Tunnels can be built using several methods, and one technique is the bottom-up method, also known as cut-and-cover.
Understanding the Bottom-Up Tunnel Construction
The bottom-up method is a straightforward approach often used for shallow tunnels, such as those for metro lines or underpasses, particularly in urban areas where the ground surface can be temporarily disturbed.
According to the reference, this method involves specific steps:
Key Steps in the Bottom-Up Method
- Excavation: The process begins by excavating a trench from the ground surface. This creates an open cut where the tunnel will be built.
- As the trench is dug, ground support may be necessary to prevent the trench walls from collapsing, especially in unstable soil conditions.
- Tunnel Construction: Once the trench is open, the tunnel structure itself is built inside.
- Various materials can be used for construction, including:
- In situ concrete (poured directly on site)
- Precast concrete sections
- Precast arches
- Corrugated steel arches
- In historical tunnel construction, brickwork was commonly used.
- Various materials can be used for construction, including:
- Backfilling: After the tunnel structure is completed, the trench around and over the tunnel is carefully back-filled with soil or other suitable material.
- Surface Reinstatement: Finally, the ground surface above the tunnel is restored to its original condition, whether it's a road, park, or building area.
This method essentially involves digging a ditch, building the tunnel within it, and then covering it back up. It contrasts with top-down methods or boring methods where the tunnel is constructed underground without significant surface excavation along its entire length.