1 Introduction

The comprehensive transportation hub has the advantages of small land area, high space utilization rate, and high traffic efficiency. The space available for development and utilization in cities is becoming increasingly scarce, requiring more and more supporting facilities, such as subway stations, transfer halls, and underground parking, to be built underground. At the same time, in order to accelerate the construction progress and shorten the construction period, adjacent foundation pits have to be constructed simultaneously, forming a deep excavation group. In adjacent excavation groups, the deformation and stress of the support structure are also exceptionally complex due to the inconsistency in excavation depth, excavation area, and excavation shape of the foundation pit [1,2,3,4,5,6,7]. In addition, the mutual interference between foundation pits and the impact of post-excavation on existing structures cannot be ignored. The mutual disturbance between excavation groups greatly increases the potential construction risks. Based on the above challenges, the independent design of the foundation pit support structure can avoid the above risks, but it significantly increases the cost of temporary support. Therefore, in order to solve the above problems, the integrated design of support structures can avoid redundant support structures. In addition, a thorough understanding of the interaction between adjacent foundation pits can greatly reduce the construction risks of such projects.

Previous research on deep foundation pits mainly focused on the interaction between a single excavation and an adjacent excavation. Tan et al. [8] studied the impact of excavation of foundation pits on existing structures or surface deformation around them. Zhou et al. [9] divided the foundation pit into multiple adjacent foundation pits and studied the deformation and stress characteristics of the support structure under different excavation schemes for foundation pits. Zeng et al. [10] studied the interaction between support structures during excavation of adjacent foundation pits with intermediate soil inclusions. Due to the crucial impact of new construction near the existing tunnel on improving the protected area, Muenpetch et al. [11] established a three-dimensional finite element analysis model considering soil structure, and found that the impact of open excavation and pipe jacking construction on the existing tunnel is relatively small. During the excavation of the foundation pit, the deformation rate of the support structure showed a downward trend. In contrast, only limited research has analyzed the impact of single or two-stage excavation on the surrounding environment.

Pit-in-pit (PIP) excavation will unavoidably result in the stress being redistributed in the outer soil, which will impact the stability of the whole foundation pit. Through on-site monitoring and indoor tests, when Sun et al. [12] investigated the retaining system’s deflection features, they found that the outer retaining system's deflection towards the inner retaining structure also had an impact on the inner retaining structure. According to Zhou et al. [9], the retaining structure’s displacement is featured during the construction of multilevel foundation pits in clay strata. The wider the steps, the smaller the interaction between multilevel retaining structure. The maximum lateral movement of the retaining structure is directly related to the excavation depth and area, and inversely related to the excavation depth and area, according to Wang et al.'s [13] investigation into how the characteristics of the construction and the retaining structure affect the movement of the retaining structure during PIP excavation. The ultimate lateral movement of the retaining structure is inversely linked to the insertion ratio and the toe coefficient of the foundation pit, and directly related to the excavation depth and area. Tan et al. [14] investigated a foundation pit consisting of a small-area rectangular inner pit and an outer pit with a big diameter, and concluded that the pit's side walls deform significantly more than other pits. Currently, research on cases of PIP excavation concentrates mainly on soft soil areas in the south of China. There is no research on the deformation characteristics of PIP excavation and multilevel retaining systems in the Beijing area.

Research has shown that when overall failure of a multi-bench retaining system occurs, the multi-bench retaining system can be equivalent to a whole unit, and the support structure deforms towards the pit as a whole. When the multi-bench retaining system undergoes separated failure, it deforms independently without any interaction. When a multi-bench retaining system undergoes associated failure, the bench has a restraining effect on the deformation of the upper support structure, and the soil reaches its limit state. The mutual influence between the two support structures results in significant differences in the soil pressure applied to the support structure. The lateral soil pressure gradually decreases with the increase of soil deformation [15, 16]. Li et al. [17] studied the migration law of soil particles during pile foundation construction and the evolution mechanism of soil pressure after piles. However, during the excavation process of multilevel foundation pits, the different deformations of the pile foundation result in significantly different evolution of soil pressure behind the piles.

