1. Research Objective

Due to the inherent characteristics of metro tunnel shield construction—being an underground project with a complex surrounding environment and high intrinsic risks—the process is prone to ground settlement and deformation incidents. These issues arise primarily from ground loss caused by shield excavation, as well as from the remolding of soil around the tunnel due to disturbance or shear failure during construction. Therefore, by analyzing the mechanisms behind soil disturbance and settlement during shield tunneling, along with the patterns of surface subsidence, and drawing on extensive literature, we can identify the key geological factors that influence both soil disturbance and ground settlement. Based on these insights, targeted measures can be developed to effectively control surface subsidence, thereby minimizing potential casualties and property damage caused by metro construction activities.

This study investigates the control of surface settlement around construction sites during tunnel boring machine (TBM) operations in Tianjin. It involves tracking the entire construction process, gathering comprehensive on-site data, and evaluating the effectiveness of key protective measures in managing ground movement. Based on these findings, the research summarizes the critical factors for controlling surface settlement during TBM excavation: 1. Selecting the appropriate TBM model based on the geological and hydrological conditions of each tunnel section; 2. Properly designing the thrust support system at the TBM launch site and optimizing TBM parameters; 3. Continuously adjusting and fine-tuning synchronous grouting parameters according to the TBM's alignment and attitude; 4. Identifying the essential principles for effectively controlling surface settlement during TBM construction.

2 Construction Process Control

This tunnel section passes beneath existing buildings and structures. During shield launching, approximately half of the cross-section lies within ⑥4 silty clay, about one-quarter is in ⑦ silty clay, and another quarter is in ⑧1 silty clay. At the shield reception stage, roughly 25% of the cross-section falls into ⑧1 silty clay, around 50% is in ⑧2–1 sandy silty soil, and the remaining 25% is in ⑧2–2 silty sand. The reception end is located at Hongqi South Road Station, which serves as a transfer station connecting Line 3 and Line 6. Currently, Line 3 is already operational. However, the geological conditions at the reception site are unfavorable, making safety assurance during reception the primary focus of this project. Any leakage at the working shaft could lead to extensive ground settlement, potentially disrupting the existing line and causing significant disruptions with far-reaching consequences.

2.1 Reinforcement Zone Control

During the construction process, the quality of the reinforced piles was monitored throughout. Prior to tunneling, core sampling tests were conducted in the reinforcement area. The test results showed excellent uniformity and self-supporting properties, with all extracted pile cores meeting the requirement of a 28-day unconfined compressive strength of at least 1.0 MPa, as well as a permeability coefficient no greater than 1.0 × 10⁻⁷ cm/s. To further verify the quality of the reinforcement zone, additional water probing will be carried out at the portal area. Specifically, nine probe holes will be arranged inside the portal: one at the center of the portal and two others positioned 30 cm away from the portal ring, spaced 2.3 meters apart along the circumference. Additionally, four probe holes will be placed at structural dead corners outside the portal ring. Detailed hole layouts are shown in Figures 3–9. Core drilling will be performed at the predetermined locations using a core drill rig, with each borehole reaching a depth of 3.0 meters and a diameter of Φ50 mm. After completing the probe holes, no visible water was observed during inspection. In summary, these findings confirm that the end-shaft reinforcement work has been successfully executed, significantly improving the soil conditions around both the entrance and exit portals. This not only validates the effectiveness of the selected technical parameters but also demonstrates the high quality of the reinforcement process.

2.2 Initial Phase Control

After the tunnel boring machine (TBM) is launched, a 100-meter trial section will be advanced to better monitor and control its various parameters. During this initial phase, special attention should be paid to setting appropriate propulsion parameters, while systematically collecting, recording, and analyzing key technical data throughout the process. This will help identify the relationship between ground settlement and construction parameters, enabling us to quickly gain insights into the operational performance of the TBM equipment and determine the optimal range of construction parameters for advancing through the geological conditions encountered in this project section.

