Deformation Control of Subway Stations under the Influence of the Construction of Deep and Large Foundation Pits with Composite Support Systems

Abstract

The excavation depth of a deep and large foundation pit of a composite support system is 25.5 m, the minimum clear distance from the main structure of the station is 10.6 m and the closest distance to the entrance and exit structure is only 3.6 m. Relying on this project, numerical simulation, field monitoring and other methods are used to study the deformation law of the self-supporting system, the ground settlement and the subway station under construction. Then, the influence of different support parameters on the deformation of the subway station and track is analyzed, and the internal relationship between the deformation value of the station and track and the support parameters is further discussed. The main work and achievements are as follows: (1) Through field measurement and analysis, the deformation history of the station and track in the excavation is studied. The obtained results show that the interbracing + pile–anchor composite support structure has better control over the deformation of adjacent stations and tracks. Comparing the simulated value of station and track deformation with the measured data, it is concluded that the simulated and measured deformation trends remain consistent, and the maximum deformation value corresponds to the middle position of the foundation pit. This confirms the rationality and reliability of the composite support system model. (2) The analysis of the influence of different support parameters on the maximum deformation value of the subway station and track shows that the deformation of the station and track is less affected by the pile diameter of the retaining pile and the size of the interbracing section. It is greatly affected by the embedding depth of the retaining pile and the number of support channels. It is suggested that the optimal parameters should be a pile diameter of 1.0 m, embedding depth of 8 m, four interbracings and an interbracing section of 1.0 m × 1.0 m.

1. Introduction

The support function of a composite support system can effectively reduce the deformation of the existing subway station structure caused by foundation pit construction. Domestic and foreign scholars have performed many studies on the safety and stability of foundation pit construction and its impact on the safety of adjacent existing subway structures based on theoretical analysis, numerical analysis and field monitoring analysis.

(1) Theoretical analysis method

Sun et al. [1] established an analytical model to study the influence of tunnel deformation caused by circular foundation pit excavation and compared it with the existing centrifugal test data and the analytical solution of square excavation. It is determined that the results of the three are within a small error range, and the reliability of the solution is verified. Based on the Mindlin solution, Zhang [2] calculated the surrounding surface settlement and the change in additional stress in the adjacent subway section during the excavation of the foundation pit and verified the influence of foundation pit dewatering and foundation pit sidewall stress release on the additional stress in the adjacent subway section. Wang et al. [3] studied an existing subway station near a deep foundation pit based on plate and shell theory, numerical simulation and field measurement methods; based on the relevant theoretical analysis, it was concluded that the excavation depth has the greatest impact on the lateral structural deformation of the station, followed by the excavation length, additional load and stiffness of the existing station, while the soil gravity has the least impact. Based on the Pasternak foundation model, Zhang et al. [4] optimized the two-stage simplified analysis method of longitudinal deformation of adjacent subway sections caused by construction for adjacent double foundation pit construction projects and analyzed the influence of simultaneous excavation of double foundation pits in soft soil on the vertical displacement of tunnels.

(2) Numerical analysis method

Zhu et al. [5] analyzed the influence of dynamic stress redistribution on deformation using a two-dimensional mathematical model. Zuo et al. [6] investigated and analyzed more than 20 domestic engineering examples of subway tunnel deformation caused by excavation and unloading of foundation pits, comprehensively analyzed the deformation law of subway tunnels adjacent to foundation pits with the excavation process using the finite element method and proposed an empirical formula for the maximum uplift and subsidence value of double-track tunnels in soft soil areas. Li et al. [7] applied the numerical simulation method to study the variation law of the structural deformation and the geometric shape and position of the existing track under the conditions of different excavation depths of the foundation pit and distances from the existing shield tunnel. Wang et al. [8], based on the foundation pit of a subway station in Xi’an adjacent to the existing subway station and section project, according to the factor of relative distance between the new foundation pit and the existing station, using the finite element method based on the parameter factors of different relative distances, analyzed the surface settlement, the deformation law of the retaining structure of the foundation pit itself and the adjacent station with the excavation process.

(3) Measurement analysis method

Kontogianni and Stiros [9] analyzed the geodetic monitoring records of some recently constructed tunnels and their deformation factors. A previous study [10] has shown that tunnel ground deformation monitoring is a common means to select and control the excavation and support methods predicted in the design and ensure the safety of tunnel excavation (including the safety of personnel in the tunnel and the safety of the ground structure) and construction quality. Three examples are given to illustrate the ground deformation monitoring and its application in tunnel construction: the Jubilee line extension of the London metro, lines 2 and 3 of the Athens Metro and the 9 km long Kallidromo railway tunnel in Greece. Another study [11] used lidar to measure tunnel deformation. The abovementioned study has shown that this method can greatly improve measurement accuracy. Using different monitoring methods, Kavvadas [10] studied the changes in the size and shape of the plastic zone during the whole excavation process. Martino and Chandler [12] found that deformation was affected by different factors, such as stress adjustment, tunnel direction, shape of excavation section and excavation method, by microvelocity probes and borehole cameras. Other studies [13,14,15] found that clear changes in plastic deformation and wave velocity decrease were observed in the surrounding rock mass area near the tunnel sidewall. Studies [16,17,18] evaluated changes in the mechanical properties of the surrounding soil when using the drill bit and blasting excavation method. Wang et al. [19] studied the reinforcement method and reinforcement parameters outside the deep foundation pit of a subway station and proposed a response scheme for the construction of foundation pits under high water levels. The deformation influence of the diaphragm wall under different retaining structure stiffness and anti-overturning stability coefficients was analyzed, and the grouting reinforcement parameters were analyzed and optimized according to the law of economic marginal decline. Studies [20,21,22] have found that the excavation of deep rock tunnels leads to the rearrangement of geological stress, resulting in a large deformation of the surrounding rock.

Currently, there are many discussions on the type selection of foundation pit support design and the verification of the application of foundation pit composite support structures in complex environments. However, there are few studies on the deformation law of its own retaining structure caused by deep and large foundation pit construction of composite support systems in complex environments, the law of surface settlement and the impact on the safety of adjacent subway stations. To ensure the safe operation of adjacent subway vehicles during the excavation of deep and large foundation pits, it is particularly important to study the influence of deep and large foundation pit construction of composite support systems on the deformation of adjacent subway stations.

