Tunnel
Top-down method: Side support walls and capping beams are made from ground level by such methods as slurry walling or neighbouring bored piling. Only a hollowed out excavation is needed to construct the tunnel roof using beams or in situ concrete sitting on the walls. The surface is then remade except for access openings. This allows early recreation of roadways, services, and other surface features. Excavation then takes place under the permanent tunnel roof, and the base slab is made.