The Influence of Complex Subway Station Construction on High Retaining Walls and Appropriate Countermeasures

Anzhen Bridge Station of Beijing metro line 12, which adopts a separated-island platform constructed by the undermining method, passes along the retaining wall of the North+ird Ring Road. A numerical simulation method is adopted to study the deformation laws of the retaining wall under the influence of station construction.+e simulation data show that the affected retaining wall will have large settlement and tilt, and countermeasures must be taken. Four countermeasures are formulated, and their control effects are analyzed. +e calculation results show that the combination of anchor cables and waist beams can effectively control the tilt of the retaining wall, but the control effect on the absolute settlement is not obvious. +e control effect of isolation piles is better than that of foundation grouting. +e joint control method of anchor cables combined with waist beams, isolation piles, and deep hole grouting makes the deformation meet the target, and according to this measure, a concept of joint control aiming at the influence of the “source,” “propagation,” and “object” is proposed, which can provide a reference for similar projects with a great difficulty in deformation control.


Introduction
In concentrated areas, subway lines are usually laid along a city's main road, so the subway tunnel or station underpasses the road frequently. In overpass sections, retaining walls are often installed, and subway construction will not only affect road safety [1][2][3], but also threaten the stability and safety of the retaining walls. At present, there have been many studies on the stability of retaining walls, including the stability of retaining walls under static loads [4][5][6], the dynamic response of retaining walls [7][8][9][10][11], the behavior of retaining walls affected by deep excavation [12,13], and the time-dependent behavior of piles and retaining walls as a result of excavationinduced soil movement [14]. ese studies have made many achievements in terms of the stability of retaining walls. However, the response of a road retaining wall with a shallow foundation under the influence of underground excavation has not been fully studied. According to the in-situ settlement laws of the retaining wall embankment, Ma et al. [15] summarized the experiences on real-time adjustment of shield construction parameters and comprehensive use of grouting. Liu [16] analyzed the settlement laws of the highway retaining wall under the influence of tunnel construction by numerical simulation. e above research only concerns the settlement of the retaining wall. Sun [17] analyzed the settlement and tilt laws of the road retaining walls affected by shallow buried tunnels. However, the influence of station construction is more complicated than that of tunnel and there is little literature on the response of the road retaining wall under the influence of subway station construction.
According to previous research on the influence of underground construction on soil movement, station construction has the following main effects on retaining walls.

Absolute Subsidence.
Soil excavation causes stratum settlement [18][19][20]. A retaining wall will settle with the ground if its foundation is in the settlement area. e resulting settlement is absolute subsidence. In this project, the minimum horizontal distance between the main body and the retaining wall foundation is only 1.05 m. Too much absolute settlement will affect the stability of the pavement structure, resulting in a poor connection between the pavement and the bridge deck and endangering traffic safety.

Differential Settlement.
Differential settlement refers to nonuniform settlement along retaining walls, the magnitude of which is the difference in the vertical displacement between two measured points. e reasons for differential settlement are complicated and include the complexity of station construction processes, the inhomogeneity of the strata and the filling, and the construction of connecting channels. Excessive differential settlement will lead to cracks in the retaining wall, which will seriously destabilize the retaining wall and lead to accidents such as soil filling collapse and road damage.

Tilt Deformation.
e closer a retaining wall is to the centerline of the main structure, the greater the surface settlement caused by construction [21][22][23]. erefore, a difference in the lateral settlement of the retaining wall foundation will cause the retaining wall to tilt and even collapse in serious cases.
However, the above deformation laws and effective control measures are currently unclear. e construction of Anzhen Bridge Station of Beijing metro line 12 will affect the retaining walls of the North ird Ring Road. Because the station adopts a separatedisland platform constructed by the undermining method, the construction of the main part of the station and the two connecting aisles will have a superimposed impact on the retaining wall, threatening the stability and safety of the retaining wall. Based on this background, this paper analyzed the deformation laws of retaining walls under the influence of station construction and proposed effective control measures and control concepts.

