The “best” tunnel shape really depends on its intended use and the surrounding conditions. In general, a "circular cross‐section" is often considered optimal for resisting external pressures because its uniform curvature distributes stress evenly, minimizes stress concentrations, and provides the greatest area for a given perimeter. This makes circular tunnels especially popular for water or sewer conduits where internal pressure is a major concern.
However, when the tunnel needs to accommodate vehicular or rail traffic, a flat floor is typically required. In those cases, a D-shaped (or segmental) tunnel—which essentially combines an arch roof with a flat base—tends to be preferred. This design maintains many of the benefits of an arch (efficient load distribution) while providing a level floor that is more practical for traffic.
Analysis of common tunnel shapes and their applications.
1. Circular Tunnels
Circular tunnels are widely used in soft soil or high-water-pressure environments. Their curved design distributes stress evenly, minimizing deformation risks.
Advantages:
① Ideal for underwater tunnels (e.g., the Channel Tunnel between England and France) due to resistance to external pressure.
② Suitable for tunnel boring machines (TBMs), which efficiently excavate circular cross-sections.
Limitations:
Wasted space in the upper sections for transportation tunnels, requiring additional lining adjustments.
2. Horseshoe (Semi-Circular) Tunnels
This shape combines a curved roof and vertical walls, balancing stability and space utilization.
Applications:
① Common in rock tunnels (e.g., Alpine mountain tunnels), where the arch shape transfers load to the surrounding rock.
② Used in metro systems (e.g., New York City Subway) to accommodate rail lines and utilities.
Engineering Considerations:
Requires robust support in fractured rock to prevent collapse.
3. Rectangular or Box Tunnels
Box-shaped tunnels are preferred for shallow urban projects.
Use Cases:
① Cut-and-cover metro tunnels (e.g., London Underground) for efficient use of limited above-ground space.
② Pedestrian underpasses and utility tunnels.
Challenges:
Less effective in high-pressure environments due to stress concentration at corners.
4. Elliptical and Composite Shapes
Hybrid designs address unique geological or functional needs.
Examples:
① The Gotthard Base Tunnel in Switzerland uses a composite shape to handle variable rock stress.
② Seismic zones may employ elliptical designs to absorb ground movement.
5. Key Factors Influencing Shape Selection
① Geology: Hard rock favors horseshoe shapes; soft soil necessitates circular TBMs.
② Hydrology: Water pressure dictates circular or oval profiles.
③ Function: Rail tunnels prioritize clearance; road tunnels require ventilation considerations.
④ Cost: TBMs reduce labor but demand high upfront investment.
The "best" tunnel shape is context-dependent. Modern engineering leverages advanced modeling tools (e.g., finite element analysis) and materials to optimize designs. As noted in structural engineering guidelines, "Simplicity in design often aligns with reliability", but innovation continues to push boundaries in challenging environments.