can be effectively controlled from the source and propagation path.
The source: (1) changes the construction method, adopts a multi-pilot tunnel excavation to change the tunnel working face from a large section to a small section, reduces the space effect caused by construction, and then reduces the deformation ( Figure 16). (2) Support measures, such as initial support, radial bolt, foot lock bolt, and advance small pipe are adopted to provide support resistance from outside, bear part of the surrounding rock pressure caused by excavation unloading, and restrain soil deformation around the tunnel excavation contour line. In the aspect of the transmission path: the cement slurry, or cement water glass double liquid slurry, is injected into the soil to improve the cohesion c and internal friction angle of the soil, to enhance the anti-deformation ability of the soil body itself, reduce the soil pressure exerted on the existing station structure due to the soil deformation, and control the deformation of the existing station structure.

Analysis on Deformation Monitoring of Existing Track Station
Whether the engineering measures adopted are effective, and whether the construction risks are controlled or not needs to be verified by the actual monitoring data. The vertical deformation of track structure in Erligou station, Baishiqiao south station-Erligou station section, and Erligou station Chegongzhuang West station section of the existing metro line 6 is monitored. The range is K5 + 009.000~K5 + 310.000 for the right line, 301 m for single track, K4 + 991.000~K5 + 310.000 for the left line, and 319 m for single track. Figures 17 and 18 show the monitoring range of the track's vertical deformation of the existing station and the data pictures taken by staff during field monitoring.  It can be seen from Figure 19 that, due to the influence of the construction of the new Erligou station, the existing line floats upward, and the deformation trend is high in the middle and low on both sides. The distance between the station and the upper undercutting section is closer than that of the sections on both sides, so the space-time effect caused by the construction is greater, and the floating degree is more obvious. The accumulated maximum uplift value of the left line is 1.34 mm, and that of the right line is 1.63 mm, which meets the requirements of the existing line structure and track deformation in the technical code for monitoring of Urban Rail Transit Engineering (GB 500911-2013) and the enterprise standard technical standard of Beijing Metro Operation Co., Ltd. guidelines for maintenance of public works (QB (J)/BDY (a) xl003-2009).

Conclusions
Through the above research, we can draw the following conclusions: (1) Construction risks may occur in metro extensions due to adjacent existing structures. The risk of underground excavation in large sections and large space subway stations has the following characteristics: classification, zoning, and time sharing.
(2) There are many types of tunnel construction risks. There are many types of tunnel construction risk. Different risk types have different quantitative evaluation indexes and different monitoring instruments. The analysis of construction risk must be specific to a certain risk event in order to be targeted. For example, the risk of structural failure of adjacent existing stations caused by the construction of new stations is studied.
(3) During the construction of subway stations, there are risk zones. The upper-pass and undercut sections are high-risk areas, which contribute 50.38% to the risk of structural failure of the existing station. It is necessary to strengthen monitoring and safety measures. The size of the construction space and the proximity distance are the two most important factors that affect risk zoning.
(4) The risk of a single sub-project during the construction process is dynamic and a function of time. In the construction process of the upper-crossing undercut section, the soil excavation of the side tunnels, 5 and 6, has the greatest impact on the deformation of the adjacent existing line, accounting for 63.4% of the cumulative deformation of the existing station. It is necessary to strengthen monitoring during the high-risk construction period. The best way to control risk is to "break the whole into parts" and prevent and control them step-by-step.

Conflicts of Interest:
The authors declare no conflict of interest.