Project Overview
Anzhen Bridge Station is at the intersection of An-Ding Road and the North ird Ring Road in an east-west alignment along the North ird Ring Road. It is a doubledeck separated-island platform station constructed by the PBA (Pile-Beam-Arc) method. e depth of the standard section is 14.9 m. ere are two overpasses named Anzhen East Bridge and Anzhen West Bridge over Andingmenwai Street running from east to west along the North ird Ring Road. ere are three sections of retaining walls: one to the east of Anzhen East Bridge, one between the two bridges, and one to the west of Anzhen West Bridge. e retaining wall is a counterfort type with a maximum height of 5.57 m.
ere are 20 retaining walls between the two bridges, and every five retaining walls share a common foundation. A deformation joint of 20 mm is set between the adjacent retaining walls. e Anzhen East Bridge and the walls are depicted in Figure 1. A plan sketch of the station and the retaining walls is shown in Figure 2. e vertical positions of the connecting aisles and retaining wall and of the main structure of the station and retaining wall are shown in Figures 3 and 4, respectively.
Because the height of the retaining wall between the East and West Anzhen Bridges is large, and it is influenced by the construction of the main structure of the station and the connection channels, this section of retaining wall is chosen as the research object. e control indexes of retaining wall deformation were formulated by comprehensively considering the structural features; design calculation results and national standard named the "Code for Monitoring Measurement of Urban Rail Transit Engineering" (GB 50911-2013) are considered comprehensively.
e control indexes of retaining wall deformation are as follows: (1) Maximum absolute settlement of the retaining wall is 15 mm (2) Maximum differential settlement of adjacent retaining walls is 8 mm Every five retaining walls share a common foundation, so there are three deformation joints in twenty retaining walls. ese deformation joints are designated as #1, #2, and #3 from west to east. Four measurement points are arranged for each deformation joint, which are on both sides of the wall top and on both sides of the wall bottom, so 12 measurement points are arranged for the 3 deformation joints.
Because the north and south retaining walls are symmetrically arranged with the station structure and the deformation laws are similar, only the north retaining wall is selected

Calculating Parameters.
e concrete structures of the station adopt the linear elastic constitutive model, and the soil materials adopt the Mohr-Coulomb constitutive model [24]. To ensure the accuracy of the simulation results, the numerical parameters of the soil layers were obtained from the geological    Advances in Civil Engineering 3 investigation report and laboratory tests. To reflect the actual soil-wall interaction behavior, interface elements are used in finite element analysis to simulate sliding between two different materials [25]. e mechanical parameters of the strata and structures are shown in Tables 1 and 2, respectively.

Absolute Settlement of the Retaining Wall.
e absolute settlement of the top of the wall is affected by the tilt deformation of the retaining wall, so an additional settlement value h(1 − cos θ) is added (where h is the height of the wall, and θ is the tilt deformation angle of the retaining wall). erefore, the displacement data of measurement points 1′-6′ in the Z-direction at the bottom of the wall are processed to obtain the settlement curves of the entire construction process, as shown in Figure 8 (the construction sequence of the PBA method is pilot tunnels, side piling, crown beam, arch, middle plate, floor, and side walls. Only the main construction procedures are shown in Figure 8, and the construction of the connecting aisles is added). (1) e construction of both the main structure and the connecting aisle will cause the settlement of the retaining wall. e calculated data show that the settlement caused by the construction of the main structure and the construction of the connecting aisle account for approximately 80% and 20% of the final settlement of the retaining wall, respectively (2) In the constructions of the pilot tunnels and arch, the curves fall quickly, indicating that these two stages have a great influence on the retaining wall (3) Among measurement points 1′-6′, the maximum settlement is 56.78 mm at measurement points 1′, and the minimum settlement is 44.19 mm at measurement points 5′, both of which exceed the settlement index of 15 mm.
According to the settlement value of the midpoint at the bottom of each retaining wall, the final settlement of each retaining wall bottom is shown in Figure 9 (the westernmost 10 10 10 10 Retaining wall panel Foundation West   1  4  3  2  5  8  7  6  9  12 11 10  13  16 15 14  17  20 19 18 Deformation joint #2 Deformation joint #1    4 Advances in Civil Engineering retaining wall is numbered 1, and the numbers increase from west to east). As shown in Figure 9, under the influence of station construction, the final settlement of the retaining wall presents a trend of "high in the east and low in the west." e maximum settlement is in retaining wall No. 1 with a settlement value of 57.39 mm, and the minimum settlement is in retaining wall No. 20 with a settlement value of 39.23 mm. e difference between the two is 18.16 mm. e reason why the retaining wall is high in the east and low in the west is that the west retaining wall is affected by the superposition of the construction of the main structure of the station and the construction of connecting aisle #1. is influence gradually decreases from the centerline of connecting aisle #1 to both sides, so the settlement value decreases gradually from west to east. is phenomenon can be more intuitively illustrated from the settlement cloud map vertically split along the retaining wall, as shown in Figure 10.

