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Tunnel shapes: advantages and disadvantages
Short answer
Circular: best for pressure, least wetted perimeter; curved floor hinders construction. Horseshoe: good arch, easier invert, not for high pressure. D‑shaped: simple flat floor, poor in weak rock. Rectangular: cut‑and‑cover only, high moments. Egg‑shaped: good self‑cleaning but complex.
Describe the advantages and disadvantages of different tunnel shapes based on geometry with neat sketches.
The shape of the tunnel cross-section is chosen according to the rock quality, the internal pressure, the construction method and the hydraulic efficiency.
Circular Horseshoe D-shaped Rectangular
___ ___ ______ _______
/ \ / \ / \ | |
| | | | | | | |
\___/ | | |________| |_______|
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| Shape | Advantages | Disadvantages |
|---|---|---|
| Circular | Best for pressure flow; resists internal and external pressure evenly; least wetted perimeter for a given area (hydraulically best); suited to TBM; minimum lining thickness | Difficult to excavate by drill and blast; the curved floor hinders the movement of vehicles, so a flat fill or extra excavation is needed during construction |
| Horseshoe (standard/modified) | Good arch action in the roof and sides; suits free-flow and low-pressure tunnels; the flatter invert makes construction and the movement of trucks easier; most common for drill and blast | Not suitable for high internal pressure; higher stresses at the corners; hydraulically slightly less efficient than circular |
| D-shaped (arched roof, flat floor) | Easy to construct and good for moving machines; suitable in good rock; economical for non-pressure tunnels | Poor in weak rock and under pressure flow; stress concentration at the corners |
| Rectangular (box) | Easy construction (cut and cover) with simple formwork; used in shallow, low-pressure sections and canals | Poor structural behaviour; high bending moments and a thick lining; not suited to deep tunnels |
| Egg-shaped / oval | Good self-cleaning at low flows; good for varying discharge | Complex construction; rarely used for hydropower |
✓ Answer checked by Claude Opus 5.5 (high effort)
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