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How do brick roofs in tunnels stick?

Question

When engineers built tunnels, they line the roof with bricks. How do they get them to stick up there before the cement dries?
Peter, Gods Oak

Answer

** Definitive **
Name: Marcus, Surbiton
Qualification: Builder
Answer: No matter how big or small an arch is, when you build it out of bricks, you build it over a former – scaffolding with ply in the shape of the arch. That creates the shape and it is supported until it’s dry. It is then built on the ground and hauled up to the roof of the tunnel.

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