Overall, the design of multilevel retaining structures is still in its early stages, and it is yet unclear how these structures evolve throughout the excavation of inner pits (third-level foundation pit and fourth-level foundation pit). The supporting transportation hub of the Beijing High-Speed Railway Chaoyang Station is the research object of this paper. To meet building standards and ensure the stability of the foundation pit, a multilevel retaining system is first advised based on the depth of the foundation pit and the characteristics of the area around it. The deformation parameters of the multilevel retaining system during the excavation of the inner pit were then examined using the monitoring data. The mechanism of soil pressure evolution during the excavation of multilevel foundation pits was discovered, and the procedure for preventing distortion of the inner pit retaining structure was explored.

2 Project Introduction

2.1 Project Overview

The supporting project, which is located next to Beijing's Chaoyang High-Speed Railway Station, is the subject of this thesis's investigation. As can be seen in Fig. 1, the total building space is 47,920 m2, including the Gong Lian hub hall, underground parking area, metro transfer hall, R4 metro station, M3 metro station, and the Bei Feng Dao area. The first floor, which measures 149.4–171.2 m in east–west direction and 173.4–220.4 m in north–south direction, is where the Gong Lian hub hall is located. The excavation depth of the first floor is 11.443 m below the ground surface (BGS). The main building of the high-speed railway station is about 10 m from the east border of the pit. The second floor is where the R4/M3 station hall level is located, and the maximum excavation depth there is 20.6 m BGS. The third floor is made up of the M3 platform floor, the R4 station hall floor, and the Bei Feng Dao area, with a maximum excavation depth of 30.27 m BGS. The Bei Feng Dao area has been combined with the design of the M3 station and R4 station areas. There is an L-shaped layout in the R4 station and M3 station sectors. The R4 platform level is situated on the fourth floor, and the total construction depth is 40.45 m BGS. The foundation pits are divided into southern and northern parts for easy explanation due to the project’s numerous foundation pits and intricate geographical interactions. The R4 station area, the Bei Feng Dao area, and the parking building area are all located in the north area. The southern region is where the M3 station, Gong Lian hub hall, and metro transfer hall are all situated. The spatial position relationship of the I–I cross-section is shown in Fig. 2.

Fig. 1
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Relationship between the Hub's location and the surrounding region

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Fig. 2
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Cross-section of the multilevel foundation pit

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2.2 Geological Conditions

The planned site is located along the old Jin Gou River’s route in the Yong Ding River’s alluvial/diluvial fan. The entire line’s terrain is generally flatter and lacks overt ups and downs, being higher in the west and lower in the east. Within 90 m BGS, the soil is dispersed as a layer of fill, silty clay, silty fine sand, fine medium sand, gravel, and pebble. The soil parameters for various layers were obtained by carrying out field and laboratory tests [18,19,20,21,22], and are presented in Table 1. The long-term phreatic water level was at 9.7–13.8 m BGS. Moreover, proper dewatering was performed before foundation pit excavation. The water level is 2.0 m lower than the elevation of the excavation.

Table 1 Physical and mechanical parameters of the layers
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3 Excavation Scheme and Multilevel Retaining Structure System

3.1 Excavation Scheme

This project's foundation pit demonstrates a complicated planar geometry, various excavation depths, incredibly stringent structural deformation, and asymmetrical extra loads close by. Table 2 shows the excavation sequence of multilayer foundation pits.

Table 2 The excavation sequence of multilayer foundation pits
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3.2 Design of the Multilevel Retaining Structure System

Considering the different excavation areas, depths, and complex surrounding environments of the foundation pit group, a single support structure type cannot meet the safe and economical excavation needs of the foundation pit. Therefore, based on the excavation area and depth of the foundation pit, an integrated design scheme for the combined support structure is proposed. The following will provide a detailed introduction to the positional relationships and settings of multilevel support structures.

3.2.1 First-Level Retaining Structure

As seen in Figs. 3 and 4, the diaphragm walls (DW) and anchor cables together form the first-level retaining structure. To control the retaining wall’s deflection and decrease the impact of foundation pit excavation on the environment, a sufficient length of anchor cables must be installed. Even though there is only 10 m between the foundation pit and the outer wall of the main structure of the high-speed railway station, lowering the anchor cable’s length will not have the same anchoring effect, which threatens the safety of the foundation pit. Cast-in-place piles are constructed to lessen the effect of excavation on the existing structure to meet the deformation control standards. The DW and cast-in-place piles are used in the retaining structure close to the high-speed railway station in the northern part of the first-level foundation pit. Three prestressed anchor cables are inserted, as shown in Fig. 3, between the cast-in-place piles and the DW to make the structure more resilient to deformation. In the southern section of the first-level foundation pit, which is close to the high-speed railway station, a number of drilled cast-in-place piles are positioned between the DW and the isolation pile. A crown beam connects the bored cast-in-place pile and the DW to form a composite retaining system, as seen in Fig. 4. The physical and mechanical properties of the anchor cables are shown in Table 3.