Based on the deep and large foundation pit with a composite support system of a new building project in Beijing, this study analyzes the deformation law of the supporting structure of the foundation pit itself in the process of foundation pit construction and its influence on the deformation of adjacent subway stations and tracks through numerical simulation, on-site monitoring and data fitting to provide some project support for the construction of similar foundation pit projects in the future.

2. Test Design and Result Analysis

2.1. Overview of New Construction Projects

Based on the project foundation pit lying longitudinally north–south and transversally east–west, the proposed structure is a five-story basement. The excavation depth is 25.5 m. The retaining structure of the foundation pit support system is a bored cast-in-place pile, and the interbracing structure is a reinforced concrete truss support. Three reinforced concrete truss supports and one prestressed anchor cable are set in the internal support system of the foundation pit, and the anchor cable is one pile and one anchor. The profile of the foundation pit is shown in Figure 1 below.

The design parameters of the interbracing, crown beam and waist beam in the foundation pit are shown in

Table 1. Supporting structure design parameters.

Name	Bracing Structure	Cross-Sectional Dimensions (m2)	Buried Depth (m)
Interbracing	The first internal support	0.8 × 0.8	3.5
	The second inner support	0.8 × 1.0	9.5
	The third inner support	1.0 × 1.0	14.5
Crown beams, waist beams	Crown beams	1.0 × 1.0	3.5
	Waist beam 1	1.1 × 1.1	9.5
	Waist beam 2	1.1 × 1.2	14.5

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The diameter of the prestressed anchor cable of the foundation pit is 0.2 m, and the total length is 30 m. The length of the free section is 5 m, and the length of the anchorage section is 25 m. The buried depth of the anchor cable is 20.5 m, and the designed inclination angle is 20°. The water–cement ratio of the cement slurry used for the anchor cable is 0.45~0.50, the strength of the cement slurry is greater than 15.0 MPa and the tensile design value is 900 kN.

The influence scope of the new foundation pit project includes the main body of the subway station, the entrance and exit and the section structure on both sides of the station. The west side of the foundation pit is adjacent to the existing station and the entrance and exit structure. The distance between the outer edge of the foundation pit retaining pile and the outer edge of the station sidewall is 10.6~13.9 m, and the distance between the outer edge of the foundation pit retaining pile and the entrance and exit structure is 3.6~4.3 m. The distance between the anchor cable of the foundation pit and the retaining pile of the existing station is approximately 1.9 m. The position relationship between the new foundation pit and the existing station is shown in Figure 2.

The groundwater in the range of depth at the construction site of the new foundation pit is phreatic water. Its stable water level is in the sixth layer (pebble stratum below the range of the foundation pit excavation), buried between 28.70 mm and 29.80 m, and the water level elevation is between 23.06 and 24.12 mm. Since the groundwater is buried at a deep depth and located in the pebble layer, the dewatering of the foundation pit was not involved during the construction. The engineering geological and hydrogeological conditions of the newly built foundation pit are shown in Figure 1 and Figure 2.

2.2. Test Scheme

This study mainly focuses on the deformation monitoring of adjacent stations and track structures. The main contents of monitoring include the following:

(a) Displacement monitoring of the foundation pit retaining pile;

(b) Displacement monitoring of the existing station main body and subsidiary structure;

(c) Displacement monitoring of the existing track structure;

(d) Track geometry monitoring.

The monitoring contents and instruments of the project are shown in

Table 2. The main content of foundation pit engineering monitoring.

Serial Number	Category	Monitoring Content	Monitoring Project	Deformation	Monitoring Equipment	Monitoring Accuracy
1	Automatic monitoring	Station structure	Station structure	Vertical	Static level	0.1 mm
2	Manual monitoring	Station structure		Vertical	Electronic level	0.3 mm
3				Lateral	Total station	0.5 mm
4		Track structure	Orbital structure	Vertical	Electronic level	0.3 mm
5			Ancillary structure	Lateral	Total station	0.5 mm
6				Vertical	Electronic level	0.3 mm
7		Orbit	Geometric bits		Track gauge	1.0 mm
8		Foundation pit retaining structure	Enclosure pile	Deformation	Clinometer	0.02 mm/0.5 m

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The abovementioned monitoring contents correspond to different monitoring point arrangements. The top horizontal displacement, vertical displacement monitoring and deep horizontal displacement monitoring of foundation pit retaining piles are arranged with 22 measuring points, for a total of 66 measuring points, numbered ZQS-1~ZQS-22, ZQC1~ZQC-22 and ZQT-1~ZQT-22, respectively. The number sequence is ordered from the northeast corner of the foundation pit along the foundation pit counterclockwise. Ordering is performed from north to south. The monitoring layout of the foundation pit is shown in Figure 3.

A total of 14 measuring points for station automatic monitoring (ZDH-1~ZDH-14) are arranged near the sidewall of the station near the foundation pit. There are 10 artificial monitoring points for station lateral deformation (CZH-1~CZH-10), 8 artificial monitoring points on the left side (ZZC-1~ZZC-8) and 14 artificial monitoring points on the right side (YZC-1~YZC-14). The right track structure has 16 vertical monitoring points (YGD-1~YGD-16) and 16 horizontal displacement monitoring points (YGS-1~YGS-16), and the left track structure has 10 vertical monitoring points (ZGD-1~ZGD-10) and 10 horizontal displacement monitoring points (ZGS-1~ZGS-10). There are nine vertical monitoring points (C-1~C-9) and six horizontal monitoring points (CH-1~CH-6) at the entrance and exit. The order of the monitoring points of the main body of the station and the track is from north to south, and the order of the vertical displacement monitoring points at the entrance and exit starts from one side of the station and rotates counterclockwise along the entrance and exit structure. The layout of the deformation monitoring points of the station and track structure is shown in Figure 4.

When the new foundation pit is adjacent to the existing subway station, the allowable limit value of the track line deformation of the existing operating station should meet the daily maintenance requirements of the track determined in the Beijing subway engineering maintenance rule, as shown in

Table 3. The management values of the allowable deviation of the static geometric dimensions of the track.

