Research on the Initial Launching Technology of Subway Shield Tunneling in Complex Terrain and Numerical Simulation of Soil Deformation

Abstract

Using the shield project of the Cai Cang Section tunnel of the Guangzhou Metro Line 13 to solve the problem that shield construction is difficult to start in a narrow space and it is easy to disturb the surrounding buildings and pipelines, the corresponding shield tunneling parameters, construction and transportation plans, residual soil management plans, and grouting reinforcement plans are designed. These are tailored according to different working conditions. Meanwhile, the MIDAS GTS 2022 numerical simulation software is applied to simulate and analyze the impact of shield tunneling construction on soil deformation, and to compare the effects before and after reinforcement of the soil layer during shield tunneling. The results show the amount of disturbance of building pipelines along the tunnel are effectively controlled by designing the corresponding shield tunneling parameters for three working conditions: contact reinforcement zone, entering reinforcement zone, and exiting reinforcement zone. In narrow spaces, three kinds of construction transportation modes (namely, horizontal transportation in the tunnel, translation transportation in the cross passage, and vertical transportation) ensure the smooth transportation of pipe segments and the smooth discharge of shield dregs. After the reinforced area is constructed, secondary grouting with cement mortar effectively reduces the erosion concrete segments by underground water. By comparing the deformation of the tunnel soil layer before and after reinforcement, it is found that the maximum surface deformation of the soil layer is significantly reduced after reinforcement. Specifically, the maximum settlement and maximum uplift are 0.782 mm and 1.87 mm respectively, which represent a reduction of 1.548 mm in the maximum surface settlement, and 0.16 mm in the maximum uplift compared with the unreinforced soil layer. This indicates that setting up a soil reinforcement zone during the initial launching stage can effectively reduce soil deformation. The Cai Cang Section tunnel shield project successfully completed the shield construction in a narrow space, which can be a reference and guide for similar projects.

1. Introduction

The shield tunneling method is a fully mechanized construction technique for underground tunnels, offering advantages such as construction safety, rapid advancement, and high automation. It has been widely applied in tunnel and underground engineering [1]. Zhou Zihao [2] established a method for the automatic recognition of soil conditioning status via laser scanning, offering key approaches for obtaining mechanical parameters for the numerical simulation of soil deformation during complex terrain shield construction. Wei’s research [3] on an automated monitoring system and deformation control strategies, offers practical engineering data for the numerical simulation of soil deformation during the initial launch of shield tunneling in complex terrain, facilitating the optimization of parameter configurations in simulation models. With accelerated urbanization, even though shield technology has advanced significantly, it has also caused environmental challenges including soil loosening, building subsidence, cracking, and ground collapse during excavation. Therefore, practical engineering must adopt specific schemes and conduct feasibility studies to ensure quality and safety [4].

Numerous scholars have analyzed the impacts of subway shield construction. Li Zhendong [5] obtained technical solutions and theoretical support for shield launching in special working conditions by studying the key technologies of multi-shield launching in narrow oblique cross passages. Li Haisheng [6] clarified the force rules and safety thresholds of embedded steel plates in lateral launching conditions by researching the force characteristics and safety of embedded steel plates for lateral shield launching.Liu Weining et al. [7] implemented the “cavern-column-beam” method in the Beijing Metro’s Fuxingmen-Bawangfen Line, ensuring the safety of underground pipelines and controlling surface deformation using a novel shallow buried excavation approach. Huang Runqiu et al. [8] addressed waterproofing in the Shanghai Metro Lines 1 and 2 by creating polyurethane water-sealing rings in silty sand strata, which enhanced concrete impermeability. For confined-space shield launching, split launching technology has gained recent prominence. For narrow-space launching, Zhan Youming et al. [9] solved deep tunnel launching challenges by using dual-split technology, but did not address dynamic parameter adjustment in composite strata. Chen Shaolin et al. [10] optimized split launching procedures but lacked a quantitative analysis of the reinforcement zone effects on soil deformation. This study addresses the combined challenges of limited space, variable strata, and a sensitive environment in the Cai-Cang Section by proposing a three-stage disturbance control parameter system and quantifying the reinforcement effects through numerical simulation; this will fill existing research gaps. Zhong Zhiquan [11] achieved successful split launching in the Singapore Metro by modifying trailing equipment, muck removal systems, and segment transfer devices. Bu Xingwei et al. [12] adapted auxiliary facilities and negative ring structures to ensure safe initiation of the Wuhan Jiangxia Qingshui River Tunnel, meeting deformation requirements. Ru Junqi et al. [13] identified three critical steps for slurry shield split launching in confined areas. Xiong Dongdong et al. [14] proposed optimizations for split launching in metro tunnels with limited space.

