Construction Scheme Effects on Deformation Controls for Open-Top UBITs Underpassing Existing Stations

Abstract

Urban rail transit networks’ rapid expansions have led to increasing intersections between existing and new lines, particularly in dense urban areas where new stations must underpass existing infrastructure at zero distance. Deformation controls during construction are critical for maintaining the operational safety of existing stations, especially in soft soil conditions where construction-induced settlement poses significant risks to structural integrity. This study systematically investigates the influence mechanisms of different construction schemes on base plate deformation when an open-top UBIT (underground bundle composite pipe integrated by transverse pre-stressing) underpasses existing stations. Through precise numerical simulation using PLAXIS 3D, the research comparatively analyzed the effects of 12 pipe jacking sequences, 3 pre-stress levels (1116 MPa, 1395 MPa, 1674 MPa), and 3 soil chamber excavation schemes, revealing the mechanisms between the deformation evolution and soil unloading effects. The continuous jacking strategy of adjacent pipes forms an efficient support structure, limiting maximum settlement to 5.2 mm. Medium pre-stress level (1395 MPa) produces a balanced deformation pattern that optimizes structural performance, while excavating side chambers before the central chamber effectively utilizes soil unloading effects, achieving controlled settlement distribution with maximum values of −7.2 mm. The optimal construction combination demonstrates effective deformation control, ensuring the operational safety of existing station structures. These findings enable safer and more efficient urban underpassing construction.

1. Introduction

Urban rail transit networks’ rapid expansions and densifications have led to increasing intersections between existing and new lines, particularly in urban core areas where limited underground space resources necessitate zero-distance underpassing of existing lines for new station construction [1]. The core challenge of such projects lies in controlling ground disturbance and structural deformation during the construction process while maintaining the normal operation of existing stations [2]. Especially in soft soil areas, tunnel construction-induced soil unloading effects, stress redistribution, and ground displacement may cause excessive settlement or differential deformation of existing structures, potentially triggering structural damage or even endangering operational safety [3,4,5].

In the field of underground engineering, the main trenchless technologies include shield tunneling, pipe jacking, and pipe roof methods [2,3,6,7,8,9]. Shield tunneling is widely applied in urban metro construction due to its high degree of mechanization and construction safety [10,11,12,13,14,15]. For instance, Fang et al. [16] successfully implemented the Beijing Qinghuayuan tunnel project beneath the existing Metro Line 10 using a 12.2 m-diameter shield machine, controlling tunnel and ground surface settlement through pre-reinforcement and synchronized grouting measures. Li et al. [17] employed centrifuge model tests to investigate the impact of new shield tunneling on existing large-diameter underpassing tunnels, simulating soil excavation, ground loss, and grouting processes during shield construction, confirming that grouting is an effective measure to mitigate responses in existing tunnels. However, shield tunneling is limited by equipment size and overburden depth requirements (generally requiring at least 1.5 times the tunnel diameter), making it unsuitable for large cross-section, shallow-buried underground space construction, particularly for large-span connecting passages in station structures [18].

Pipe jacking, as a non-excavation underground construction technique, is well-established in small and medium-sized pipeline projects [19,20,21]. Ren et al. [22] developed an analytical method for predicting ground deformation induced by pipe jacking based on engineering practice, comprehensively considering factors such as bulkhead additional thrust, jacking machine friction, grouting pressure, and ground loss, and providing practical reference for deformation control in underground pipeline applications through actual project validation. Ma et al. [23] conducted detailed research on the influence of pipe geometric parameters on ground response and jacking force in rectangular pipe jacking construction, emphasizing the need to comprehensively consider cross-sectional design and geological conditions for large cross-section applications, balancing surface displacement control and jacking force consumption. As a trenchless technology, pipe jacking is increasingly popular in urban utility construction for water, electricity, and gas, particularly slurry pipe jacking which utilizes peripheral slurry to stabilize surrounding soil, effectively avoiding environmental impacts and traffic delays caused by surface excavation. Nevertheless, pipe jacking faces technical bottlenecks such as insufficient rigidity and excessive thrust when applied to large cross-sections, making it difficult to meet the construction requirements of large underground structures, with particularly high risks in soft soil areas [24,25]. Traditional pipe roof method, as a tunneling technology for special geological conditions, can to some extent overcome cross-sectional size limitations [6]. However, its construction process relies on box culvert jacking or temporary steel supports combined with horizontal soil reinforcement to maintain stability. Steel supports combined with multiple pilot tunnel excavation methods suffer from low excavation efficiency and complex construction procedures; box culvert jacking technology requires large-scale working shafts and stable backing measures, increasing engineering difficulty and cost [26].