	Project		Scheduled Maintenance (mm)		Regular Maintenance (mm)
			Main Line	Other Lines	Main Line	Other Lines
Line tracks	Gauge		+4, −2	+5, −2	+6, −3	+7, −3
	Level		4	5	6	8
	Height		4	5	6	8
	Rail direction (straight line)		4	5	6	8
	Triangle pit (twist)	Easement curve	4	5	6	8
		Branch lines and circular curves	4	5	6	8

Note: ① The gauge deviation does not contain the gauge widening value set according to regulations on the curve; however, the maximum gauge (including widening value and deviation) should not exceed 1456 mm. ② The orbit deviation and height deviation are the maximum vector values measured by a 10 m chord. ③ The deviation of the triangle pit does not contain the distortion caused by the curve superelevation along the slope. When checking the triangle pit, the base length is 6.25 mm, but there is no triangle pit beyond the table within a distance of 18 m.

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2.3. Analysis of Monitoring Results

The construction duration of the new foundation pit protection structure is 60 days. The construction duration of foundation pit excavation is approximately 160 days. The specific excavation construction time of the foundation pit is shown in

Table 4. Excavation time of each stage of the foundation pit.

Construction Phase (Excavation)	Construction Time	Construction Progress	Excavation Volume (m)
1	15 November 2018~15 December 2018	Earth excavation	3.5
2	15 December 2018~15 January 2019		6
3	15 January 2019~15 Febrary 2019		5
4	15 Febrary 2019~15 March 2019		6
5	15 March 2019~1 May 2019		6

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2.3.1. Deformation of the Foundation Pit Retaining Structure

It can be seen from the time-history curve shown in Figure 5 of the vertical displacement of the retaining pile top at typical positions ZTC-5, ZTC-9, ZTC-16 and ZTC-20 measuring points that the vertical displacement of the retaining pile top presents a floating trend in the excavation stage of the foundation pit. The main reason is that the foundation pit soil excavation is unloaded, and the bottom of the pit soil is uplifted and causes the pile to float. The maximum vertical displacement of the pile top of the retaining pile occurs at measuring point ZQC-16 (i.e., the position far from the station side), and the maximum vertical displacement is 2.1 mm. In the pouring stage of the bottom slab, the top of the retaining pile shows an obvious subsidence trend, mainly because the pouring of the bottom slab concrete is equivalent to the application of the load to inhibit the uplift of the foundation pit bottom, resulting in the downward settlement trend of the retaining pile.

The measured horizontal displacement data of the four direction measuring points ZQS-3, ZQS-7, ZQS-17 and ZQS-20 of the retaining pile are selected for analysis, as shown in Figure 6. During the excavation and construction of the foundation pit in each stage, the horizontal deformation trend of the retaining pile top is toward the inner side of the foundation pit (the value toward the inner side of the foundation pit is positive, and the value toward the outer side of the foundation pit is negative), and it shows a continuous rise. The maximum horizontal displacement of the retaining pile top occurs at the ZQS-3 measuring point, which is 6.2 mm. In the floor pouring stage, the horizontal displacement of the retaining pile top has a sudden decrease trend, and the maximum change value reaches 3.5 mm. Horizontal displacement deformation gradually converges, which inhibits the drift of the retaining structure to the inside of the pit.

To further obtain the horizontal deformation law of the retaining pile in deep foundation pit excavation construction, the horizontal displacement monitoring data of the retaining pile at each time point of ZQT-4, ZQT-9, ZQT-15 and ZQT-21 in four directions and four measuring points in the southeast and northwest of the retaining pile are analyzed, as shown in Figure 7. The deformation trend of each retaining pile measuring point position at different excavation time nodes is large in the middle and small at both ends. With the passage of time, the maximum horizontal deformation position of the retaining pile gradually moves downward along the pile body.

Comparing the pile displacement of the four measuring points at different construction times, it can be seen that the horizontal deformation of the ZQT-9 retaining pile on the south side of the foundation pit is slightly smaller than that on the north side of the ZQT-21 measuring point. The main reason is that the excavation interface of the north side of the foundation pit is large, and the contact surface between the retaining pile and the lateral active earth pressure is large during the excavation of the foundation pit, resulting in large deformation of the retaining pile on the north side. The retaining pile at measuring point ZQT-4 on the west side of the foundation pit is close to the side of the subway station, and the pile deformation is slightly smaller than that at measuring point ZQT-15 on the east side of the foundation pit. The main reason for the deformation of retaining piles is that the stiffness of subway station structure is large, and the disturbance of station structure caused by foundation pit excavation is far less than that of the stratum.

2.3.2. Deformation of the Main Body and Subsidiary Structure of the Station

(1) Deformation of the main station structure

The deformation of each measuring point of the main structure of the subway station at different construction nodes is shown in Figure 8. During the excavation of the foundation pit from October 2018 to February 2019, the vertical deformation of adjacent subway stations was small. The main reason is that the influence of retaining pile construction and shallow soil excavation on adjacent stations is small in the early stage of foundation pit construction. During the construction period from February 2019 to May 2019, the vertical deformation of the station structure was large, and the excavation of the foundation pit had a great influence on the central position of the main structure of the station. The overall deformation trend of the station is large in the middle and small in the end. The maximum vertical deformation of the main structure of the station occurs at measuring point ZDH-7, i.e., the central position of the station. This deformation trend is consistent with the deformation trend of the numerical simulation. The maximum vertical deformation of the station is 1.43 mm.

To study the vertical deformation law of the main structure of the station in the process of foundation pit construction, the time-history data of the station vertical deformation monitoring points ZDH-3, ZDH-6 and ZDH-11 are selected for analysis, as shown in Figure 9.

According to Figure 9, during the construction of foundation pit retaining piles and temporary columns, the station has a slight subsidence trend. The deformation of the vertical displacement of the main structure of the station in the excavation stage is divided into three parts:

(a) When the excavation depth of the foundation pit is 3.5 and 9.5 m in excavation stages 1 and 2, respectively, the deformation of the station remains stable, and there is a slight downward trend. The soil outside the foundation pit should be squeezed inward due to the excavation and unloading of the foundation pit soil, but the soil outside the pit is squeezed due to interbracing, resulting in a slight downward movement of the vertical deformation of the station structure.

(b) In the third stage of excavation, i.e., the excavation depth from 9.5 to 14.5 m, the vertical displacement deformation of the station began to increase considerably. With the increasing excavation depth of the foundation pit, the retaining piles of the foundation pit are more exposed inside the foundation pit, while the active earth pressure outside the pit increases, and the supporting effect of interbracing weakens. The soil outside the foundation pit has an inward inclination trend, and the station has an upward displacement trend.