This study focuses on the shield tunneling project of the Caihongqiao Station–Congbian Road Station section (abbreviated as Cai-Cang Section) of the Guangzhou Metro Line 13 to analyze key challenges. In the Cai-Cang Section project of the Guangzhou Metro Line 13, the shield must be launched under conditions of eccentric shaft positioning and limited space. The tunnel traverses interbedded fully to slightly weathered argillaceous siltstone (saturated uniaxial compressive strength 5–30 MPa) and a pore-fissure composite aquifer, while closely passing 16 buildings/structures laterally or vertically (minimum horizontal distance 0.181 m, minimum vertical distance 1.116 m). Existing studies lack systematic research on parameter matching for split launching in narrow spaces, the quantitative evaluation of the reinforcement effects in composite strata, and disturbance control for ultra-close buildings/structures, which all necessitate targeted technical solutions.

2. Project Overview

2.1. Alignment Overview

The Cai-Cang Section Line starts from E21 Shield Shaft, runs along Dongfeng West Road, underpasses Renmin North Road and the Panfu Interchange, passes through buildings such as the Jiaheyuan Community, Children’s Palace, Pacific Center, and Times International Tower, traverses the tunnel-first, station-later section of Memorial Hall, underpasses Metro Line 2 and the Xiaobei Road Viaduct, and ends at Cangbian Road Station. Memorial Hall Station is located at the intersection of Jiefang North Road and Dongfeng Road, running east-west along Dongfeng Road. It is a two-story underground island platform station. The main structure of the station was constructed using the drift-pile method for underground excavation, with the station built after the shield tunneling passed through. The station spans from YDK18 + 468.075 (ZDK18 + 468.085) to YDK18 + 777.095 (ZDK18 + 777.095), with a central mileage of YDK18 + 658.895 (ZDK18 + 658.895). The total length of the station is 309 m, with a burial depth of 13.2 m.

The Caihongqiao Station-Memorial Hall Station Section (abbreviated as Cai-Ji Section) shield tunneling section has two horizontal curves on the left main line, with a minimum curve radius of 1000 m. After exiting the cross passage, the left line reaches Memorial Hall Station with gradients of −3‰ → −8.448‰ → +6‰ → −5‰; the right line follows the same gradient sequence (−3‰ → −8.448‰ → +6‰ → −5‰) to Memorial Hall Station, with a maximum vertical curve radius of 5000 m. The design scope of the Cai-Ji Shield Section is as follows:

• Left line: Starting/ending mileage ZDK17 + 464.160~ZDK18 + 658.895, short chainage 2.513 m, and a total length of 1192.222 m.
• Right line: Starting/ending mileage YDK17 + 464.986~YDK18 + 658.895, short chainage 0.084 m, and a total length of 1193.825 m.

The Memorial Hall Station-Cangbian Road Station Section (abbreviated as Ji-Cang Section) runs along Dongfeng Middle Road, passes by Sun Yat-sen Memorial Hall, and reaches Cangbian Road Station after underpassing the Xiaobei Road-Dongfeng Road Interchange Project. The section has a burial depth of 23.151 m~31.434 m, a track spacing of 14.4~16.18 m, a minimum horizontal curve radius of 800 m, and a “V-shaped” longitudinal slope with a maximum gradient of 25‰. The design scope is as follows:

• Left line: Starting/ending mileage ZDK18 + 658.895~ZDK19 + 669.658.
• Right line: Starting/ending mileage YDK18 + 658.895~YDK19 + 669.658.

