The shield tunneling method is an advanced tunnel construction method commonly used in urban subways, railways, highways, and underwater tunnels. It utilizes a shield machine (TBM) to perform excavation, support, and lining within the ground, achieving efficient, safe, and automated tunnel construction. The following is a detailed introduction:
I. What is the Shield Tunneling Method?
Simply put, the shield tunneling method uses a large cylindrical steel machine (TBM) to excavate the soil in front of it while assembling prefabricated segments at the rear to form the tunnel lining. Jacks propel the machine forward.
II. Basic Principles of the Shield Tunneling Method
The shield tunneling method is a mechanized construction method that uses a shield machine (TBM) to advance underground, excavating the soil, supporting the surrounding rock, and assembling lining segments to form the tunnel structure.
The shield machine has a cutterhead at the front to cut the soil; a shield in the middle supports soil pressure and prevents ground collapse; and at the rear, concrete segments are assembled to form the tunnel structure.
III. Advantages of Shield Construction
1. High Safety: The shield machine has a built-in steel casing (shield) that supports the surrounding soil while excavating, effectively preventing landslides.
2. Low Environmental Impact: Construction is primarily carried out underground, where ground subsidence is manageable, making it suitable for operations beneath dense urban buildings and major transportation routes.
3. High Degree of Automation: Key processes such as excavation, mucking, and segment assembly are largely automated, reducing manual intervention and improving construction efficiency.
4. High Tunnel Quality: The prefabricated segments are highly precise, the lining quality is stable, and the waterproofing is excellent.
IV. Disadvantages of Shield Construction:
1. High Initial Investment: The design, manufacturing, and procurement costs of the shield machine are extremely high.
2. Limited Adaptability: The tunnel is less adaptable to complex geological conditions (such as isolated boulders and high-strength rock formations), requiring auxiliary construction methods.
3. Curve Radius Restrictions: Sharp turns are impossible, requiring a larger curve radius.
4. Single cross-sectional shape: Typically, only circular cross-sections can be excavated.
V. Basic Construction Process of the Shield Tunneling Method
1. Shield Launch: The shield machine is assembled in a pre-constructed "launch shaft." After commissioning, the cutterhead begins cutting soil, gradually advancing underground.
2. Tunnel Excavation and Support: The excavated soil from the cutterhead is transported to the surface via a conveyor belt (discharging). Simultaneously, the assembly mechanism at the rear of the shield machine joins the precast concrete segments to form the tunnel lining, creating a permanent support structure.
3. Synchronous Grouting: After the segments are assembled, grout is injected into the gaps between the segments and the soil to fill the gaps, control ground subsidence, and enhance tunnel stability.
4. Shield Receiving: When the shield machine reaches the pre-determined "receiving shaft," tunnel breakthrough is complete. The shield machine is then dismantled and hoisted out.
VI. Main Application Scenarios of the Shield Tunneling Method
① Ground Conditions: Suitable for soft soil (such as clay, silt, and sand) and soft rock formations, and particularly for complex geological conditions where mining methods (blasting excavation) are unsuitable.
② Project Types: Widely used in urban subway tunnels, river/sea crossing tunnels, highway/railway tunnels, and municipal underground utility corridors.
The shield tunneling method is a mature, safe, and efficient underground space development technology. By constructing the tunnel structure simultaneously during excavation, it perfectly solves the global challenge of tunnel construction in soft, unstable formations. With the development of technology, new equipment such as dual-mode shield machines and rectangular shield machines have emerged, continuously expanding its application areas and making it an indispensable core method for modern urban underground engineering construction.