Differential Settlement of the Retaining Wall.
By subtracting the settlement value of measurement point No.
1′ from the settlement value of measurement point No. 2′, the difference of each time is obtained to form the diachronic differential settlement curve of deformation joint #1, and the differential settlement curves of deformation joints #2 and #3 are also obtained in a similar way, as shown in Figure 11 (the positive and negative signs of the differential settlement value in Figure 11 represent the height difference between the left and right sides of the deformation joint, not the size).

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As shown in Figure 11, the maximum differential settlement does not occur at the end of construction but rather during construction. e maximum differential settlements of deformation joints #1, #2, and #3 are 1.04 mm, 1.27 mm, and 0.63 mm, respectively, all of which are less than the index value of 8 mm. erefore, the station construction has little influence on the differential settlement of the retaining wall, so the differential settlement of the retaining wall can be regarded as a nonkey control index when formulating protection measures.

Tilt Rate of the Retaining Wall.
e tilt rate of the retaining wall is the ratio of the horizontal displacement difference between the wall top and the wall bottom to the height of the wall-that is, where t is the tilt rate of the retaining wall, x is the horizontal displacement of the wall top, x′ is the horizontal displacement of the wall bottom, and h is the height of the wall. According to the calculation results of the tilt rate, the changing curves of the tilt rate with construction progress for the six measurement lines from 1-1′ to 6-6′ are obtained, as shown in Figure 12 (the positive and negative signs of the tilt rate in the figure represent the tilt direction, not its size).
In the constructions of the pilot tunnels and arch, the curves show obvious peak values, indicating once again that these two stages have a great influence on the retaining wall. e maximum tilt rates of the measurement lines from 1-1′ to 6-6′ are 4.13‰, 3.62‰, 3.62‰, 3.29‰, 3.16‰, and 3.06‰, respectively, all of which exceed the index value of 1‰.
e simulated data show that the final tilt rate of each retaining wall decreases gradually from west to east. To better reflect this phenomenon, the final tilt rate of each retaining wall is shown in Figure 13 (the westernmost retaining wall is numbered 1, and the numbers increase from west to east).
As shown in Figure 13, due to the construction of connecting aisle #1, the tilt rate of the retaining wall shows a trend of "small in the east and large in the west," corresponding to the settlement deformation of "high in the east and low in the west," and the final tilt rates of all 20 retaining walls exceed the index. e final tilt rate curve of the retaining wall from west to east presents a "step-down" shape comprising four "steps," and the dividing line between adjacent steps is the position of the deformation joint of the retaining wall foundation. e tilt rate of the five retaining walls sharing one foundation is at the same level, which indicates that the retaining wall foundation features an antitilt ability for the panels.

Formulation of the Control Measures.
After the above analysis, the absolute settlement and the tilt of the affected retaining wall both exceed the index values, so corresponding control measures must be taken to ensure safety. According to the conditions of this project, several countermeasures such as anchor cables with waist beams, foundation grouting, isolation piles, and deep hole grouting inside the tunnel are proposed to control the retaining wall deformation.

Anchor Cables with Waist
Beams. e retaining walls are connected by waist beams to form a whole, and the anchor cables penetrate the waist beams and are set inwards. e length of the anchor cable is 11.5 m; the length of the free section is 5 m, the length of the anchorage section is 6.5 m, and the inclination angle is 15°downward. One row of anchor cables is set with 10 cables. To analyze the control effect of the anchor cables with waist beams, two cases are arranged: one row of anchor cables and two rows of anchor cables, as shown in Figure 14.