Total length: 1010.763 m. A No. 1 connecting passage combined with a wastewater pump room is located at YDK19 + 283.200.

2.2. Engineering Hydrogeology

According to geological survey data, the strata along the Cai-Cang Section from top to bottom are, as follows: artificial fill layer, alluvial-proluvial fine sand, medium-coarse sand, silty clay layer, a fluviolacustrine deposit layer, a residual soil layer, completely weathered clastic rock, a strongly weathered layer, a moderately weathered layer, and a slightly weathered layer. The shield tunnel passes through completely weathered argillaceous siltstone, strongly weathered argillaceous siltstone, moderately weathered argillaceous siltstone, and slightly weathered argillaceous siltstone. Groundwater in the surveyed area is classified into three types by occurrence, as follows: quaternary loose layer pore water, layered bedrock fissure water, and networked bedrock fissure water. The Pearl River is located approximately 2.2 km south of the project site, with a complex estuarine topography, abundant upstream inflow, a long flood season, and a typhoon influence, resulting in complex annual tidal variations. The fully weathered argillaceous siltstone has the following characteristics: natural water content 25%, compression modulus 4 MPa, permeability coefficient 0.1 m/d, saturated uniaxial compressive strength 30 MPa, and elastic modulus 5 GPa.

2.3. Surrounding Environment

The positional relationship between the major buildings/structures and the shield section along the Cai-Cang Section is shown in

Table 1. The section of Cai Cang Section mainly relates to the position of structures and shield sections.

Buildings/Structures	Starting/Ending Mileage	Relationship with Shield Section
Metro Line 11 Section Tunnel	YDK17 + 465~YDK17 + 480 ZDK17 + 472~ZDK17 + 508	Overpasses (with a clear tunnel spacing of approximately 2.773 m)
Sima Yong Box Culvert	YDK17 + 465~YDK17 + 492 ZDK17 + 464~ZDK17 + 542	Underpasses (vertical spacing of 15.15 m)
Guangzhou Children’s Palace Gymnasium & Swimming Pool	ZDK17 + 578~ZDK17 + 615	Passes by (approximately 0.181 m from the left tunnel)
Dongfeng West Viaduct Widening Project No. 1 Pedestrian Overpass	YDK17 + 713~YDK17 + 760 ZDK17 + 672~ZDK17 + 774	Passes by (3.8 m from the right tunnel)
Renmin Road Viaduct Project (North Section)	YDK17 + 858~YDK17 + 868 ZDK17 + 865~ZDK17 + 869	Passes by (1.45 m from left tunnel, 1.96 m from right tunnel)
Renmin North Road-Dongfeng West Road Pedestrian Overpass	YDK17 + 900~YDK18 + 000 ZDK17 + 912~ZDK17 + 980	Passes by (pile foundation is 3.732 m from the crown of the right tunnel)
Jintongtai Building	YDK18 + 200~YDK18 + 240	Passes by (4.44 m from the right tunnel)
Guangzhou Municipal People’s Hospital Outpatient Building	YDK18 + 256~YDK18 + 290	Passes by (3.399 m from the right tunnel)
Dongfeng Middle Road-Panfu Interchange	YDK18 + 193~YDK18 + 471 ZDK18 + 193~ZDK18 + 471	Underpasses and passes by (pile foundation is 0.46 m from the crown of the left tunnel)
No. 9 Civil Air Defense Project	ZDK18 + 364.329 YDK18 + 361.365	Overpasses (vertical clear distance of 1.116 m)
Jiefang North Road Viaduct Project No. 1 Pedestrian Overpass	YDK18 + 600~YDK18 + 650 ZDK18 + 591~ZDK18 + 640	Passes by (7.662 m from the right tunnel)
Buildings/Structures	Starting/Ending Mileage	Relationship with Shield Section
Jiefang North Road Viaduct	YDK18 + 651~YDK18 + 665 ZDK18 + 650~ZDK18 + 661	Passes by (1.55 m from left tunnel, 1.5 m from right tunnel)
Metro Line 2		Underpasses (clear spacing from tunnel structure edge is 2.987 m)
Guangzhou Municipal People’s Congress Standing Committee Office Building		Passes by (clear distance of 4 m from right tunnel)
Guangdong Building		Passes by (clear distance of 18 m from right tunnel)
Jiaye Building		Passes by (clear distance of 6 m from right tunnel)
Guangdong Environmental Protection Building		Passes by (clear distance of 6 m from right tunnel)