(c) During excavation stages 4 and 5, the vertical deformation of the station has a clear upward trend; that is, when the excavation depth of the foundation pit is greater than 14.5 m, the excavation depth of the foundation pit further increases, and the vertical deformation of the station is more pronounced.

In the stage of bottom slab pouring and structure construction, the station structure generally presents a floating trend, in which the bottom slab pouring stage fluctuates and sinks, and the maximum vertical deformation of the main structure of the station is 1.29 mm.

To study the horizontal deformation law of the main structure of the station during foundation pit construction, the time-history data of the horizontal displacement monitoring points CZH-2, CZH-5 and CZH-8 of the main structure of the station are selected for analysis, as shown in Figure 10. The horizontal displacement of the main structure of the station shifts to the inside of the foundation pit at all stages of excavation and gradually increases. The maximum horizontal displacement deformation of the main structure of the station is 1.7 mm. Compared with the vertical displacement, the maximum horizontal displacement of the main structure of the station is 1.3 times its maximum vertical displacement. In the construction of foundation pit excavation, the horizontal displacement of the station is more sensitive than its vertical displacement. The foundation pit retaining pile plays a great role in the deformation isolation of the station. In the construction, the measurement frequency of the horizontal displacement of the station should be strengthened.

(2) Structural deformation of the entrance and exit

To obtain the vertical deformation law of the entrance and exit structure in the foundation pit construction process, the time-history data of the vertical monitoring points C-4, C-5, C-8 and C-9 at the entrance and exit are selected for analysis, as shown in Figure 11. The vertical displacement of the entrance and exit structures has a slight downward trend in the construction stage of the foundation pit retaining structure. In the excavation stage of the foundation pit, the phenomenon of subsidence and then floating is presented, which is opposite to the deformation of the main structure of the station. The main reason is that the entrance and exit structure is close to the foundation pit and is in the position of the surface settlement trough. In the process of excavation and unloading of the foundation pit, the soil near the foundation pit is squeezed to produce downward displacement, which drives the entrance and exit structure to move downward. The cumulative deformation value of the vertical displacement of the entrance and exit in the excavation stage is 1.7 mm. In the stage of floor pouring and structure construction, the structural deformation of the entrance and exit has an obvious downward trend, and the maximum displacement reaches 2.9 mm. More attention should be given to the monitoring and measurement of the vertical deformation of the entrance and exit in the construction process.

The measured data of CH-1, CH-2, CH-3 and CH-4 at the horizontal deformation monitoring points at the entrance and exit are selected for analysis, as shown in Figure 12. The horizontal displacement deformation trend of the entrance and exit gradually increases, which is consistent with the horizontal displacement deformation trend of the station, but the growth rate is more obvious than that of the station. The maximum horizontal displacement is 2.8 mm, which is 1.7 times the maximum horizontal displacement of the station. The horizontal displacement trend of each measuring point is biased toward the inside of the foundation pit.

2.3.3. Deformation of the Track Structure

To ensure the normal operation of subway vehicles, real-time follow-up displacement monitoring of station track structures is carried out in the process of foundation pit construction. The measured vertical displacement data of the right track structure at each measuring point at different construction time nodes are analyzed, as shown in Figure 13. During the construction period from October 2018 to February 2019, the deformation of the track lines was small, and there was a downward trend. During the subsequent construction period, the deformation of the track line considerably increased, and the overall deformation of the line showed a trend of small deformation at both ends and large deformation in the middle, which was consistent with the deformation trend of the station. The maximum cumulative vertical displacement of the track structure was 1.24 mm.

The time-history displacement data of the right track measuring points YGD-4, YGD-7 and YGD-12 are analyzed, as shown in Figure 14. The right track has a stable subsidence trend in the construction stage of the foundation pit retaining structure, and the subsidence error of each measuring point is small. When the vertical displacement of the track is in excavation stages 1 and 2, that is, the excavation depth of the foundation pit is 3.5 and 6.5 m, the vertical displacement of the track maintains a stable downward trend. In excavation stages 4 and 5, when the excavation depth is less than 14.5 m, the vertical displacement of the track has an obvious upward trend. In the pouring stage of the bottom slab, the cumulative displacement increases first and then decreases, and the vertical deformation of the right track is consistent with the deformation trend of the station measuring point.

Figure 15 shows that the horizontal displacement of the right track continues to increase at all stages of foundation pit excavation construction, and the horizontal displacement deformation trend of each measuring point shifts to the pit. The maximum horizontal displacement of the track is 1.4 mm.

In the process of foundation pit excavation, by extracting the vertical displacement data of the left and right tracks and subtracting the left and right rail data, the horizontal time-history changes of the left and right rails are obtained. As shown in Figure 16 and Figure 17, the horizontal deviation of the right rail is between −0.8 and 0.6 mm, and the maximum deviation occurs in the fourth excavation stage of the foundation pit (that is, the excavation depth is 20.5 m), which is located at the position of the YGD-8 measuring point (that is, the middle of the right rail). The horizontal deviation of the left rail is −0.4 to 0.6 mm, and the maximum horizontal deviation occurs in the fifth excavation stage of the foundation pit (that is, the excavation depth of the foundation pit is 25.5 m), which is located at the ZGD-4 measuring point (that is, the middle of the left rail).

Through the analysis of the manual monitoring data of the track distance of the left and right lines, the maximum deviation value of the track distance in the construction process is +2 mm, less than the deformation control requirements, which meets the allowable deformation control requirements of the track geometry. In the process of foundation pit construction, the deformation control of the track structure is better, and the safe operation of subway trains is not affected in the process of foundation pit construction.

3. Stratum–Foundation Pit–Station–Track Interaction Model

3.1. Establishment of the Finite Element model

In this paper, the new and existing subway structures are optimized. The basic assumptions are as follows:

(a) The retaining structure of the new foundation pit and the adjacent existing stations and retaining structures are all linear elastic structures.

(b) Soil is an ideal elastic–plastic material with isotropy and mean value. The stratum is simplified by the model. The influence of groundwater is not considered in the model, and the initial in situ stress of the model only considers the self-weight of the soil and ignores the structural stress of the soil.

(c) The whole excavation process of the foundation pit is simulated through the construction sequence. In the excavation process, only the change in the spatial displacement of part of the foundation pit is considered, and its time effect is not considered.