Fig. 3
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Cross-section of the first-level retaining structure at the III–III location

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Fig. 4
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Cross-section of the first-level retaining structure at the II–II location

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Table 3 Physical and mechanical parameters of anchor cables
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3.2.2 Second-Level Retaining Structure

Because there are no drainage control requirements for the second-level retaining structure, and the excavation depth of the second-level foundation pit is only 8.45 m, double rows of continuous bored piles are used as the retaining system for the southern part of the second-level foundation pit, as shown in Fig. 5. The second-level retaining structure must also be used as a temporary retaining structure for the third-level foundation pit and the fourth-level foundation pit in the R4 station area, as shown in Fig. 6. As a result, the northern portion of the second-level retaining structure uses a combined retaining structure of DW and bored piles. To lessen the impact of inner pit excavation on surrounding structures, high-pressure jet-grouted technology is also used to strengthen the soil between the double-row continuous bored piles, as shown in Fig. 6. Other parts of the second-level foundation pit continue to use the combined retaining structure of the DW and anchor cables.

Fig. 5
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Cross-section of the second-level retaining structure at the II–II location

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Fig. 6
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Cross-section of the Second-level retaining structure at the III–III location

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3.2.3 Third-Level Retaining Structure

The third-level foundation pit is an excavation group. If distributed excavation is adopted, although it reduces the impact of foundation pit excavation on the existing structure, it cannot meet the requirements of the construction period. In light of this, the third-level retaining structure employs an integrated design, but the retaining structure in the northern portion of the third-level foundation pit uses a DW and a double circular concrete structure in Fig. 7a. A DW and concrete retaining structures are used in the retaining system for the third-level foundation pit’s southern part, shown in Fig. 7b. As depicted in Fig. 8, the third-level foundation pit’s southern and northern portions share a DW.

Fig. 7
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Plan view of the third-level retaining structure

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Fig. 8
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Cross-section of the third-level retaining structure at the IV–IV location

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Fig. 9
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Cross-section of the third-level retaining structure at the III–III location

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Figure 10 illustrates the retaining structure cross-section of the M3 area. The retaining structure has a cross-sectional size of 1 m × 1 m, and the DW thickness in the M3 area is 0.8 m. In the M3 area, a row of steel columns is installed with a cross-sectional size of 0.55 m × 0.55 m, a longitudinal spacing of 9 m, and angle steel with a cross-sectional size of L200 mm × 200 mm × 20 mm. The cross-sectional dimensions of the concrete waler beam are 1.2 m × 1.0 m, 0.6 m × 0.6 m for the steel waler beam, and 1 m × 1 m for the concrete diagonal structure.

Fig. 10
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Cross-section of the third-level retaining structure at the II–II location

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Figure 9 depicts the retaining structure cross-section of the R4 area. The circular concrete retaining’s cross-sectional dimensions in the R4 area are 2.2 m × 1.0 m or 2 m × 1 m. The steel column has a 0.55 m × 0.55 m cross-sectional dimension. The concrete waler beam cross-sectional dimensions are 1.2 m × 1.0 m. The concrete retaining structure cross-sectional dimensions are 1.2 m × 1.0 m. The concrete connection beam cross-sectional dimensions are 0.6 m × 0.8 m. The diaphragm wall is 1.2 m thick.

Figure 8 depicts the structural cross-section of the Bei Feng Dao area. In the Bei Feng Dao region, the DW and circular concrete structure system is utilized. The DW has 0.8 m thickness. The circular concrete structure has the same dimensions as the above-described parameter.

The following measures were performed to improve the structure’s overall stability: (1) Between the steel columns, install diagonal braces. The dimensions of the vertical steel cross-bracing are L200 mm × 150 mm × 12 mm. One set of vertical steel cross-bracing is spaced every two spans. (2) In the area of the circular concrete structure, install a 0.3-m-thick concrete slab in the circular concrete structure area.