. Special-grade risk (distance <1 m): the Children’s Palace (0.181 m) and the civil air defense project (1.116 m); these require real-time settlement monitoring (control value ≤ 3 mm). First-grade risk (distance 1–3 m): the Renmin Road Viaduct (1.45 m) requiring control of the shield lateral deviation (≤20 mm) (Section 3.2).

3. Analysis of Construction Difficulties and Key Points

3.1. Narrow Shield Launching Space

The shield launching site faces spatial constraints, necessitating the adoption of a split-start method. Trailers #5 and #6 of the shield machine must remain aboveground to extend the pipelines for initial excavation. These trailers are lowered and connected in the shaft only after the shield advances approximately 70 rings and the negative rings are dismantled.

Since the shield shaft is offset from the tunnel centerline, precise horizontal positioning of the launching frame is critical. Before tunneling commences, the launching frame must be translated to its designated position. Significant deviation between the shield axis and the tunnel alignment may cause segment misalignment or portal seal failure, underscoring the importance of accurate positioning for both the launching frame and the reaction frame. After assembling and aligning the pre-launch negative rings, the cutterhead must reach the portal chainage (tolerance: ±15 mm), and a shield attitude recheck (yaw ≤0.1°, pitch ≤0.05°) must be conducted before propulsion begins. The launching frame translation error should be ≤±10 mm, and attitude deviation ≤0.1° (heading)/0.05° (pitch). The negative ring assembly axis deviation from the design axis should be ≤±15 mm, and the shield tail gap uniformity error ≤±10 mm.

3.2. Shield Machine Passing Through Surrounding Buildings/Structures

During shield launch and tunneling, varying degrees of disturbance may occur to nearby buildings, structures, and underground pipelines, as follows:

(1)

Underpassing: improper operation during underpassing may cause settlement/cracking of adjacent buildings and ground settlement.

(2)

Overpassing: inadequate parameter control during overpassing may increase soil disturbance leading to soil chamber pressure loss and ground collapse.

(3)

Side passing: when passing pile foundations laterally, the lateral pressure from the tunneling may cause pile deformation, endangering bridge/building structures.

4. Shield Launch and Tunneling Technology

4.1. Pre-Tunneling Preparations

Preparations include temporary site construction, portal reinforcement, shield hoisting/translocation, shield assembly/commissioning, portal sealing, reaction frame/launch frame installation, negative segment installation, construction logistics, and portal removal.

4.2. Tunneling Parameter Design

Based on the classifications defined in the Code for Construction and Acceptance of Shield Tunnelling Method [15], the actual construction process can be categorized into three distinct working conditions, as follows: the approach zone, the entering zone, and the exiting zone of the ground improvement area. The tunnelling parameters designed for these conditions are presented in

Table 2. Design of driving parameters.