(d) Due to the large excavation area of the foundation pit and many model element nodes, some structures of the model are simplified to improve the calculation speed. According to the principle of equivalent stiffness, the bored pile structure of the new foundation pit is replaced by the elastic plate element structure, and the track rail is replaced by the elastic beam.

According to the foundation pit design, station design drawings, geological survey reports and other relevant data, the overall stratum–foundation pit–station–track interaction model is established using the Midas GTS NX three-dimensional finite element software. The size of the model is determined according to the location relationship between the new foundation pit and the adjacent subway station, the influence range of foundation pit excavation and the relevant theory of surface settlement caused by the foundation pit. The size of the model is 280 m long (along the direction of the station), 200 m wide (along the transverse direction of the station) and 70 m down from the ground surface. The overall model diagram is shown in Figure 18.

The model has 802,309 units and 1,403,010 nodes, including five layers of stratum soil, soil within the excavation range of the foundation pit in layers and blocks, a grouting equivalent layer, a foundation pit support system structure, station and tunnel interval soil and a tunnel and track structure system. The soil within the excavation range of the foundation pit is divided into four parts longitudinally along the foundation pit, and the parts are divided into nine layers from top to bottom. The grouting equivalent layer is the layer below 6 m between the foundation pit and the station, and its depth range is 20 m. The foundation pit support system structure includes an enclosure pile, temporary steel lattice column, temporary concrete column, three concrete truss internal supports, three waist beams and one anchor cable. The structures of the station and tunnel are obtained by extracting the soil of the station and interval tunnel and giving the structural attributes of the station and tunnel. The track bed is simulated by a solid element. A schematic diagram of the model position relationship between the new foundation pit and the existing station is shown in Figure 19.

The new model simplifies the stratum soil according to the stratum parameters, uses 3D solid elements for simulation and endows different parameter attributes to different soil layers in the process of establishing the grid. The soil constitution adopts the Drucker–Prager yield criterion, referred to as the D-P criterion, considering the effect of hydrostatic pressure and reflecting the real yield and failure state of soil and thus eventually ensuring the speed and accuracy of calculation to analyze the deformation near the foundation pit better.

The model mainly considers the self-weight conditions of the soil and structure. Grouting pressure is added during grouting reinforcement, and anchor prestress is applied during anchor rod construction. The model boundary constraint adopts a normal constraint on the bottom surface in the X, Y and Z directions and adopts a free boundary on the top surface in the Z direction. In the model, the friction contact between pile and soil was established by setting the pile interface, and nodes of the soil layer and other structures were connected by coupling.

3.1.1. Simulation of Foundation Pit Support System

The foundation pit support system includes a foundation pit enclosure pile, reinforced concrete internal support, prestressed anchor rod, steel lattice column and concrete column structure. Due to the large number of elements and nodes in the three-dimensional simulation model, to better simulate the actual working conditions and simplify the calculation difficulty of the model, the foundation pit enclosure pile is simulated by a 2D plate element structure using the principle of equivalent stiffness; the prestressed anchor rod, concrete internal support, steel lattice column and concrete column are simulated by a 1D embedded beam element structure; and the corresponding prestressed constraints are applied. The main reason is that the beam element is an isotropic and nonyielding linear elastic material; thus, it can simulate well the characteristics of the structure, and the embedded beam element does not need to consider coupling with the soil structure grid, which simplifies the calculation process and greatly shortens the calculation time of the model. The simulation of the foundation pit support system is shown in Figure 20.

3.1.2. Simulation of the Existing Station Structure System

The existing station structure system includes the station main structure, station column, section structure, track slab and track. The main structure and section structure of subway stations are simulated by 2D slab elements. The track slab and platform are simulated by 3D solid elements, and the station column and rail are simulated by 1D embedded beam elements. The simulation of the station structure system is shown in Figure 21.

3.1.3. Grouting Reinforcement Simulation

The grouting reinforcement construction is simulated by providing the grouting attribute of the original soil layer in the model grouting reinforcement area. First, the range of grouting reinforcement is determined in the modeling process, and then the grouting parameter attribute of the range is given. Before the foundation pit enclosure structure and column construction, the grouting reinforcement construction is completed by activating the soil of the grouting reinforcement layer and giving the attribute of grouting reinforcement parameters.

3.2. Model Calculation Parameters and Construction Simulation Process

3.2.1. Soil Parameters

According to the actual engineering geological survey report, the soil parameters of the numerical simulation are obtained, and the difficulty of model construction and calculation is simplified by combining the soil layers with similar physical properties. The parameters of the soil layer are shown in

Table 5. Finite element model to simulate soil parameters.

Serial Number	Name	Thickness (m)	Density (g/cm3)	Internal Friction Angle (°)	Force of Cohesion (kPa)	Poisson Ratio	Elastic Modulus (MPa)
1	Miscellaneous soil filling	2.0	1.70	5	10	0.3	12.0
2	Silty clay	5.0	1.96	15	35	0.3	40.0
3	Fine Sand	8.0	1.95	28	--	0.3	75.0
4	Pebble⑤	15.0	2.10	35	--	0.25	135.0
5	Pebble⑥	32.0	2.15	40	--	0.25	180.0

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3.2.2. Structural Parameters

After collecting and consulting the structural design data of new projects and built subway stations, it was determined that the concrete strength of the enclosure structure, temporary concrete column pile foundation of the foundation pit, concrete internal support, subway entrance and exit and other structures is C30, and the main structure of the station, tunnel and track slab is made of C40 strength concrete. Anchor rods and steel lattice columns are made of steel. The structural parameters are shown in

Table 6. Finite element model to simulate material parameters.

Serial Number	Name	Poisson Ratio	Density (g/cm3)	Elastic Modulus (MPa)
1	C30	0.25	2.3	3.0 × 104
2	C40	0.20	2.3	3.25 × 104
3	Steel	0.20	7.5	2.1 × 105
4	Grouting	0.20	2.1	2.0 × 103

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The new foundation pit is divided into five stages of excavation, and the depth and earthwork of each stage are shown in

Table 7. Excavation parameters of foundation pit soil.