In recent years, the UBIT method has gained widespread attention as a novel underground excavation structure type [6]. Compared to traditional pipe roof methods, the UBIT completes pipe jacking and then uses pre-stressing tendons to connect discrete pipe sections into an integrated load-bearing system, forming a unified structure that combines advance supports with a permanent structure, offering high bearing capacity and construction flexibility, particularly suitable for complex scenarios involving large cross-sections, shallow burial, and zero-distance underpassing [27]. He et al. [6] proposed an intelligent optimization design method UBIT based on the Shanghai Metro Line 20 Pingli Station project, achieving automatic generation of pipe arrangement through multi-objective optimization algorithms, improving structural design efficiency and performance. Zhang et al. [27] first proposed and successfully applied a new UBIT method for the Shanghai Metro Wudinglu Station Exit 1 project, using transverse tensioned pre-stressing tendons to integrate discretely jacked square steel pipes into a cohesive load-bearing structure, achieving construction without temporary supports, soil reinforcement, and with full-section excavation, verifying the reliability of the structural system through joint tests and full-scale tests. However, the influence mechanisms of different pipe jacking sequences, pre-stress tension levels, and soil chamber excavation schemes on base plate deformation during UBIT construction have not been systematically studied, and optimization of construction schemes still requires further exploration to meet the stringent deformation control requirements in engineering practice [28].

This research, based on the Ningbo–Cixi Line Kongpu Station UBIT underpassing project, employs an open-top UBIT structure to conduct in-depth analysis on base plate deformation control issues during zero-distance underpassing of existing stations. Through refined three-dimensional numerical simulation, this study systematically investigates the influence patterns of different pipe jacking sequences, pre-stress tension levels, and soil chamber excavation schemes on base plate deformation, focusing on analyzing stress redistribution mechanisms, soil unloading uplift effects, and deformation evolution characteristics during construction. It reveals the action mechanisms of different construction schemes on existing station base plate deformation, providing a theoretical basis and technical support for construction scheme optimization in UBIT zero-distance underpassing projects.

2. Project Overview

2.1. Project Background

The Ningbo to Cixi Urban Railway extends from Kongpu Station (interchange with Ningbo Metro Line 2) to Cixi High-speed Railway Station, with a total length of approximately 64 km. The planned Kongpu Station is located at the intersection of North Ring Road and Meizhu Road in Jiangbei District, forming a T-shaped interchange with the existing Line 2. The station has a length of 429.35 m and features a three-level underground structure with double columns and three spans of cast-in-place reinforced concrete, constructed using the cut-and-cover method. The line underpasses Meizhu Bridge within the range of DK64 + 150 to DK64 + 207, as shown in Figure 1a. Figure 1b illustrates the cross-sectional view of the underpassing structure, showing the steel UBIT system, MJS (Metro Jet System) reinforcement, diaphragm walls for both the new line and Line 2, and the interlayer soil between them. The UBIT section is 26.4 m long with cross-sectional dimensions of 30.4 × 11.1 m, comprising 29 standard pipe sections and 3 working chamber pipe sections. The launching shaft (foundation pit C) and receiving shaft (foundation pit D) have net widths of 36 m and 32 m, respectively.

The Kongpu Station of the Ningci Line is located in the Ningbo Plain subarea, an alluvial-lacustrine plain with relatively flat terrain and ground elevations generally ranging from 2.25 to 4.26 m. According to the geotechnical investigation report, the soil layers at the UBIT underpassing section are, from top to bottom: ①_1a miscellaneous fill, ②₂ silty clay, ②_2c mucky silty clay, ②_2t silty sand, ③_1a clayey silt, ④_2b silty clay, ⑤_1b silty clay, ⑤₂ silty clay, ⑤_3a clayey silt, ⑥₂ silty clay, ⑥_3a clayey silt, and ⑥_4a silty sand [29,30,31]. The cross-sectional profile of the underpassing section is shown in Figure 2, and the relevant physical and mechanical parameters of the soil layers are listed in

Table 1. Soil properties of the construction site.