Working Condition	Parameter	Design Value
Approaching Reinforcement Zone	Advance Speed	10~20 mm/min
	Thrust Force	600~700 t
	Cutter Head Torque	2000~2500 kN·m
	Earth Pressure	0
	Muck Volume	0
	Grouting Volume	0
Entering Reinforcement Zone	Advance Speed	10~20 mm/min
	Thrust Force	600~700 t
	Cutter Head Torque	2000~2500 kN·m
	Earth Pressure	0~1.5 bar
	Muck Volume	61.8~63.1 m3
	Grouting Volume	10~20 mm/min
Exiting Reinforcement Zone	Advance Speed	10~20 mm/min
	Thrust Force	700~900 t
	Cutter Head Torque	2000~2500 kN·m
	Earth Pressure	1.5~1.8 bar
	Muck Volume	61.8~63.1 m3
	Grouting Volume	5.6~7.8 m3

. The three-stage soil chamber pressure values (0/0–1.5/1.5–1.8 bar) are calculated using Terzaghi’s theory and dynamically adjusted through field trial boring (the first 10 rings), ensuring a cutterhead torque fluctuation of ≤10% (Table 2).

4.3. Construction Transportation

(1)

The construction transportation during the right line launch phase consists of three main parts, as follows: horizontal transportation inside the tunnel, transverse passage shifting transportation, and vertical transportation through the shaft.

Horizontal Transportation Inside the Tunnel: a single track with 43 kg/m steel rails is laid inside the tunnel for transporting muck, segments, and other materials. The shield machine uses small muck buckets for muck removal, with horizontal transportation carried out by a 1-set marshalling train composed of a 45 t traction locomotive, one flatbed car for segment transportation, and one flatbed car for general cargo, as shown in Figure 1.

(2)

Shifting Transportation in the Transverse Passage: during the right line launch phase, materials and muck required for shield tunneling are connected to the ground via the muck shaft and the No. 1 transverse passage. Since the muck shaft is located outside the main tunnel alignment, direct vertical transportation at the muck shaft is infeasible. Therefore, lifting devices and transportation devices must be installed within the No. 1 transverse passage to ensure material/muck transportation, as shown in Figure 2.

(3)

Vertical Transportation: a 16 t gantry crane is installed above the muck shaft to transport materials required for shield tunneling, and to dump the muck into the muck pit.

4.4. Segment Installation

The segment installation procedure is illustrated in Figure 3. Segment selection prioritizes alignment with the tunnel design geometry, while ensuring the shield tail clearance meets the requirements for subsequent excavation cycles. Installation begins at the tunnel invert, followed by adjacent segments, and concludes with the keystone. Prior to keystone installation, the waterstop gasket must be lubricated. The keystone is initially inserted radially by two-thirds, adjusted, and then longitudinally pushed into place. Upon segment positioning, thrust jacks at corresponding locations are extended to secure the segments with a force exceeding the stabilization requirements, before retracting the erector. The bolts are tightened immediately after installation, followed by secondary tightening after the ring exits the shield tail.

4.5. Synchronous and Secondary Grouting

After the shield fully enters the tunnel portal past three rings of the post-reinforcement zone, portal sealing commences. Synchronous and secondary grouting initiates once the injected grout solidifies. The synchronous grouting filling rate should exceed 95%. After secondary grouting, the soil permeability coefficient decreases from 10⁻⁴ m/s to 10⁻⁶ m/s (field pumping test data).

(1)

Synchronous Grouting

Cement mortar is used as a synchronous grouting material. P.O 42.5 ordinary Portland cement is employed to enhance the corrosion resistance of the grout stone body. This ensures the segments are encapsulated within a corrosion-resistant grout matrix to mitigate groundwater corrosion of the segment concrete. Based on previous similar tunnel projects, the synchronous grouting mix proportion is determined as shown in

Table 3. The mixture ratio of slurry for synchronous grouting.

Cement (kg)	Fly Ash (kg)	Bentonite (kg)	Sand (kg)	Water (kg)
80~200	241~380	50~60	600~780	460~600

.

Synchronous grouting is conducted simultaneously with shield tunneling using four pipelines (four injection points) symmetrically injected by A-fluid piston pumps and B-fluid screw pumps.