3.2.4 Fourth-Level Retaining Structure

The foundation pit for the fourth-level is thin and elongated, with a buried depth of 40.45 m. The fourth-level retaining structure, as illustrated in Fig. 11, employs a combination structure of a double steel structure and steel diagonal structure. The cross-section of the fourth-level retaining structure at the III–III location is shown in Fig. 12. The steel retainer is 0.8 mm in diameter and 0.02 m thick.

Fig. 11
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Plan view of the fourth-level retaining structure

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Fig. 12
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Cross-section of the third-level retaining structure at the III–III location

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4 Field Instrumentation

Real-time deformation information on the enclosure structure during pit-in-pit excavation was collected [23]. Figure 13 shows the instrumentation layout of the PIP. The monitoring items, control standards, and instruments for PIP are listed in Table 4. These include ZQT, which keeps track of the retaining structure's horizontal deflection, ZQC, which keeps track of the retaining structure’s vertical displacement, and ZZC, which keeps track of the retaining structure’s vertical displacement.

Fig. 13
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Instrumentation layout of the PIP

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Table 4 On-site monitoring items and instruments of the PIP
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5 Analyses of Field Measurements

5.1 Lateral Wall Displacement

Due to the limited space of this article, 11 representative monitoring points were selected as a response to the changing characteristics of the supporting structure during the excavation of the inner pit. Figure 14 shows the lateral movement of the retaining structure as well as the excavation of the third-level and fourth-level foundation pit. If the value is positive, the structure is deforming toward the pit’s interior. If the value is negative, it is deforming toward the pit's exterior. The diaphragm wall is barely affected by the PIP excavation. On the east side of the pit, the retaining structure has a lateral distortion between 8 and 10 mm. On the west side, it has a lateral deformation of about 6 mm. The primary reasons for this are as follows: (1) The reinforced concrete retaining structure has high stiffness, and the circular retaining structure has good compressive performance. (2) According to Zheng et al.'s [24] model test results, the bench between the retaining structure and the first- and second-level structures prevents lateral distortion.

Fig. 14
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Horizontal displacement of the third-level retaining structure in the northern area

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At monitoring site ZQT-73, the east side of the first-level bench is 36 m in width, and the pile's greatest horizontal movement is 6.5 mm. At the monitoring site ZQT-5, the east side of the second-level bench is 18 m wide, and the pile’s maximum lateral movement is 12 mm. The most lateral movement of the double-row piles at this location is merely 4.2 mm, which is much less than at other monitoring stations. The width of the second-level western bench is 65 m at the monitoring site ZQT-25. Naturally, it follows logically that as the soil bench's breadth increases, the effect of inner pit excavation on the outer structure will diminish. Additionally, the wall top’s displacement at monitoring point ZQT-15 exhibits an opposite tendency, with deformation moving away from the pit’s edge. The main justification for this is that under the model test results of Fan et al. [25], the bench with a smaller width between the retaining structure will result in a “push back displacement” on the bench with a larger width.

Combining the aforementioned deformation rules, it can be said that the width of the bench is the primary factor causing the retaining structure's asymmetric deformation to differ. However, the width of the bench is frequently fixed based on the positional relationship between the excavation groups. Casting a base slab may be able to successfully control the movement of side walls, according to Tan et al. [14, 26, 27], to further examine how the foundation slab and third-level retaining structure affect side wall displacement during the inner pit’s excavation.

During the excavation of multilevel foundation pits, the bottom plate and inner pit support structure have a good restraining effect on the outer pit enclosure structure. However, as a permanent structure, the bottom plate can reduce the use of temporary support, accelerate construction progress, and save construction costs. Likitlersuang et al. [5] showed that the type of structural unit has a significant impact on the finite analysis structure, but the following analysis mainly focuses on the deformation differences of the structure under different simulation conditions. Therefore, in this article, the diaphragm wall is simplified as a pile element, and the internal support is simplified as a support force. The model size is 135 m × 100 m, a total of 42,396 elements. This article uses Midas software and adopts a modified Mohr–Coulomb constitutive model to simplify the foundation pit into a two-dimensional plane strain model, and investigates the influence of bottom plate and inner pit support on the deformation of the retaining structure. The next focus will be on exploring the constraint effect of the bottom plate and internal support on the deformation of the enclosure structure. The simulation of the excavation process of the foundation pit is similar to Chen et al. [28, 29], and the detailed modeling process can be referred to Hsiung et al. [30] and Iwata et al. [31]. The selection of soil parameters during the modeling process can refer to Table 1. The four cases are as follows:

Case 1: The original working condition, which is consistent with the pit-in-pit building method described in this research.