(1) The objectives of conducting the 100m trial drive are: ① To familiarize ourselves with the operation methods and mechanical performance of the tunneling machine in the shortest possible time; ② To gain a thorough understanding of the geological conditions specific to this project, identify the construction characteristics of earth-pressure balanced tunneling in this region, and quickly determine the optimal settings for various parameters; ③ To master the proper methods for adjusting key construction parameters, we will employ multiple monitoring techniques during the trial section—such as ground settlement monitoring and deep settlement monitoring—to systematically collect comprehensive data that will guide the tunneling operations; ④ Through this trial segment, we also aim to analyze the collected monitoring data, enabling us to fine-tune the tunneling parameters and optimize the synchronous grouting volume. During the construction of the trial section, monitoring frequency must be increased, and real-time feedback of monitoring data should be provided promptly to construction technicians for analysis. Based on these insights, adjustments to all relevant parameters can be made continuously until both ground settlement and tunneling machine posture are brought under strict control.

(2) Key construction points for the first 100-meter trial advancement segment: ① When advancing to the 20th ring, grout the tunnel portal to prevent potential soil loss; ② After completing the 100-meter trial advance and ensuring the tunnel has sufficient frictional resistance to counteract the reaction force from shield excavation, promptly carry out the removal of negative-ring segments and the shield base during construction breaks. Meanwhile, set up a working platform at the bottom of the shaft, lay out diverging tracks, and install dual parallel motorized rail systems.

2.3 Shield Machine Parameter Control

(1) Advancing through the reinforcement zone: ① When the shield machine passes through the reinforcement zone, closely monitor parameters such as cutterhead torque, screw conveyor torque, oil temperature, and water temperature. ② Strictly control the soil pressure by setting the initial pressure between 0.00 and 0.10 MPa, and make reasonable adjustments based on factors like propulsion oil pressure and cutterhead oil pressure. Simultaneously, analyze and adjust these parameters in conjunction with settlement reports and other construction data, providing timely feedback to the propulsion team to ensure safe start-up operations. ③ Carefully manage the excavated muck volume, keeping it within the range of 46.99 m³ to 47.95 m³ per ring. Through continuous analysis and adjustment, identify the most *optimal* values. ④ Maintain a slow propulsion speed, limiting it to no more than 1 cm/min. Additionally, introduce foam or bentonite into the soil ahead of the shield machine as needed to improve soil conditions. ⑤ Install monitoring points at close intervals and increase the frequency of measurements. ⑥ Ensure dynamic information is promptly communicated throughout the process. ⑦ Key precautions when passing through the reinforcement zone: a. After the negative-ring segments have fully exited the tail shield, reinforce the area immediately. b. Keep the total jacking force within an appropriate range to prevent excessive deformation of the rear support structure; ideally, the jacking force should not exceed 1000 tons. c. As the shield machine enters the tunnel portal, pay extra attention to the integrity of the portal’s waterproofing equipment.

State—when in poor condition, reinforcement measures should be taken.

(2) Propulsion after exiting the reinforcement zone: ① Pressure balance setting principles: During the initial launching phase, the theoretical soil pressure is 0.152 MPa. ② Muck removal control: Theoretical muck volume per segment is 47.95 m³, with actual muck removal maintained between 98% and 100%. ③ Advancement speed: Ideally controlled within 1–2 cm/min, adjusted appropriately based on various influencing factors. ④ Shield alignment accuracy and ground settlement requirements: Alignment deviation must not exceed 50 mm, while ground settlement should be kept within +10 mm to –30 mm. For specific areas requiring enhanced protection, settlement control standards may be slightly tightened as necessary.