Soil Number	γ (kN/m3)	e	ccu (kPa)	φcu (°)	Es (MPa)
①1a	19.0
①2	18.7	0.95	25.6	34.2	4.0
②2c	17.3	1.35	13.9	19.0	2.2
②2t	19.8	0.69	7.0	25.8	3.5
③1a	19.8	0.72	16.0	36.3	2.5
④2b	18.1	1.11	15.9	28.1	3.5
⑤1b	19.2	0.86	29.3	27.7	6.0
⑤2	18.9	0.91	20.5	22.3	5.0
⑤3a	19.0	0.87	25.9	31.8	8.0
⑥2	18.8	0.93	25.5	28.2	4.2
⑥3a	19.6	0.76	38.5	24.9	5.0
⑥4a	20.0	0.65	19.7	30.2	8.0

.

2.2. Construction Technology Features

In this project, the UBIT method employs an open-top structure, suitable for the complex conditions of zero-distance underpassing of the existing Line 2 station at Kongpu Station of the Ningbo–Cixi Line. The construction technology primarily includes launching and receiving shaft construction, pipe jacking, installation of corrugated pipes and pre-stressing tendons, concrete pouring inside and between pipes, pre-stressing and anchoring, soil excavation, and main structure backfilling. With cross-sectional dimensions of 30.4 × 11.1 m and a mining section length of 26.4 m, this method uses pre-stressing tendons to connect the pipe sections into an integrated load-bearing structure, achieving the integration of advance support and permanent structure. The pipe jacking process induces compression and disturbance of the surrounding soil, pre-stress tensioning enhances overall structural stability while affecting surrounding soil displacement, and soil excavation leads to stress release and redistribution. These construction phases have significant impacts on base plate deformation and are the core focus of this study on the influence mechanisms of different construction schemes [32].

3. Model Building and Validation

3.1. Model Establishment

A comprehensive three-dimensional finite element model was developed using PLAXIS 3D to simulate the UBIT zero-distance underpassing of an existing operational station. The model incorporates the existing station structure (118 m × 24.5 m), launching and receiving shafts (28 m depth), and the UBIT structure (30.2 m × 10.6 m cross-section), including pipe segments, inter-pipe concrete, intra-pipe concrete, and pre-stressing tendons. The analysis employs drained conditions, which is appropriate for the relatively rapid UBIT construction process compared to consolidation time scales in the predominant silty clay layers. Construction processes are idealized assuming precise jacking alignment and uniform pre-stress application to enable systematic evaluation of construction scheme effects. Model boundaries are positioned at 3–5 times excavation depth (200 m × 200 m × 60 m) based on Saint-Venant’s principle and standard practice in underground engineering [33,34]. While formal convergence testing was not conducted for boundary spacing, this approach has been validated through successful comparison with field measurements from the Wuding Road project, indicating that boundary effects do not significantly influence results in the critical deformation zone. Mesh refinement uses element sizes of 0.1 m in critical UBIT and station areas, 0.2 m in existing station regions, and 0.5 m elsewhere, generating 395,125 elements and 622,025 nodes as shown in Figure 3.

The selection of soil constitutive model is crucial for simulation accuracy [35]. The traditional HS (Hardening Soil) model, as a strain-hardening elastoplastic model that adopts the Mohr–Coulomb criterion and accounts for both shear and compression hardening, is suitable for excavation and foundation bearing capacity analysis, but has notable limitations. Field measurement data indicates that soil deformation during subway construction is primarily concentrated in the small-strain range, where the HS model’s stress–strain relationship description is insufficiently precise, often leading to the overestimated prediction of influence zones [36]. To address this issue, Benz [37] made key improvements to the HS model, introducing two important parameters, $G_0^{ref}$ and ${\gamma }_{0.7}$, enabling the model to more accurately describe soil stiffness and its nonlinear variation characteristics under small-strain conditions, thereby precisely reflecting the hardening process of soft clay and the degradation pattern of shear modulus with increasing strain. This improvement resulted in the HSS (Hardening Soil with Small-strain Stiffness) model, which was selected for this study due to its excellent performance [3]. Compared to traditional models, the HSS model can more realistically simulate the influence mechanisms of UBIT underpassing on existing stations under complex and sensitive engineering conditions, providing higher calculation accuracy and more reliable prediction results for subsequent analysis [38,39,40]. The HSS model parameters for the Ningbo site were calibrated following established procedures for soft soils in the Yangtze River Delta region. The small-strain shear modulus $G_0^{ref}$ was determined from shear wave velocity measurements during site investigation and validated using empirical correlations developed by Gu et al. [36] for Shanghai soft clays. The threshold shear strain ${\gamma }_{0.7}$ was calibrated based on laboratory triaxial test results following the methodology proposed by Benz [37], with values ranging from 1 × 10⁻⁴ to 3 × 10⁻⁴ for the various soil layers.