Foundation Grouting.
e grouting position is directly below the retaining wall foundation, and the range is 4.3 m × 42 m × 3 m. When simulating, the stratum parameters after grouting are consistent with those of the advanced   Advances in Civil Engineering small pipe grouting. A sketch of the foundation grouting is shown in Figure 15.

Isolation Piles.
e isolation pile is in the form of a composite anchor pile, which is designed to bear the bending moment and shear caused by soil movements and is arranged between the main structure of the station and the retaining wall in three rows with a diameter of 0.15 m, an interval of 0.8 m, a row spacing of 1 m, a length of 27 m, and a setting angle of 17°. Based on the principle of equivalent stiffness, the isolated pile is simplified into a continuous structure for simulation. e model diagram is shown in Figure 16.

Deep Hole Grouting inside the Tunnel.
Before the excavation of the main structure of the station, deep hole grouting should be carried out to reinforce the soil in front of the tunnel face. e grouting range is 1.5 m outside and 0.5 m inside the excavation contour. e grouting section diagram is shown in Figure 17. e protective effect of each measure is analyzed below. Because the differential settlement of the retaining wall under the condition of no measures meets the index requirement, analyzing the differential settlement of the retaining wall under various measures is not necessary.

Control Effect on the Absolute Settlement.
According to the settlement value of the midpoint at the bottom of each retaining wall, the final settlement curves of 20 retaining walls under the condition of one row of anchor cables and two rows of anchor cables are compared with the settlement curves without measures, as shown in Figure 18. Figure 18 shows that the final settlement curves of one row of anchor cables and two rows of anchor cables almost coincide, indicating that there is no significant difference in the control of the absolute settlement between them. e maximum settlement value of one row of anchor cables is 55.34 mm, and the maximum settlement value of two rows of anchor cables is 55.43 mm. Compared with the situation without measures, the settlement value is reduced by 2.04 mm and 1.95 mm, respectively, so the absolute settlement control effect is not obvious.

Control Effect on Tilt Deformation.
e final tilt rate curves of the retaining wall under the two conditions of one row of anchor cables and two rows of anchor cables are compared with those without measures, as shown in Figure 19.
As shown in Figure 19, the maximum tilt rate of the retaining wall with one row of anchor cables is 2.08%, which is 49.27% lower than that of the retaining wall without measures. e maximum tilt rate of the retaining wall with two rows of anchor cables is 1.31%, which is 68.05% lower than the maximum tilt rate of the retaining wall without measures.
erefore, the use of anchor cables with waist beams has a significant control effect on the tilt of the retaining wall, but the tilt rate still exceeds the requirements of the index.
It is also found that the step height difference of the curve after setting the anchor cables is reduced, which indicates that the use of anchor cables with waist beams reduces the deformation difference among the retaining walls and forms a whole to jointly resist the influence of station construction.    Advances in Civil Engineering

