Cut and cover

Cut-and-cover is a construction method for shallow tunnels, culverts, and immersed-approach structures in which a trench is excavated from the surface, the structure is built inside it, and the excavation is then backfilled and the surface reinstated. In port and waterway works it builds road and rail underpasses beneath quays, service culverts, and the land approaches to immersed-tube tunnels. The two variants are bottom-up, where the structure is cast in an open braced excavation, and top-down, where the roof slab is cast first on diaphragm walls and excavation proceeds beneath it. Temporary support uses sheet piles, secant piles, or diaphragm walls, often inside a cofferdam where the work is below water level.