3.2. Numerical Simulation Validation

The UBIT method was first applied to the project of Wuding Road Station on the Shanghai Metro Line 14 [28]. This study selected measured data from this project as a validation benchmark to evaluate the reliability of the established finite element model and its calculation results. Using the same modeling method as in this paper, a precise finite element model of Wuding Road Station was constructed, with the electrical culvert monitoring point serving as a key comparison point. As shown in Figure 4, the numerical simulation results and field measurements demonstrate highly consistent deformation trends across various construction stages (top pipe section construction, vertical pipe construction, bottom pipe section construction, pre-stressed tension, and after excavation completion), with close numerical values and maximum differences controlled within a reasonable range. This validation result fully confirms that the finite element model established in this paper possesses high reliability and accuracy, laying a solid foundation for subsequent in-depth studies of the force–deformation patterns of existing operational station floor slabs during UBIT underpassing.

3.3. Construction Scheme Design

To systematically study the impact of different construction schemes on floor slab deformation during UBIT underpassing of existing stations, this paper designs three categories of construction schemes based on numerical simulation methods, focusing on pipe jacking sequence, pre-stress tension level, and soil chamber excavation sequence [41,42,43]. Given the sensitive nature of underpassing an operational metro station, deformation control is critical for this project. Chinese rail transit engineering standards (GB 50911-2013) [44] provide guidelines for 10–20 mm settlement thresholds to ensure continued safe operation of existing infrastructure.

The pipe jacking sequence has a significant influence on ground stress redistribution and floor slab deformation. This paper designs 12 pipe jacking sequence schemes divided into two categories, Schemes 1–6 which feature early jacking of the bottom horizontal pipe (from bottom to top), and Schemes 7–12 which feature later jacking of the bottom horizontal pipe (from top to bottom), as shown in

Table 2. Different pipe jacking schemes of UBIT.

Scheme Number	Bottom Horizontal Pipe Jacking Timing	Pipe Jacking Sequence
Scheme 1	Later jacking of bottom horizontal pipe	1-2-3-4-5
Scheme 2		1-3-2-4-5
Scheme 3		1-4-2-3-5
Scheme 4		2-3-1-4-5
Scheme 5		2-4-1-3-5
Scheme 6		3-4-1-2-5
Scheme 7	Early jacking of bottom horizontal pipe	5-1-2-3-4
Scheme 8		5-1-3-2-4
Scheme 9		5-1-4-2-3
Scheme 10		5-2-3-1-4
Scheme 11		5-2-4-1-3
Scheme 12		5-3-4-1-2

. The pipe section numbering is illustrated in Figure 5, which shows the cross-sectional arrangement with dimensions of 30.4 m × 11.1 m, including key components such as concrete corbels, station floor, MJS reinforcement, pre-stress wires, C50 concrete lining, and five main pipe sections (Pipe-1 to Pipe-5) with sizes of 2 m × 2 m and 1.6 m × 1.6 m.

The pre-stress tension level directly affects the overall performance of the UBIT structure and the deformation control effect of the surrounding soil. This study systematically establishes three gradient levels of pre-stress, low pre-stress (1116 MPa), medium pre-stress (1395 MPa), and high pre-stress (1674 MPa), aiming to quantify the influence mechanisms of different pre-stress intensities on structural stability and deformation control [45].

The soil chamber excavation sequence has a decisive impact on the floor slab deformation pattern. Targeting this key factor, this paper carefully designs the following three typical excavation schemes: (1) left-to-right sequential excavation scheme, simulating the unidirectional cumulative deformation effect that may occur during construction; (2) side-first-center-later excavation scheme, strategically utilizing the stable boundary conditions formed by preliminary side excavations to effectively control deformation development in the central core area; (3) center-first-sides-later excavation scheme, focusing on analyzing the impact of premature failure of the core support area on overall structural stability. During the simulation of each excavation scheme, the study focuses on the maximum settlement characteristics of the finally excavated area [46].