Comparison of the Protection Effect between Foundation
Grouting and Isolation Piles. e absolute settlement of the retaining wall cannot be controlled effectively by means of two rows of anchor cables with waist beams, and the tilt rate still exceeds the index value. erefore, other means should be combined to control the retaining wall deformation. Grouting and isolation piles are two common means to protect adjacent structures in close construction [26][27][28][29][30][31]. However, in past engineering practices, the selection of grouting and isolation piles was often blind. us, scientific protective means were not selected according to the engineering situation, preventing the expected effect from being achieved or causing material waste. is paper compares and evaluates the protection effect of grouting and isolation piles on the premise of applying two rows of anchor cables with waist beams and analyzes the differences of the protection effect between the two measures. e final absolute settlement and final tilt rate of the retaining wall under the conditions of grouting and isolation piles are shown in Figure 20.
By comparing settlement and tilt rate curves between the two cases, it is indicated that the control effect of the foundation grouting is not ideal. e maximum settlement of the retaining wall is 50.30 mm, and the maximum tilt rate is 1.21‰, exceeding the index value by 235% and 21%, respectively. Relatively speaking, the control effect of the isolation piles is remarkable. e maximum settlement of the retaining wall is 19.24 mm, and the maximum tilt rate of the retaining wall is 0.97‰, which meets the index requirements, but the absolute settlement still exceeds the index value by 28%. us, reducing the settlement value of the retaining wall is the most difficult problem in this project, and other means should be combined to control the settlement.
To better explain the reason for the large gap between the protection effects of grouting and those of isolation piles, the stratum settlement cloud maps of the two situations are compared, as shown in Figure 21.
Because the retaining wall foundation is close to the centerline of the main body of the station, the scope of grouting is completely within the scope of the setting tank, and the construction of the station causes the overall settlement of the reinforced soil, which explains why the grouting effect is not significant. In the case of isolation piles, the width of the setting tank decreases obviously, and the impact of the stratum settlement caused by station construction on the retaining wall is weakened.
is is because the lower part of the isolation pile goes deep into a stable stratum, and the structure of the isolation pile is stable, which can have an obvious partitioning effect on the transmission of the impacts of construction. In addition, there is no plastic zone in the pile during construction, so the settlement of the retaining wall decreases obviously.
It can be seen from the protection effect of grouting that when grouting reinforcement is carried out around a shallow foundation, if the grouting scope is completely within the construction influence scope, the grouting reinforced body will settle as a whole, and the protection effect on the foundation is not good. A large number of engineering examples have proven that grouting the soils surrounding an excavation space inside a tunnel can effectively control stratum settlement [32][33][34], so the grouting effect is closely related to the grouting position, and grouting measures are more suitable to solve the settlement problem from the "influence source"; that is, the reasonable grouting range should be the soils around the excavation face.

Deep Hole Grouting Protection Effect.
e differential settlement and tilt rate of the retaining wall meet the index requirements based on the combination of two rows of anchor cables and isolation piles, but the absolute settlement still exceeds the index value. To further reduce the settlement value, deep hole grouting is performed in addition to the aforementioned combination of measures, and the final settlement of each retaining wall is obtained, as shown in Figure 22.
According to the calculation results, the maximum settlement of the retaining wall is 13.87 mm, which meets the index requirement, and the maximum tilt rate of the retaining wall is further reduced to 0.60%.

Determination of the Final Protection Measure.
After analyzing the effect of each control measure, the final protection measure is determined as a combination of two rows of anchor cables, waist beams, isolation piles, and deep hole grouting. To demonstrate the control effect of each measure on the retaining wall deformation, the settlement and tilt rate changing curves of the retaining wall are drawn according to the final deformation of the retaining wall under each measure, as shown in Figure 23 (the measures are gradually accumulated from left to right, and the settlement value is the absolute value). e joint measures adopted for retaining wall deformation cover the entire transmission of the impacts of the construction: the deep hole grouting aims at the source of   the influence, the isolation piles aim at the propagation of the influence, and the anchor cables with waist beams aim at the object of the influence. For similar projects with a great difficulty in settlement control, the combination of "source," "propagation," and "object" can effectively protect adjacent structures.

Conclusions
is paper analyzes the retaining wall deformation under the influence of station construction by a numerical simulation method. Four countermeasures are formulated and their control effects are analyzed. e final protection measures and the protection concept are proposed. e main conclusions are drawn as follows: (1) e construction of the station has little influence on the differential settlement of adjacent retaining walls, but the affected retaining walls have large absolute settlement and tilt deformation values, forming a trend of "high in the east and low in the west." (2) Anchor cables with waist beams can effectively control the tilt of the retaining wall, but the tilt rate is still beyond the target in this case. e simulation data show that the control effect of anchor cables on the absolute settlement is not obvious, but this measure has a significant control effect on the tilt of the retaining wall. (3) e differences in the protection effect between foundation grouting and isolation piles are analyzed. e calculation results show that the foundation grouting control effect is not ideal because of the oversettlement of the reinforced soil, while the control effect of the isolation piles is significant. e tilt rate meets the index requirements when isolation piles is adopted, but the absolute settlement still exceeds the index. (4) After the analysis, a combination of anchor cables, waist beams, isolation piles, and deep hole grouting is adopted to jointly control the retaining wall deformation. e maximum settlement and the maximum tilt rate of the retaining wall in this case are both less than the deformation index. According to this measure, a concept of joint control aiming at the influences of the "source," "propagation," and "object" is proposed, which can provide a reference for similar projects with a great difficulty in deformation control.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.