To ensure effective filling of the annular gap while preventing segment deformation/damage from grouting, the grouting pressure is set at 0.2~0.6 MPa based on calculations and experience. For full-face rock strata, the grouting volume is 1~1.1 times the theoretical volume of the annular gap, resulting in a per-ring (1.5 m) grouting volume Q = 4.31~4.74 m³. The actual grouting volume is adjusted according to the settlement monitoring data measured during tunneling. The synchronous grouting rate must match the tunneling speed, with the average grouting rate determined by completing the grouting volume for one 1.5 m ring within the time taken for shield advancement. A dual-parameter control criteria is adopted, that is, grouting is deemed compliant when both the grouting pressures reach the set value and the grouting volume exceeds 90% of the design value.

4.6. Shield Alignment Control and Adjustment

Due to the uneven hardness of the strata, the changes in the tunnel curve and slope, and operational factors, the shield cannot advance exactly along the designed tunnel axis and will deviate to some extent. When this deviation exceeds a certain limit, it will cause the tunnel lining to encroach on the limit, reduce the gap behind the shield tail, deteriorate the local stress on the segments, and increase ground loss, thereby causing greater surface settlement. Therefore, effective technical measures must be taken during shield construction to control the direction of excavation and correct the deviation in a timely and effective manner.

(1)

Control of the shield direction. The shield attitude is monitored using a millimeter measurement system and a manual measurement as an auxiliary. This system is equipped with guidance, automatic positioning, excavation program software, and a display. This can dynamically display at all times, the deviation and the trend of the shield’s current position from the designed tunnel axis in the shield machine’s main control room. Based on this, the excavation direction of the shield machine is adjusted to keep it within the allowable deviation range. As the rear reference point of the shield guidance system moves forward with the shield advancement, precise positioning must be carried out through a manual measurement. To ensure the accuracy and reliability of the advancement direction, a manual measurement is conducted twice a week to verify the measurement data of the automatic guidance system, and to recheck the position and attitude of the shield machine. This ensures the correct excavation direction of the shield.

The shield’s excavation direction is controlled by operating the shield’s thrust cylinders in different zones. When advancing on an uphill section, the thrust of the lower cylinders of the shield is appropriately increased; when advancing on a downhill section, the thrust of the upper cylinders is appropriately increased; when advancing on a left-turn curve, the thrust of the right cylinders is appropriately increased; when advancing on a right-turn curve, the thrust of the left cylinders is appropriately increased; when advancing on a straight and level section, the thrust of all cylinders should be kept as consistent as possible i.e., the same.

(2)

Adjustment and correction of the shield’s excavation attitude. The shield’s attitude is adjusted and deviations are corrected by operating the thrust cylinders in the different zones. For sharp curves and slope change sections, if necessary, the shield’s overcutting tools can be used for local overcutting to correct the deviation. When the rolling exceeds the limit, the shield will automatically alarm. At this time, the shield’s cutterhead should be reversed to correct the rolling deviation.

4.7. Portal Sealing

To ensure the safe entry of the shield machine into the tunnel, the self-supporting and compactness of the soil at the tunnel exit must be good. To prevent soil collapse and groundwater intrusion when the shield machine passes through the portal, and to ensure a smooth reception, the portal must be sealed and reinforced. After the shield body fully exits the portal, the leakage rate should be <0.1 L/(m·min) to meet the requirements of the Shield Tunnel Engineering Construction and Quality Acceptance Standards.

When the shield machine is 5 to 7 rings away from the portal, secondary double-liquid grouting is carried out behind the segments, 3 to 5 rings away from the shield tail, to cut off the water source behind the shield machine and to form an effective water-stop curtain for the entire tunnel. This prevents water and sand leakage from behind the shield during its entry into the tunnel. While secondary grouting is being injected, synchronous grouting is also carried out simultaneously to prevent the shield tail brush and the grouting holes at the shield tail from becoming blocked, which could affect subsequent construction. After the shield machine hits the wall, radial holes and shield tail grouting are used to prevent water from flowing into the front chamber from the head and tail of the machine.