Construction Phase (Excavation)	Excavation Depth (m)	Amount of Excavation (m³)
1	3.5	44,000
2	6.0	58,000
3	5.0	48,000
4	6.0	53,000
5	5.0	47,000

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The construction simulation steps include initial in situ stress, foundation pit enclosure structure and temporary column construction, five-layer soil excavation and support and anchor rod erection, with a total of 42 steps. The construction simulation process is shown in

Table 8. Finite element model numerical simulation procedure.

Construction Phase	Serial Number	Construction Name
Phase 1	Stage 0	Initial ground stress balance
	Stage 1	Grouting properties of grouting layer soil
	Stage 2	Drilling piles, temporary columns and pile foundations for foundation pit envelopes
	Stages 3–6	Earth 3.5 m in the first stage of excavation
	Stage 7	Poured crown beams and first internal supports
Phase 2	Stages 8–15	Earth 6 m in the second stage of excavation
	Stage 16	Pouring the waist beam and the second inner support
Phase 3	Stages 17–24	Earth 5 m in the third stage of excavation
	Stage 25	Pouring the waist beam and the third inner support
Phase 4	Stages 26–33	Earth 6 m in the fourth stage of excavation
	Stage 34	Laying the anchor cable and applying preaxial force
Phase 5	Stages 35–42	Excavation of the fifth stage of the foundation pit earthwork 5 m to the bottom of the pit

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3.3. Comparative Analysis of Monitoring Results and Numerical Simulation

3.3.1. Comparison of the Horizontal Displacement of the Retaining Pile

The measured and simulated data of horizontal displacement monitoring measuring points ZQT-4, ZQT-9, ZQT-15 and ZQT-21 of foundation pit retaining piles at the completion of foundation pit excavation are selected for analysis. As shown in Figure 22, the actual measured deformation trend of each measuring point is consistent with the simulated calculated deformation trend. By comparing the numerical simulation with the actual measurement data, the numerical simulation can basically reflect the deformation trend of the retaining structure in the construction of deep and large foundation pits.

As seen in Figure 22a, after the foundation pit excavation is completed, the position of the maximum horizontal displacement of the retaining pile at measuring points ZQT-4 and ZQT-9 is basically consistent. Similar to the longitudinal measuring points of the foundation pit, at both positions, the maximum value is 0.6 times the excavation depth along the vertical direction of the pile. The simulated value of the maximum deformation of measuring points ZQT-4 and ZQT-9 is slightly different from the measured value. The main reasons are as follows: (a) The interlayer of the ground surface at measuring points ZQT-4 and ZQT-9 has the effect of grouting reinforcement, which reduces the horizontal displacement of the retaining pile in the process of foundation pit excavation. (b) The actual construction of the inner support in the foundation pit is more complex; thus, it is difficult to simulate the entire construction process of the internal support by numerical simulation, and there are certain errors in the equivalent substitution of the support structure simulation, leading to certain deviations between the calculated results and the actual measurement results. Figure 22b shows that after the foundation pit excavation is completed, the maximum horizontal deformation position of the retaining pile at measuring points ZQT-15 and ZQT-21 is basically the same, being 0.6 times the excavation depth of the foundation pit at both positions. The simulated value of the maximum deformation of the retaining pile at measuring points ZQT-15 and ZQT-21 is basically the same as the measured value.

3.3.2. Comparison of the Deformation Results of the Main Structure of the Station

The simulation and measured data of station vertical displacement monitoring points ZDH-1~ZDH-14 in the model after the completion of foundation pit excavation were selected for comparative analysis. Figure 23 shows that the simulation of the vertical displacement of the station structure is basically consistent with the measured deformation trend, and the position of the simulated maximum deformation value point is consistent with the position of the measured maximum deformation value point, both of which occur at measuring point ZDH-8 (i.e., the middle of the station). The three-dimensional simulation model of deep and large foundation pit excavation based on the composite support system established by the new foundation pit project can reflect the deformation law of adjacent existing subway stations under the influence of foundation pit construction and has a high reference value for similar projects in the future. The maximum deformation of the station vertical displacement simulation is 1.95 times the actual measured maximum deformation, mainly for the following reasons:

(a) Because the construction quality of the complex supporting system of the foundation pit is high and the construction requirements of the deformation control of the adjacent station are stricter, the control of the adjacent station in the construction process of the foundation pit is better.

(b) The model is calculated according to the most unfavorable factors and cannot overcome the time–space effect in the process of foundation pit excavation. The environment of the foundation pit construction site is complex, and the three-dimensional simulation cannot fully simulate the actual situation; thus, there is an idealized state.

(c) There is a certain error between the hypothesized structure simulation and the real structure in the three-dimensional finite element, and the calculation process is not completely consistent with reality.

3.3.3. Comparison of the Deformation Results of the Track Structure

Through the analysis of the measured data of station and track deformation, it can be seen that the foundation pit has little impact on the existing subway line in the first two excavation stages (i.e., when the excavation depth is 0~9.5 m), and significant impact occurs in the fourth and fifth excavation stages. In this section, the vertical deformation data of YGD-1~YGD-16 corresponding to the right track of the model in the third excavation stage, the fourth excavation stage and the fifth excavation stage are selected for comparative analysis.

From the comparison between the measured and simulated values of the settlement of the existing track structure in Figure 24a, it can be seen that the measured and simulated values of the vertical displacement of the existing track structure caused by the completion of the third excavation stage of the foundation pit are similar in trend, and there is a certain error in the value, with an error value between 0.04 and 0.61 mm. The measured maximum deformation value occurs at measuring point YGD-7, and the simulated maximum deformation value occurs at YGD-9. The measured maximum value of the vertical displacement of the overall track structure is 0.45 mm; the simulated maximum value is 1.01 mm, and the maximum error difference is 55%. As the model is an ideal state with some assumptions, there will be a difference (about 0.5 mm) to an extent compared with the measured data. This condition is in the normal range of similar projects in other studies, and the trend and the order of magnitude between the measured data and simulated data are consistent, so it is not such a big problem in the calculation.

Figure 24b shows that the measured and simulated vertical displacements of the existing track structure caused by the completion of the fourth excavation stage of the foundation pit have similar trends, and the error value of the measurement value is between 0.28 and 0.99 mm. The measured maximum deformation value occurs at measuring point YGD-8, which deviates southward compared with the third stage, and the simulated maximum deformation value occurs at YGD-9. The measured maximum value of the vertical displacement of the overall track structure is 0.84 mm; the simulated maximum value is 1.73 mm, and the maximum error difference is 51%.

