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Trapezoidal tunnel linings were developed for use with tunnel boring machines and have been used successfully with a range of different types of machine. Each ring usually has 6 nominally identical segments without the need for the top and key segments required for expanded linings.
A taper can be provided across the ring width, allowing the tunnel to be driven around a curve. The linings are generally designed for a specific contract and the chosen boring machine. Gaskets are used to provide a water-tight seal.
Acknowledgement: The Concrete Society