What is the difference between single shield TBM and double shield TBM? Tunnel Boring Machines (TBMs) are critical for efficient tunneling in challenging geological conditions. Among their variants, single shield and double shield TBMs serve distinct purposes based on their design and operational mechanisms. The following is a detailed comparison of the main differences between the single shield TBM and the double shield TBM:
1. Structural Design
Single Shield TBM:
① Features one protective shield covering the entire machine, allowing workers to install precast concrete segments (lining) within the shielded area while tunneling proceeds.
② Relies on thrust cylinders that push against the installed lining segments to advance, requiring intermittent pauses for segment installation.
Double Shield TBM:
① Equipped with two shields: a front shield (for excavation and cutterhead support) and a rear shield (for segment installation).
② Uses gripper shoes to anchor against the tunnel walls, enabling simultaneous excavation and segment installation without stopping.
2. Operational Workflow
Single Shield:
① Sequential process: Excavation → Thrust using installed segments → Pause for segment installation → Repeat.
② Suited for unstable or fractured rock where immediate lining is necessary to prevent collapse.
Double Shield:
① Continuous process: Gripper shoes anchor the machine to the tunnel walls, allowing the cutterhead to advance while the rear shield installs segments.
② Ideal for mixed geology (stable and unstable zones) due to its ability to switch between gripper and thrust modes.
3. Geological Adaptability
Single Shield:
① Best for soft ground, highly fractured rock, or areas requiring frequent segmental support.
② Struggles in hard rock or large fault zones due to reliance on segment thrusting.
Double Shield:
① Excels in hard rock and heterogeneous formations. Its gripper system provides stability in solid rock, while the rear shield ensures continuous lining in weaker zones.
② Example: Achieved a record 608 meters/month in hard rock at a coal mine in China.
4. Efficiency and Speed
Single Shield:
Lower efficiency due to frequent pauses; typical progress ranges from 200–400 meters/month.
Double Shield:
Higher efficiency with uninterrupted tunneling; speeds can exceed 500 meters/month in optimal conditions.
5. Key Advantages and Limitations
Single Shield:
① Pros: Simplified maintenance, effective in unstable ground.
② Cons: Slower progress, segment-dependent thrusting.
Double Shield:
① Pros: Continuous operation, adaptability to varied geology.
② Cons: Complex mechanics, higher upfront cost.
The choice between single and double shield TBMs hinges on geological conditions and project requirements. Single shield machines prioritize safety in fragile rock, while double shield models maximize efficiency in mixed or hard rock environments.