2.4 Synchronous Grouting and Secondary Grouting

(1) Synchronous Grouting. Synchronous grouting during shield tunneling is a key method for filling the annular gaps between the surrounding soil and the segment rings, as well as for minimizing post-construction deformation. It also serves as an essential procedure in shield tunneling operations. During shield tunneling, grouting must be carried out promptly, evenly, and in sufficient quantities to ensure that all construction voids are adequately and timely filled. This, in turn, helps reduce both surface settlement and segment misalignment to minimal levels, while simultaneously preventing water leakage at segment joints. Grout performance requirements: The synchronous grout (refer to Table 1) must meet pumping specifications. Specifically, its bleeding rate, as well as its 1-day and 28-day compressive strengths, should all comply with design and relevant standards. To satisfy these criteria, this project has chosen to use a hardening grout. Hardening grouts have been successfully employed in numerous projects, consistently delivering strength characteristics that meet design requirements and proving highly effective in maintaining tunnel stability. Additionally, this type of grout exhibits low shrinkage, minimal dilution effects from groundwater, and poses no environmental hazards.

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(2) Secondary Grouting. Behind the last segment of each shield machine, a 2.5m x 1m flatbed cart is equipped to carry one grouting machine and a complete mixing system, ensuring that all tunnel segments within the area behind the shield tail can be promptly subjected to either intermittent or secondary grouting as needed. Based on actual site conditions, grouting intervals may be further refined for enhanced pressure application. Typically, secondary grouting for segment-to-segment construction gaps is performed every 5 rings. The grout used is a rigid-type slurry; however, in particularly critical areas, a dual-component slurry—combining cement and water glass—may be employed for secondary grouting. During the grouting process, a dedicated personnel must be assigned to meticulously record details such as grouting locations, grouting volumes, and pressure readings. These records should be continuously updated in alignment with real-time ground deformation monitoring data, allowing for timely adjustments to maintain optimal construction quality. Additionally, supplementary grouting may be required based on observed post-construction settlement patterns.

2.5 Reception Phase Control

The first 50 rings before shield reception mark the receiving phase. During shield construction in this segment, it is crucial to strictly control the cutterhead's earth pressure balance, ensuring a slight uplift of the surrounding ground at the cutterhead opening. This uplift helps counteract the ground settlement that occurs as the shield advances and pushes against the soil behind it. At the same time, construction parameters closely tied to the cutterhead pressure balance—such as muck removal volume, advancing speed, total thrust force, and the fluctuation range of actual earth pressure around the set target pressure—must also be tightly monitored. This approach minimizes both over-excavation and under-excavation, while keeping fluctuations in balancing pressure to a minimum. Once the shield enters the reinforcement zone, the earth pressure and total thrust should be appropriately reduced to ensure the safe opening of the tunnel portal. Key control points are illustrated in Figure 1.

This time, the tunnel portal excavation will be carried out using a crushing method, cutting off both the inner and outer rows of reinforcing bars, and thoroughly clearing any remaining reinforcement inside the portal to ensure unobstructed access upon arrival. To verify the effectiveness of ground reinforcement directly ahead of the TBM's arrival, a suitable number of sample holes will be drilled at strategic locations within the tunnel crown area. Initially, nine sample holes are planned—three each at the top/bottom, left/right, and center positions of the portal—though this number may be adjusted upward depending on the conditions observed in the sample holes. The portal excavation can only proceed once the sample holes confirm that the ground conditions are optimal, as illustrated in Figures 2 and 3.

3 Monitoring and Analysis

The settlement monitoring for the tunnel boring machine (TBM) section between Yizhongyuan Hospital Station and Hongqinanlu Station lasted a total of 240 days, during which more than 6,500 sets of observation data were collected. The monitoring was divided into three phases: the TBM launching phase, lasting 21 days; the normal TBM excavation phase, spanning 146 days; and the TBM reception phase along with subsequent monitoring, which extended for 73 days. In the first phase, the maximum settlement recorded was 20.55 mm, occurring at observation point DBC-09-08 on January 20, 2015. In the second phase, the largest settlement measured was 15.37 mm, observed at point DBC-35-04 on June 20, 2015. Finally, in the third phase, the maximum settlement reached 17.93 mm, noted at observation point DBC-43-18 on August 30, 2015. The statistical results are summarized in Table 2.