After the shield machine’s cutterhead hits the wall and enters the tunnel, another grouting is carried out at 10 rings behind the shield tail, using double-liquid grout with an initial setting time of 2 to 3 min. As much grout as possible should be injected to tightly seal the gap between the segments and the rear part to prevent groundwater from flowing back to the machine head. The secondary grouting slurry uses double-liquid grout. The materials are 42.5 cement, tap water, and water glass, with a slurry ratio of liquid cement (mainly cement slurry): water = 100:100 (mass ratio); B liquid (mainly water glass solution) water: water glass = 1:0.7.

After the concrete at the portal is removed, the shield should be advanced and the segments assembled as soon as possible to minimize the time for the shield to enter the tunnel. After the shield body completely exits the portal ring, the portal is sealed. At this time, the steel wire rope on the portal flap is tightened to ensure that all the gaps are closed and a complete seal is formed. Double-liquid grouting is carried out through the grouting holes inside the segments, starting from the bottom and moving upwards so the secondary grouting forms a sealed and waterproof curtain between the segments and the reinforced soil. This achieves the effect of sealing the portal. Meanwhile, the single-liquid grouting system is applied for synchronous grouting to fully fill the gap between the segments at the tail of the shield and the reinforced body. Then, grouting is carried out at the portal, as needed, to seal the entrance.

5. Numerical Simulation Analysis of Soil Layer Deformation

5.1. Model Establishment

Before the shield starts, grouting reinforcement is required for the tunnel portal. In this paper, models with, and without, reinforcement zones are established respectively to study the influence of the reinforcement zone on soil layer deformation. In the calculation model of this paper, the model size is 60 m × 50 m × 45.17 m, and the grid is divided with a size of 0.8 m per cell. The grid size of the soil layer between the tunnels is 1 m, and the grid size of the remaining soil layer is 2 m. The calculation model is shown in Figure 4. The model bottom is fixed in three directions, side faces are constrained horizontally, and the surface is a free boundary. The initial stress field is calculated by self-weight stress, and groundwater uses a hydrostatic pressure boundary (water table depth 3 m). Soil parameters are from in-situ tests (standard penetration test, direct shear test). For

Table 4. Table of physical and mechanical parameters of the soil and structures.

Material Name	Young’s Modulus (MPa)	Poisson’s Ratio	Unit Weight (kN/m3)	Cohesion (kPa)	Internal Friction Angle (°)
Artificial Fill	18	0.4	17.5	17.5	22
Silt Sand	23	0.35	18	4	30
Medium Coarse Sand	70	0.35	19	8	34
Silt	4	0.33	15.5	14	12
Silty Clay 11	13	0.3	18	30	22
2 Silty Clay 2	13	0.3	19	33	24
3 Silty Clay	25	0.3	19	55	26
Fully Weathered Clayey Siltstone	17	0.3	18	35	25
Material Name	Young’s Modulus (MPa)	Poisson’s Ratio	Unit Weight (kN/m3)	Cohesion (kPa)	Internal Friction Angle (°)
Strongly Weathered Coarse Sandstone	60	0.28	20	50	30
Strongly Weathered Clayey Siltstone	70	0.3	20	40	28
Moderately Weathered Coarse Sandstone	80	0.25	23	120	37
Moderately Weathered Clayey Siltstone	30	0.25	20	70	30
Slightly Weathered Clayey Siltstone	50	0.28	21	110	33
Concrete	35,500	0.2	25
Shield Shell	208,000	0.2	78

, the reinforcement zone parameters assume the elastic modulus is increased to 150 MPa (reference value for cement soil), a Poisson’s ratio of 0.25, and a permeability coefficient of 10⁻⁷ m/s.