Figure 24c shows that the measured and simulated vertical displacements of the existing track structure caused by the completion of the fifth excavation stage of the foundation pit have similar trends, and the error value of the measurement value is between 0.27 and 1.22 mm. The measured maximum deformation value occurs at measuring point YGD-8, and the simulated maximum deformation value occurs at YGD-9. The measured maximum vertical displacement of the overall track structure is 1.24 mm, and the simulated maximum is 2.42 mm. The maximum error difference is 49%.

In conclusion, through the comparative analysis of the simulated value and the measured data, it is concluded that the deformation calculation trend of the retaining pile and the main structure of the station is basically consistent with the measured result trend. The measured horizontal displacement of the retaining pile is close to the simulated value, and the measured vertical deformation of the main structure of the station is approximately 0.5 times the simulated value. The measured and simulated deformation trends of the track structure are consistent in all stages of foundation pit excavation. The deep and large foundation pit model of the composite support system can reflect the influence of construction on the deformation of adjacent existing stations and track structures and verify the applicability of the model to supporting projects.

4. Parametric Analysis

4.1. Influence of the Parameters of the Retaining Pile on the Deformation of the Existing Subway Station and Track

4.1.1. Influence of the Embedded Depth of Different Retaining Piles

To analyze the influence of the embedded depth of the retaining pile on the deformation of the existing subway station and track, according to the different embedded ratios of the retaining pile, five working conditions of the embedded depth of 6 m, 7 m, 8 m, 9 m and 10 m were set to simulate the excavation of the foundation pit, and the embedded depth of the retaining pile was 8 m in the initial condition. The maximum vertical and horizontal deformations of the station and track structures under various conditions are shown in Figure 25.

With the increase in the embedded depth of the retaining pile, the maximum vertical displacement of the adjacent subway station and track gradually decreases, and the amplitude of the change in the deformation value gradually decreases. When the embedded depth of the foundation pit retaining pile increases from 6 to 8 m, the maximum vertical deformation of the station and track clearly changes, and when the embedded depth of the foundation pit retaining pile increases from 8 to 10 m, the maximum vertical deformation changes little. When the embedded depth of the foundation pit retaining pile increases from 6 to 10 m, the maximum deformation values of the station and track structure decrease by 2.18 and 2.15 mm, respectively. The maximum vertical displacements of the station and track structures decreased by 28%, 39%, 45% and 48% and 32%, 43%, 48% and 51%, respectively.

The maximum horizontal deformation of the station and track decreases with increasing embedment depth of the retaining pile. When the embedded depth of the foundation pit retaining pile increases from 6 to 10 m, the maximum horizontal displacement of the station and track structure decreases by 27%, 36%, 41% and 43% and 31%, 39%, 42% and 45%, respectively. The horizontal displacement of the station and track structure is more sensitive to the change in the embedded depth of different retaining piles.

Through the deformation analysis of station and track structures under different embedded depths of retaining piles, it can be concluded that increasing a certain embedded depth of retaining piles can reduce the deformation of station and track structures by nearly 50%. When the embedded depth of the retaining pile is 8 m, the deformation of the station and track structure is no longer obvious. Considering the economic benefits of foundation pit construction and reducing the difficulty of construction, the embedded depth of the foundation pit retaining pile can be selected as 8 m; i.e., the embedded ratio is 0.31.

4.1.2. Influence of Different Pile Diameters of Retaining Piles

To analyze the influence of the diameter of the retaining pile of the foundation pit on the existing subway station and track structure, the diameter of the retaining pile near the station side is set to be 0.6, 0.8, 1.0, 1.2 and 1.4 m for foundation pit excavation simulation. The diameter of the retaining pile near the station side is taken as the initial condition (i.e., the pile diameter is 1.2 m). The maximum deformation curves of the station and track structures under various conditions are shown in Figure 26.

The maximum vertical deformation of the station and track structure decreases with the increasing diameter of the retaining pile. The maximum vertical deformation of the station is between 2.68 and 2.93 mm, and the maximum deformation of the track structure is between 2.41 and 2.62 mm. The station and track structure are less affected by the diameter of the retaining pile. When the diameter of the retaining pile increases from 0.6 to 1.4 m, the maximum vertical displacement of the station and track decreases by 4%, 5%, 8% and 9% and 4%, 8%, 9% and 8%, respectively. It is worth noting that when the pile diameter increases from 1.2 to 1.4 m, it has little impact on the station structure and basically no impact on the track deformation. The track deformation runout in the chart is a normal calculation error that does not represent an upward trend.

The maximum horizontal deformation of the station and track decreases with the increasing diameter of the retaining pile. The maximum vertical deformation of the station is between 3.35 and 3.61 mm, and the maximum deformation of the track is between 3.10 and 3.39 mm. When the diameter of the retaining pile increases from 0.6 to 1.4 m, the maximum vertical displacement of the station and track structure decreases by 2%, 5%, 6% and 7% and 4%, 6%, 7% and 9%, respectively.

Through the analysis of foundation pit retaining pile parameters (retaining pile embedded depth and diameter), it can be seen that the embedded depth of the new deep foundation pit retaining pile has a great influence on the adjacent subway station. When the retaining pile embedded depth increases to 8 m, the maximum displacement of the station and track decreases more. The maximum displacement of the station and track changes little when the embedded depth of the retaining pile increases from 8 to 10 m. When the diameter of the retaining pile increases from 0.6 to 1.4 m, the maximum displacement of the station and track structure changes little, which indicates that the diameter of the retaining pile has little effect on the deformation of the adjacent subway station and track.

4.2. Influence of Internal Support Parameters on the Deformation of the Existing Station and Track

4.2.1. Influence of Different Support Section Sizes

To explore the influence of the interbracing stiffness of the deep foundation pit on the deformation of adjacent existing subway stations and track structures, the stiffness of interbracing is controlled by changing the cross-sectional area of interbracing based on the influence parameters of interbracing stiffness. To better study the relationship between the deformation of the station and track and the section size of the interbracing, the section size of the third interbracing is taken as the initial parameter (that is, the section size is 1.0 m × 1.0 m). On this basis, one side of the section is changed as the comparison condition. It is assumed that the sections of the third interbracing under five conditions are A1 (0.8 m × 1.0 m), A2 (0.9 m × 1.0 m), A3 (1.0 m × 1.0 m), A4 (1.1 m × 1.0 m) and A5 (1.2 m × 1.0 m). The maximum deformation curves of the station and track structures under various conditions are shown in Figure 27.