Based on the monitoring data from the tunnel boring machine (TBM) construction section between Yizhongxin Hospital Station and Hongqinanlu Station, settlement-time curves for the settlement monitoring points are presented in three distinct phases.

Phase 1 (Tunnel Boring Machine Launch Phase): During the construction of the TBM launch phase, data from each monitoring point along the DBC05 to DBC09 sections—specifically the ground settlement points—was organized. The resulting settlement-versus-time curves are shown in Figure 4.

Phase 2 (Tunnel Boring Machine Normal Advancement Phase): During the construction of the tunnel boring machine's normal advancement phase, data from each monitoring point along cross-sections DBC16, DBC17, DBC18, and DBC35—specifically the ground settlement points—was organized. The resulting settlement-versus-time curves are shown in Figure 5.

Phase 3 (Tunnel Boring Machine Reception Phase): During the construction of the TBM reception phase, data from each monitoring point along cross-sections DBC43 and DBC44—specifically the ground settlement points—was organized. The resulting settlement-versus-time curves are shown in Figure 6.

According to the ground settlement-versus-time curves for the three stages of shield tunneling construction, it can be observed that even with various settlement-control measures implemented during shield driving, ground settlement is inevitably induced. The largest settlement occurs during the shield launching phase, followed by the shield receiving phase, while the smallest settlement is seen during the normal tunneling phase.

4 Conclusion

This paper, based on Tianjin's hydrogeological conditions and combined with the actual construction of the tunnel boring machine (TBM) section between Yizhong Hospital Station and Hongqi Nanlu Station on Tianjin Metro Line 6, investigates the settlement patterns of the surrounding ground surface caused by TBM-driven tunneling. The study analyzes various phenomena observed during settlement monitoring along the Yizhong Hospital Station–Hongqi Nanlu Station TBM section, comparing them with theoretical calculations. The research yields the following key conclusions: ① Over a period of more than 240 days, surface settlement and deformation were monitored throughout the construction of the TBM section on Tianjin Metro Line 6 between Yizhong Hospital Station and Hongqi Nanlu Station. The results showed that the maximum surface settlement reached 20.55 mm. Notably, all settlement values remained within the allowable deformation limits specified by relevant standards, indicating no significant deformation or excessive subsidence occurred around the TBM tunneling area. This confirms that the TBM model, end-face reinforcement strategy, TBM reception plan, and precise control of tunneling parameters employed in this project are highly effective, providing valuable guidance for future metro construction projects. ② In strata composed of silty sand and sandy silt, effective measures to control surface settlement during TBM excavation include: accurately determining the excavated material volume; conducting trial sections to establish the loose coefficient for silty sand and silty clay, thereby preventing over-excavation; carefully controlling grouting volumes to ensure complete filling of voids around the tunnel segments with slurry; and prioritizing muck conditioning. Field observations revealed that sand喷out can occur when excavating through these strata, underscoring the importance of thorough muck improvement. Laboratory tests have demonstrated that using foam alone or in combination with sodium-based bentonite significantly enhances muck stability and reduces settlement risks. ③ To minimize surface settlement around the TBM tunneling area in Tianjin, the following strategies are recommended: - Ensure high-quality reinforcement of the ground at the launching and receiving shafts to stabilize the end faces. - Reasonably adjust the cutterhead design and opening ratio of the TBM based on Tianjin’s unique geological characteristics. - During excavation through silty sand and sandy silt layers, maintain optimal soil pressure and grouting volumes while emphasizing muck conditioning to effectively manage surface settlement. - Strengthen comprehensive quality control across all aspects of TBM operations, establishing a networked system for real-time monitoring, measurement, and data analysis to guide and optimize construction practices.