4. Analysis of the Impact of Different Construction Schemes on Floor Slab Deformation

4.1. Influence of Pipe Jacking Sequence

The pipe jacking sequence has a decisive impact on the deformation of existing station floor slabs [47,48,49,50]. Through refined numerical simulation, this study systematically analyzes 12 different jacking schemes, categorizing them into early jacking of bottom horizontal pipe (Schemes 1–6) and later jacking of bottom horizontal pipe (Schemes 7–12), deeply examining the influence mechanisms of each scheme on floor slab settlement characteristics. As shown in Figure 6, the early jacking of bottom horizontal pipe schemes causes greater disturbance to the bottom soil during the early construction stage, triggering an uneven stress release process, resulting in an expanded settlement trough distribution with maximum settlement values reaching −7.3 mm and significantly higher deformation gradients, indicating that the bottom pipe priority jacking strategy significantly exacerbates stress concentration phenomena. In contrast, the later jacking of bottom horizontal pipe schemes adopts the strategy of prioritizing upper pipe section jacking, achieving progressive release and rational redistribution of soil stress, with more uniform settlement distribution and overall maximum settlement values significantly lower than the early jacking of bottom horizontal pipe schemes. Particularly, Scheme 4 (jacking sequence: 2-3-1-4-5) demonstrates optimal performance, with a maximum settlement of only −5.3 mm, the optimal value among all schemes, fully demonstrating the importance of a rational jacking sequence for controlling floor slab deformation.

Furthermore, through in-depth comparative analysis of Scheme 4 (2-3-1-4-5) and Scheme 5 (2-4-1-3-5), this section reveals the intrinsic influence mechanism of the pipe jacking sequence on station floor slab deformation. As shown in Figure 7, Scheme 4 adopts a continuous jacking strategy for adjacent pipes (pipe-3 immediately follows pipe-2), successfully forming a continuous and complete support structure system; meanwhile, Scheme 5 adopts an intermittent pipe jacking strategy (jumping to pipe-4 after pipe-2), resulting in a discontinuous early support structure. This strategic difference in jacking sequence directly determines the floor slab deformation distribution characteristics; Scheme 4 forms a more uniform and symmetrical deformation distribution with balanced and coordinated deformation on both sides, while Scheme 5 exhibits obvious uneven deformation characteristics with significant rightward offset tendency.

As the jacking process continues, the performance difference between the two schemes becomes increasingly prominent, with Scheme 4 ultimately controlling maximum deformation within a safe range of −5.2 mm, while Scheme 5 reaches −5.8 mm. The deformation curves further reveal that Scheme 4 maintains relatively balanced deformation distribution at each pipe jacking stage, especially after completing the jacking of pipe-3, providing effective support conditions for the central area of the station structure; meanwhile, Scheme 5 exhibits localized deformation concentration on the right side after the second step of jacking pipe-4. Research results indicate that a scientifically rational jacking sequence can form an efficient support structure system in the early construction stage, effectively controlling the range of soil disturbance and significantly reducing deformation increments during subsequent jacking processes.

Figure 8 quantitatively compares the deformation contribution characteristics of Scheme 4 and Scheme 5 at various jacking stages. Scheme 4 adopts a continuous jacking strategy for adjacent pipes which, although showing slightly higher cumulative deformation after the second step, exhibits significantly reduced subsequent deformation growth rates, with the deformation increment approaching zero between the third and fourth steps. This deformation control advantage stems from the formation of a continuous and complete support structure framework in the early stage, creating stable conditions for subsequent jacking. In contrast, Scheme 5 employs an intermittent pipe jacking strategy with initially small deformation that then experiences continuous rapid growth from the third step onwards, ultimately reaching a cumulative deformation of −5.8 mm, which is significantly higher than Scheme 4’s −5.3 mm. Its deformation increments show irregular distribution, maintaining high values in the third to fifth steps, indicating ineffective control of deformation development even in later stages. This finding provides a key guiding principle for jacking sequence design in UBIT underpassing projects: priority should be given to ensuring continuity and integrity of the support structure, avoiding local support weaknesses caused by intermittent jacking.