The maximum vertical deformation of the station and track structure decreases with increasing interbracing section size. The maximum vertical deformation of the station is between 3.10 and 3.56 mm, and the maximum deformation of the track structure is between 2.79 and 3.20 mm. When the interbracing section of the foundation pit supporting system increases from A1 to A5, the maximum vertical displacement of the station and track structure decreases by 6%, 10%, 11% and 13% and 6%, 9%, 11% and 13%, respectively.

The maximum horizontal deformation of the station and track structure decreases with increasing interbracing stiffness. The maximum vertical deformation of the station is between 3.75 and 4.31 mm, and the maximum deformation of the track structure is between 3.43 and 3.94 mm. The maximum horizontal displacement of the station and track structure decreases by 4%, 8%, 11% and 13% and 6%, 9%, 11% and 13%, respectively, when the interbracing section increases from A1 to A5.

The vertical and horizontal displacements of station and track structures are less affected by the interbracing section size in the foundation pit. In the actual foundation pit construction process, considering the economic benefits of foundation pit construction, the interbracing section size of the new foundation pit can be selected as A3 (1.0 m × 1.0 m).

4.2.2. The Influence of Different Support Channels

To explore the influence of the number of supporting lanes of deep and large foundation pits on the deformation of adjacent existing subway stations and tracks, five working conditions of three, four, five, six and seven supporting lanes were set for foundation pit excavation simulation. The maximum deformation curves of the station and track structures under various conditions are shown in Figure 28.

The maximum vertical deformation of the station and track structure decreases with the increase in the number of interbracing channels. When the interbracing of the foundation pit increases from four to seven, the deformation of the maximum vertical displacement of the station and track structure is no longer significantly increased. The maximum vertical deformation of the station is between 1.82 and 4.10 mm, and the maximum deformation of the track is between 1.64 and 3.51 mm. Compared with the deformation of the station and track under the influence of foundation pit construction of three interbracings and one anchor cable support system, when the foundation pit adopts four interbracings, the maximum vertical displacement of the station and track structure deformation is reduced by 21% and 24%, respectively. When five interbracings are adopted, the maximum vertical displacement deformation decreases by 24% and 22%; when six interbracings are adopted, the maximum vertical displacement deformation decreases by 30% and 29%; when seven interbracings are adopted, the maximum vertical displacement deformation decreases by 30% and 24%.

The maximum horizontal deformation of the station and track structure decreases with the increase in the number of interbracings. The maximum horizontal deformation of the station is between 2.28 and 4.68 mm, and the maximum deformation of the track structure is between 1.97 and 4.21 mm. The horizontal displacement deformation of the station and track structure is sensitive to the number of supporting paths in the foundation pit. Compared with the three interbracings and one anchor cable support structure of the foundation pit, when the foundation pit adopts four interbracings, the maximum horizontal displacement deformation of the station and track structure is reduced by 12% and 20%, respectively. When the foundation pit adopts five interbracings, the maximum horizontal displacement deformation is reduced by 17% and 18%, respectively. When the foundation pit adopts six interbracings, the maximum horizontal displacement deformation is reduced by 18% and 15%, respectively. When the foundation pit adopts seven interbracings, the maximum horizontal displacement deformation is reduced by 24% and 28%, respectively.

The deformation of the station and track is greatly affected by the number of interbracings in the foundation pit. When the number of interbracing paths is greater than four, the maximum vertical displacement decreases less, and the maximum deformation of the station and track changes by approximately 20% compared with the three interbracings and one anchor cable support system. Considering the economic cost of construction, three interbracings and one cable support system can be selected for foundation pit support design, but further protective measures should be taken for foundation pits and station structures.

5. Conclusions

Taking the deep and large foundation pit of a composite supporting system in Beijing adjacent to the existing subway station as the project background, this study evaluates the deformation law of the existing subway station and track caused by deep and large foundation pit excavation by combining numerical simulation and field measurement. The specific conclusions are as follows:

(1) Based on the analysis of the field measured data, the retaining pile rises continuously with the excavation of the foundation pit, and the horizontal deformation of the retaining pile is inclined to the inner convex drum of the foundation pit. With the advancement of the construction process of the foundation pit, the position of the maximum horizontal displacement of the pile gradually moves downward; the entrance and exit structure of the station is greatly affected by the foundation pit excavation. After the foundation pit excavation is completed, the maximum horizontal and vertical displacement deformation values of the entrance and exit are 3.6 and 1.7 mm, respectively. When the excavation depth of the foundation pit is 14.5~25.5 m, the vertical displacement of the station and track begins to significantly change, and the maximum vertical displacement is 1.43 and 1.24 mm, respectively. The horizontal displacement of the station and track increases with the excavation of the foundation pit.

(2) Through the comparative analysis of the simulated value and the measured data, it is concluded that the trend of the deformation calculation results of the retaining pile and the main structure of the station is basically consistent with the measured results. The measured horizontal displacement of the retaining pile is close to the simulated value, and the measured vertical deformation of the main structure of the station is approximately 0.5 times the simulated value. The measured and simulated deformation trends of the track structure are consistent in all stages of foundation pit excavation. The deep and large foundation pit model of the composite support system can reflect the influence of construction on the deformation of adjacent existing stations and track structures and verify the applicability of the model to supporting projects.

(3) Through the model calculation and analysis under the working conditions of different support parameters, it can be seen that the support parameters that can be adopted for construction are a pile diameter of 1.0 m, embedded depth of 8 m, four internal supports and 1.0 m × 1.0 m inner support section. The deformation of the existing station and track is less affected by the pile diameter of the retaining pile and the cross-section size of the internal support and more affected by the embedded depth of the retaining pile and the number of internal support tracks. When the embedded depth of the retaining pile is greater than 8 m, which is the optimal embedded depth of this project, and when there are more than four internal support tracks, the reduction in the maximum deformation of the station and track is no longer clear.

(4) The general trend is that the greater the embedded depth, the smaller the influence is on the subway. However, it is found that when the embedded depth increases to 8 m and above, the influence of the further increase in the depth on the deformation tends to be less obvious. Therefore, the inflection point of 8 m can be a certain reference value for engineering foundation pit design.