Case 2: The third-level foundation pit is dug without a horizontal retaining structure, and the other conditions are the same as in case 1.

Case 3: The first-level foundation pit and the second-level foundation pit are both used to place the base slab, and the remaining settings are identical to case 2.

Case 4: A fixed base slab and horizontal retaining system are used, and the other situations are consistent with case 1.

The monitoring data shown in Fig. 14 is related to the horizontal displacement of the retaining structure shown in Fig. 15a. The second-level and first-level retaining structures gently deform into the pit during excavation of the third-level pit. In contrast to the first-level and second-level structures, the bench is a passive area that restrains the deformation. This phenomenon will be elaborately discussed in Section 6.2. In Fig. 15b, the maximal horizontal deformation of the third-level retaining structure is observed at the wall’s top. The deformation trajectory of the second-level and first-level retaining structures is similar to that of case 1. The retaining structure's lateral distortion does, however, rise by 63.6% in comparison to scenario 1. The inner pit’s retaining structure is crucial for preventing the outer pit from deforming.

Fig. 15
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Horizontal deformation of the multilevel retaining structure

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In Fig. 15c, the third-level structure’s distortion path is similar to that of case 1. The third-level structure’s top is constrained by the base slab of the second-level pit. In contrast, the deformation pattern of the first-level and second-level structures is drastically different from that of case 2 and case 1, with the maximal horizontal deformation occurring at the structure’s base. The horizontal displacement of first-level and second-level structures, however, was less at the top and more at the bottom due to the base slab’s restricting influence on the top of the structures. Although the first-level and second-level retaining structures suffered significant kicking deformation as a result of the excavation of the inner pit, the maximum lateral distortion was only 8 mm, which was 36.4% and 61.1% less than cases 1 and 2, respectively. According to Tan et al. [14], in Fig. 15d, the retaining structure’s deformation control is flawless. The retaining structure’s horizontal deformation pattern is comparable with case 3, although the value dropped by 20%.

As a result, we can choose to set a fairly rigid retaining structure in the PIP or quickly build the base slab during the construction process, thus controlling the deformation of the outer retaining structure by limiting the deformation of the inner pit.

5.2 Vertical Movement of Retaining Walls

The release of soil stress during foundation pit excavation is the main factor contributing to the vertical displacement of the retaining wall. The third-level retaining wall's vertical movement as the inner pits were excavated is shown in Fig. 16. As can be observed, there are three stages to the retaining wall's vertical movement: (1) during the initial phase of excavation, the retaining wall is dominated by settlement; (2) once the excavation is complete, the retaining wall shows uplift; (3) the vertical distortion of the wall tends to reduce and stabilize as the base slab and building are constructed. The evolution process of the third-level retaining wall is similar to that of Gao et al. [32]. This phenomenon demonstrates that the friction force operating on the pile body exceeds the total of the self-gravity of the retaining wall and the friction force of the soil settlement on the wall due to the rebound of the pit bottom [33].

Fig. 16
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Vertical movement of the retaining wall

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The vertical movement of the double-row pile structure is depicted in Fig. 16d. It is evident that settlement dominates the vertical deformation and that the excavation of the inner pit has little bearing on the double-row piles. The main reason is that the minimum distance is 19 m between the third-level foundation pit and double-row piles, and the release of earth stress has little effect on double-row piles.

5.3 Vertical Movement of the Pile

Uneven vertical movement of the pile is caused by various excavation depths in multilevel foundation pits. The upward motion of the pile during the excavation of the third-level foundation pit is shown in Fig. 17. The coupling effect of the self-weight and the frictional force bearing on the pile body has the greatest impact on the pile after the soil stress has been relieved [32]. The vertical deformation of the pile is similar to that of the retaining wall, although the vertical displacement of the pile construction is lower. The installation of numerous uplift piles at the bottom of the foundation pit significantly restricts the movement of the soil there.