4.2. Influence of Pre-Stress Tension Level

To systematically analyze the regulatory mechanism of the pre-stress level on structural deformation, this study established three gradient levels of pre-stress tension (1116 MPa, 1395 MPa, and 1674 MPa, corresponding to 0.6, 0.75, and 0.9 times the tensile strength of 1860 MPa for 15.2 mm steel strands), systematically analyzing their effects on station floor slab deformation characteristics and UBIT structure force performance.

As shown in Figure 9, although floor slab deformation exhibits similar distribution patterns in form under different pre-stress levels, significant differences exist in deformation magnitude and local characteristics. Under 1395 MPa, the floor slab forms a typical “alternating concave-convex” deformation pattern between the UBIT area and soil chamber area: maximum settlement occurs directly above the middle-right vertical row of jacked pipes. Meanwhile, in the middle chamber soil area, settlement noticeably decreases, forming characteristic reverse bending deformation. Compared to standard conditions, under low pre-stress conditions, the concave–convex feature significantly weakens, resulting in a more gradual overall deformation distribution; whereas under high pre-stress conditions, the “alternating concave-convex” deformation feature further intensifies, creating a more pronounced deformation gradient.

Figure 10 shows the deformation cloud diagrams of the UBIT structure under three different pre-stress levels. After pre-stress tensioning activation, the rebound contraction force generated by the pre-stressing tendons creates an inward compression effect on the entire UBIT structure. Due to the asymmetry in structural boundary conditions, where the bottom of the UBIT is tightly connected to the horizontal UBIT forming rigid constraints while the top area remains relatively free, this difference in constraint conditions causes the vertical pipe rows to contract inward producing negative displacement, while the bottom UBIT exhibits upward bulging deformation. Under low pre-stress conditions, both the maximum contraction and maximum bulging amounts are controlled at approximately 0.45 mm; as pre-stress increases to medium level, the corresponding deformation increases to about 0.6 mm; and when pre-stress reaches high level, the maximum deformation further increases to 0.7 mm but the growth rate significantly slows. This phenomenon indicates the diminishing marginal utility of increased pre-stress, meaning that the deformation control benefits brought by higher pre-stress levels gradually decrease, suggesting that a reasonable pre-stress level should be selected in engineering practice to balance deformation control effects and construction costs.

Notably, the UBIT structure deformation exhibits significant spatial distribution differences. The inward contraction deformation of the vertical pipe rows on both sides is larger, while the middle area shows smaller deformation; similarly, the bulging deformation at the bottom edges exceeds that in the middle area. This non-uniform distribution characteristic mainly stems from the constraint effects of the structural geometric form. The stiffness around the perimeter of the UBIT structure is relatively lower, making it more susceptible to deformation under load; meanwhile, the middle region, constrained by surrounding structures in multiple directions, has enhanced stiffness and therefore smaller deformation.

4.3. Influence of Soil Chamber Excavation Sequence on Floor Slab Deformation

Figure 11 displays a detailed comparison of floor slab deformation curves under three different soil chamber excavation schemes. Figure 11a shows the characteristics of sequential excavation. Left, middle, and right chambers excavation, respectively, produce maximum settlements of −7.5 mm, −7.8 mm, and −7.9 mm at distances of 10 m, 5 m, and 15 m from the center, with each soil chamber excavation creating interactive effects on other areas through stress redistribution. Figure 11b demonstrates the side-first–center-later scheme. Excavation of both side chambers forms symmetrical deformation distribution, followed by middle chamber excavation creating a peak value (−7.0 mm) in the central area but with limited influence range, indicating this strategy effectively controls deformation in the middle region. Figure 11c presents the center-first–sides-later scheme. Middle chamber excavation generates maximum settlement of −8.2 mm with a wide influence range, followed by side excavations leading to the final maximum settlement reaching −7.8 mm.

Research findings indicate that maximum deformation typically occurs in the last excavated area across all schemes, suggesting that earlier excavations create unfavorable stress conditions and deformation environments for subsequent excavations. Meanwhile, the excavation process not only causes settlement in the excavated area but also induces uplift in surrounding soil, verifying the existence of soil unloading effects and their influence on deformation distribution. In the side-first–center-later scheme, side excavations provide “accommodation space” for the middle chamber through unloading effects, reducing final deformation in the central region; whereas the center-first excavation scheme, by destroying the core support area, results in reduced overall stability and poorer deformation control.

