The cut and cover method is a simple method of construction for shallow tunnels where a trench is excavated and roofed over with an overhead support system. This system is strong enough to carry the load of what is to be built above the tunnel.
There are two basic forms of cut-an-cover tunneling:
1. Bottom-up Method
In the bottom-up method a trench is excavated, with ground support as necessary, and the tunnel is constructed in it. This tunnel can be formed from a variety of materials including; in situ concrete, precast concrete, precast arches, corrugated steel arches or brickwork. The trench is then back-filled and the surface is reestablished.
2. Top-Down Method
In this method side support walls and capping beams are constructed from ground level. With a shallow excavation, tunnel roofs of precast beams or in situ concrete are made and then the surface is reinstated (except for access openings). This allows for the early replacement of surface features such as roads and services. Excavation is then continued under the permanent tunnel roof and the base slab is constructed.
Some underground subway stations, such as Canary Wharf tube station, are made using large cut-and-cover boxes. This form of construction has two levels to allow for the separation of programs. For example this allows the economical arrangement for ticket hall, station platforms, passenger access and emergency aggress, ventilation and smoke control, staff rooms and equipment rooms. The interior of Canary Wharf station has been described as an underground cathedral as a result of the sheer size of the excavation. Most traditional stations on the London Underground used bored tunnels for stations and passenger access.