General overview on tunnel boring machines
Tunnel boring machines (TBMs) are large, specialized machines used to excavate tunnels through a variety of geologies, including rock, soil, and mixed ground. These machines are widely employed in civil engineering projects, such as the construction of subway systems, railway tunnels, highways, and utility tunnels for water and sewer systems.
Key Components of TBMs
1. Cutterhead: The front part of the TBM, equipped with cutting tools like disc cutters (for hard rock) or teeth (for softer ground). The cutterhead rotates to break up the ground, and its design depends on the type of soil or rock being excavated.
2. Shield: A cylindrical structure that surrounds the machine and protects the workers and equipment during the excavation process. It also helps support the tunnel walls during construction.
3. Conveyance System: This system removes the excavated material (called muck) from the tunnel, usually through conveyor belts, screw conveyors, or rail systems.
4. Gripper and Propulsion Systems: TBMs are equipped with hydraulic grippers or legs that press against the tunnel walls to provide forward thrust, allowing the machine to advance incrementally.
5. Support System: As the TBM advances, it may install tunnel lining segments to provide permanent structural support to the tunnel walls.
Types of Tunnel Boring Machines
1. Earth Pressure Balance Machines (EPB): These are used in soft, loose soils. They control the pressure at the tunnel face by using the excavated material to support the front of the machine, preventing the collapse of the tunnel during excavation.
2. Slurry Shield Machines: Suitable for water-logged or unstable soil conditions, these TBMs inject a slurry (a mixture of water and bentonite) into the cutting chamber to balance the pressure and support the tunnel face.
3. Hard Rock TBMs: These machines are used for boring through solid rock. They employ disc cutters that grind and break the rock, and are typically used in projects like mountain tunnels or underground metro systems.
4. Hybrid TBMs: Capable of working in both rock and soil conditions, hybrid TBMs combine features from different types of machines, making them versatile for mixed ground environments.
Advantages of TBMs
- Efficiency: TBMs can continuously excavate and support tunnels, making them faster and more efficient than traditional methods like drill-and-blast.
- Safety: They provide a safer working environment by shielding workers from unstable ground, water ingress, and other hazards.
- Precision: TBMs create smooth, uniform tunnel walls, reducing the need for additional support or finishing work.
Challenges
- Cost: TBMs are expensive to manufacture, transport, and assemble, making them suitable only for large-scale projects.
- Limited Flexibility: Once the TBM starts boring, it’s difficult to change its direction or adapt to unexpected conditions without significant modifications.
Applications
- Subway systems: Cities like London, New York, and Shanghai have used TBMs to construct expansive underground rail networks.
- Hydroelectric projects: TBMs are used to bore tunnels for water conveyance in dam projects.
- Road tunnels: TBMs help create tunnels under mountains or rivers for highways and expressways.
Tunnel boring machines have revolutionized the way tunnels are built, offering a safe, efficient, and precise method for underground construction projects across the globe.