Fig. 17
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The vertical movement of the pile

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The vertical displacement of the pile has a different trend at measurement points ZZC-33 and ZZC-34 shown in Fig. 18. The building and the base slab of the third-level foundation pit strike the pile at monitoring point ZZC-34, causing settlement deformation. The fourth-level pit excavation has an impact on the pile at monitoring site ZZC-33, but soil unloading is what causes the piles to rise and deform.

Fig. 18
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Simulated vertical movement of the pit bottom

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The fact that the pile at monitoring point ZZC-33 had been elevated while the pile at monitoring point ZZC-34 had subsided suggests that excavation of the inner pit would result in the pit's bottom rebounding, which would intensify the pile uplift inside the rebound zone [34]. Midas GTS software is used to simulate the deformation of the foundation pit bottom during the excavation of the inner pit in order to confirm the aforementioned result. Figure 18 shows the foundation pits’ vertical distortion after the third-level foundation pit’s excavation is complete. The positive numbers indicate advancement, and negative values indicate stagnation. Naturally, the inner pit has the highest uplift value, and the pit bottom can deform to a maximum of 16.6 mm. The deformation value of the second-level pit was lowered by 71% in comparison to the inner pit. The uplift value of the second-level foundation pit is 3 mm. The first-level foundation pit is not significantly impacted by the excavation of the inner pit, and the highest settlement deformation is 2.1 mm. Only the nearby foundation pit is impacted by the inner pit's stress release.

6 Lateral Earth Pressure

The evolution mechanism of soil pressure after multistage support structures has important reference significance for the design of support structure systems. However, the interaction mechanism between multilevel support structures is still unclear, greatly limiting the promotion and application of multilevel support structure systems. Due to insufficient soil pressure monitoring data, it is not possible to obtain the evolution mechanism of soil pressure during excavation in real time. The discrete element method (DEM) not only can reveal the evolution process of soil pressure after the support structure, but also can explain the distribution law of the contact force chain from the perspective of microscopic particles [35,36,37]. In contrast, the two-dimensional (2D) DEM model has a higher computational efficiency and a lower computational cost when compared to the 3D DEM model. As a result, the 2D DEM model is frequently used to analyze the evolution of the earth pressure law and the process of how the earth arch forms.

6.1 DEM Modelling

The DEM approach unquestionably has an advantage in modelling large deformation and pile–soil interaction since the active and passive lateral soil pressure of a multilevel foundation pit excavation is highly complicated [18, 28, 36, 38, 39]. In order to examine the characteristics of lateral earth pressure distribution and their evolution mechanism, this work simulates the excavation of the multilevel retaining foundation pit using the PFC2D model. The objective of the numerical simulation is to determine the distribution of the lateral earth pressure behind the multilevel retaining structure. As shown in Fig. 19, the foundation pit retaining structure is simplified in this simulation as cantilever piles for modeling convenience without affecting the simulation findings. Additionally, because the length of this paper is limited, the work of [35, 36] on the modeling of the foundation pit and the location of monitoring points should be consulted. Table 5 displays the microscopic parameters of particles.

Fig. 19
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DEM modelling

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Table 5 Micromechanical parameters in the DEM model
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6.2 Result Analysis

Different evolving laws are presented by the earth pressure in the first-level excavation and the second-level excavation. As can be seen in Fig. 21a, during the first-level pit excavation, the upper earth pressure behind the first-level pile is decreasing, while the lower earth pressure behind the first-level pile remains constant with the static earth pressure. As illustrated in Fig. 20b, the soil pressure on the higher part before the first-level pile increases. When the first-level foundation pit is excavated, the revolution of the pile body compresses the soil in front of the first-level pile, making the soil more likely to be in a passive state. When the first-level foundation pit is excavated, the distribution of earth pressure is consistent with that seen by Li et al. [17, 40, 41].