Comprehensive analysis indicates that the side-first–center-later excavation strategy can effectively utilize soil unloading effects and stress redistribution characteristics, forming a relatively balanced “triple-peak” deformation distribution, achieving effective control of overall deformation, and providing reliable technical reference for similar projects.

4.4. Discussions

The systematic comparison of different construction schemes reveals several important mechanisms governing deformation control in UBIT underpassing projects. The superior performance of Scheme 4 (continuous jacking sequence 2-3-1-4-5) can be attributed to the formation of a continuous support structure in the early construction stage, which effectively distributes soil stress and minimizes localized deformation. This finding aligns with the support mechanism principles observed in conventional pipe roof methods [25] but demonstrates enhanced efficiency through the integrated UBIT approach. The pre-stress level analysis reveals an optimal balance at 1395 MPa, where the balanced deformation pattern provides effective deformation control without excessive structural stress. This phenomenon occurs because the pre-stressing tendons create inward compression effects that redistribute soil stress around the UBIT structure [45].

Numerical model validation against field measurements from the Wuding Road Station project demonstrates satisfactory agreement (R² = 0.81), confirming the applicability of the modeling methodology for similar soft soil conditions in the Yangtze River Delta region. While the study employs a single set of soil parameters derived from site-specific geotechnical investigation to ensure consistency in construction scheme comparisons, the relative performance ranking of construction schemes is expected to remain consistent under reasonable parameter variations as the underlying deformation mechanisms are governed by fundamental soil–structure interaction principles.

The current analysis employs drained conditions, which represents a simplification of the complex hydro-mechanical behavior in soft soils. As demonstrated by Su et al. [30], hydro-mechanical coupling effects can significantly influence deformation evolution in soft media, particularly under complex loading conditions such as excavation and pre-stressing. While this approach is appropriate for the relatively rapid UBIT construction process and enables systematic comparison of construction schemes, future investigations incorporating coupled analysis would provide a more comprehensive understanding of pore pressure effects and long-term deformation behavior. The analysis is based on idealized construction conditions to isolate the fundamental mechanisms governing deformation control. Although real-world construction tolerances may introduce additional variability, the relative performance characteristics identified in this study provide valuable guidance for construction scheme selection.

5. Limitations

The study presents several limitations that warrant consideration. Parameter sensitivity analysis of key soil properties (elastic modulus, cohesion, and friction angle) and hydrogeological variations (groundwater level changes) were not conducted, which could influence deformation predictions under varying site conditions. The absence of site-specific monitoring data from the Ningbo–Cixi project constrains direct model validation, although cross-validation with the Wuding Road Station project provides confidence in the modeling approach. The analysis focuses exclusively on construction-induced deformation without incorporating long-term settlement assessments involving consolidation and creep phenomena. Furthermore, economic evaluation of construction alternatives was not conducted, which would enhance the decision-making framework for practical implementation.

6. Conclusions

Through refined numerical simulation, this paper systematically investigates the influence of different construction schemes for UBIT underpassing on the deformation of existing station floor slabs, reaching the following main conclusions:

(1) Adopting a continuous jacking strategy for adjacent pipes can form a continuous and complete support structure system, effectively controlling the range of soil disturbance and limiting maximum settlement to a safe level of −5.2 mm. Among all schemes tested, Scheme 4 (2-3-1-4-5) demonstrates optimal performance with minimum settlement; whereas the intermittent jacking strategy leads to discontinuous early support structures, significantly reducing deformation control effectiveness.

(2) Pre-stress tensioning creates an inward compression effect on the UBIT structure, causing the UBIT structure to exhibit characteristic deformation patterns with bottom bulging and top settlement. The medium pre-stress level (1395 MPa) provides optimal balance between structural stiffness and cost efficiency. The settlement of the UBIT top causes increased settlement in the floor slab area above it, while bottom bulging reduces settlement in the soil chamber area floor slab through soil force transmission, forming typical “alternating concave-convex” deformation characteristics.

(3) The excavation process of various soil chamber areas not only causes settlement in the excavated area but also induces uplift in surrounding soil due to stress redistribution. The side-first–center-later excavation strategy is the recommended approach that effectively utilizes the unloading uplift effects generated by side excavations, providing appropriate deformation space for the middle chamber and forming a relatively balanced “triple-peak” deformation distribution; whereas the center-first excavation scheme, by disrupting the core support area early in the process, reduces overall stability and results in poorer deformation control.