Figure 5 shows a cross-sectional view of the model with the reinforcement zone. The dimensions of the established model are 50 m in width and 45.17 m in thickness. The outer diameter of the tunnel is 6.4 m, and the inner diameter is 5.8 m. The center of the tunnel is 25.5 m away from the ground surface. The reinforcement width of the reinforcement zone is 3 m outside the tunnel, the reinforcement height is from 3 m above the tunnel top to 0.5 m below the tunnel bottom, and the reinforcement length is 7 m. The 26.8 m × 9.9 m area shown in the figure is the soil reinforcement zone.

In this simulation, 3D solid elements were adopted for the soil layer and the lining structure, while 2D plate elements were used for the grouting material and the shield shell to establish the model. The soil layer was modeled using the Mohr-Coulomb constitutive model, and the grouting material, segments, and shield shell were modeled using the elastic model. The specific parameters of the materials are shown in Table 4.

5.2. Construction Stages

This model is set up with 28 construction steps. In line with the actual project, the shield construction step in the model is set at 1.5 m. The right tunnel is constructed first, and the left tunnel construction begins after the right tunnel has been constructed for 100 m. To enhance computational efficiency, the shield construction process is simplified in the simulation. The right tunnel is regarded as an existing tunnel, and the left tunnel is constructed by excavating two rings each time, with each ring being 1.5 m wide, for a total of 40 rings.

5.3. Analysis of Soil Layer Deformation Model

To study the influence of reinforcement on the surface deformation of the soil layer before and after, the vertical displacement cloud map of the last construction stage of the model is extracted for analysis, as shown in Figure 6. The model was validated using field monitoring data. After the left tunnel, the maximum measured surface settlement was 0.82 mm (simulated value 0.782 mm, error 4.6%), and the maximum heave was 1.95 mm (simulated value 1.87 mm, error 4.1%). The results show the model can well reflect actual deformation patterns.

As shown in Figure 6, the blue areas in the cloud map represent the locations of soil settlement, while the red areas represent the locations of soil uplift. The overall model analysis indicates that the maximum surface settlement of the soil layer with the reinforcement zone is significantly less than that of the unreinforced soil layer, and the maximum surface uplift of the soil layer is also less than that of the unreinforced soil layer. As can be seen from Figure 6a, the maximum surface settlement of the soil layer in the left line area is 2.33 mm, and the maximum surface uplift of the soil layer is 2.03 mm. As can be seen from Figure 6b, after the portal reinforcement treatment, the maximum surface settlement of the soil layer around the tunnel is significantly reduced compared with the unreinforced soil layer (only 0.782 mm), and the maximum surface uplift of this reinforced soil layer is also reduced, to 1.87 mm. Through comparison, it can be found that after the portal reinforcement construction, the problem of soil layer surface deformation caused by shield construction can be effectively reduced.

6. Conclusions

This study, based on the shield tunneling project of the Cai-Cang Section of the Guangzhou Metro Line 13, successfully achieved shield launching in confined spaces and derived the following conclusions:

• Tailored Shield Parameters: for three working conditions (contact reinforcement zone, entry reinforcement zone, and exit reinforcement zone) shield tunneling parameters were specifically designed to control any environmental disturbances within acceptable limits.
• Multi-Modal Transportation: three transportation methods (in-tunnel horizontal transport, cross-passage transfer transport, and vertical transport) ensured efficient segment delivery and muck disposal in spatially constrained environments.
• Grouting Optimization: post-reinforcement synchronous grouting using cement mortar (P.O42.5 cement) effectively mitigated groundwater erosion of the concrete segments, enhancing construction safety.
• Soil Reinforcement Efficacy: numerical modeling revealed that soil reinforcement significantly reduced surface deformation. The maximum settlement decreased from 2.33 mm (unreinforced) to 0.782 mm (reinforced), a reduction of 1.548 mm, while the maximum uplift decreased from 2.03 mm to 1.87 mm (a reduction of 0.16 mm). These results validate the effectiveness of soil reinforcement for minimizing ground deformation.