Fig. 20
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Lateral earth pressure development from the standpoint of micro-analysis

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As the second-level pile generates rotational displacement into the pit during the excavation of the second-level pit, the soil pressure behind the first-level pile drops once more, resulting in a relaxation zone, as depicted in Fig. 20c. The bottom of the structure, where the cantilever retaining structure's rotation point is located, compresses the soil in front of the second-level pile, resulting in the compression zone depicted in Fig. 20d, which raises soil pressure. Due to the bench's restraint effect [24], as depicted in Fig. 20e, the displacement of the first-level pile is relatively minimal. In contrast, a relaxation zone forms, and the soil pressure decreases, as a result of the second-level pile’s larger displacement. Additionally, behind the second-level pile is where the active earth pressure is present. The excavation of the inner pit has less of an effect on the deformation of the retaining structure than the traditional retaining structure does. This is due to the bench's back-pressure action, which prevents structures from deforming.

The distribution of the indirect contact force chain of particles during the excavation of the second-level foundation pit is depicted in Fig. 20g. Due to the proportionate scaling of the contact force, a line of the same width denotes the same contact force size. The density of the contact force chain behind the first-level piles drastically decreases when the second-level foundation pit's excavation is complete, which is consistent with the downward trend in soil pressure behind the pile top seen in Fig. 20a. After the first-level pile, the soil pressure switches from being static to being active. The rise in earth pressure distribution shown in Fig. 20d is consistent with the increase in contact force chain density in front of the first-level pile.

Through the analysis of the above numerical simulation and monitoring data, it is shown that the existence of the second-level support can effectively constrain the deformation of the first-level support. There is a correlation between the passive lateral soil pressure of the first-level support and the active soil pressure of the second level support. The smaller the distance between the two rows of support structure, the more complex the stress. As the bench width increases, the correlation between the soil pressure decreases. According to the contact force chain and soil pressure in the bench, the soil between the two rows of the structure can be divided into a “relaxation zone” and a “compression zone.” The soil in the relaxation zone is in an active state, while the soil in the compression zone is in a passive state, as shown in the Fig. 21. Therefore, by increasing the retaining structure to reduce soil deformation in the loose area or setting a bottom slab to increase soil stiffness between the two rows of support structure, the impact of internal pit excavation on the surrounding area of the foundation pit can be reduced, ensuring the stability of the foundation pit.

Fig. 21
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Distribution characteristics of soil pressure in multilevel foundation pits

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7 Conclusions

Based on the multilevel deep foundation pit project at Beijing Chaoyang Railway Station, this research examines the retaining structure's distortion properties when the inner pit is excavated. A different strategy for preventing distortion during the inner pit excavation was analyzed. The mechanism for the evolution of soil pressure was revealed during the construction of multilevel foundation pits. The following conclusions have been reached:

  1. (1)

    Based on the spatial position relationship between foundation pit groups, a combined support structure system that meets the excavation requirements of foundation pit groups has been proposed. The integrated design of the support structure not only avoids the repeated setting of different foundation pit support structures but also reduces the multiple disturbances to the surrounding environment caused by the excavation of the foundation pit in sequence while saving the construction period. It successfully solved the problem of a traditional single support type not suitable for multilevel foundation pit excavation.

  2. (2)

    The excavation of the inner pit will exacerbate the horizontal displacement of the outer support structure. Compared to the outer support structure, the excavation of the inner pit reduces the passive earth pressure, but the constraint effect of the inner support structure on horizontal displacement makes the deformation of the outer support structure relatively limited. The asymmetry of local horizontal loads causes a push-back displacement between the large side of horizontal deformation and the small side of horizontal deformation.

  3. (3)

    The inconsistent excavation depth of multilevel foundation pits results in differences in vertical deformation of the support structure. The excavation of the inner pit causes a large uplift value of the support structure near the foundation pit, while the impact on the structure far away from the foundation pit is relatively small. Attention to the vertical differential deformation of the support structure is needed during the excavation process of the foundation pit.

  4. (4)

    Excavation of the inner pit will change the distribution characteristics of soil pressure between the two structures. According to the difference in relative horizontal displacement between the two levels of support structures, the soil between the two levels of support can be divided into a “relaxation zone” and a “compression zone,” with the soil pressure being active and passive, respectively. It can provide reference for the design of support structures before excavation of foundation pits.

    This study offers a design scheme for a multi-level foundation pit support structure and validates the system's practicality using on-site monitoring data. The numerical simulation method was utilized to reveal earth pressure evolution characteristics in multi-level foundation pits. The future study should focus on examining the mutual influence of pile length and multi-level structure spacing in different foundation pits in order to offer more specific parameters for multi-level foundation pit design.