The difference between hard rock tunnel boring machine and shield machine
Hard rock tunnel boring machines (TBM) and shield machines are both heavy machinery used for underground tunneling, but they differ significantly in design, applicable environments, and operating principles. Understanding these differences is crucial for selecting the appropriate tunneling equipment.
1. Hard Rock Tunnel Boring Machine (TBM)
Hard rock tunnel boring machines, particularly full-face TBMs, are specifically designed for excavating tunnels through hard rock. Key features include:
- Cutterhead Design: The cutterhead of a hard rock TBM is equipped with tungsten carbide or diamond-tipped cutting tools to break rock. The cutterhead is usually disc-shaped with a large diameter to facilitate rock cutting.
- Applicable Environment: TBMs are ideal for stable geological conditions with high rock strength. In such conditions, they can efficiently and continuously tunnel through rock, producing debris that is easy to manage.
- Excavation Speed: Under suitable geological conditions, TBMs offer high excavation speed and efficiency.
2. Shield Machine
Shield machines, particularly earth pressure balance (EPB) or slurry shield machines, are mainly used for tunneling through soil, mud, and softer rock layers. Key features include:
- Cutterhead and Cutting System: The cutterhead of a shield machine is different from that of a TBM. It is typically equipped with scrapers, disc cutters, and mixing devices to adapt to softer geological conditions. Behind the cutterhead, the shield body provides support to the excavation face to prevent soil collapse.
- Applicable Environment: Shield machines are used in soil, mud, softer rock, or unstable geological conditions. They effectively support the excavation face to prevent tunnel collapse in these environments.
- Excavation Method: As the shield machine excavates, the cut soil or rock debris is discharged through a screw conveyor while the machine pushes forward, forming the tunnel.
3. Key Differences
- Operating Principle: TBMs usually use disc cutters or cutting tools to break rock by pushing the tools against the rock surface with hydraulic cylinders. Shield machines, on the other hand, excavate using a rotating cutterhead and shield shell, cutting soil or rock and discharging it through a spoil removal system.
- Design and Structure: TBM cutting tools are more robust, suited for rock breaking, while shield machines have a supporting shield body suitable for unstable geological conditions.
- Applicable Geology: TBMs are suited for hard rock, while shield machines are designed for soil and softer rock layers.
- Applicable Projects: TBMs are used in hard rock projects such as mining, railway, and highway tunnel excavation, while shield machines are used for projects in soft soil or mixed strata, such as urban subway and underground utility tunnels.
- Construction Method: TBMs typically use the drill-and-blast or mechanical excavation method, requiring drilling, blasting, or mechanical work inside the tunnel. Shield machines usually use the shield method, which involves assembling and testing the shield machine above ground, then advancing it into the tunnel for excavation and lining work.
- Excavation Efficiency: In favorable geological conditions, TBMs typically have faster excavation speeds than shield machines.
- Construction Costs: TBMs are generally used for long-distance, large-diameter tunnel excavation, offering higher efficiency but at higher costs. Shield machines are more suitable for short-distance, small-diameter tunnel excavation, with lower efficiency but also lower costs.
The choice between a TBM and a shield machine depends on the specific geological conditions of the tunnel, tunnel design requirements, project cost, and time constraints.