Although many of the major subway systems have significant portions of their track running on the surface, the main sections are run in tunnels. Often these tunnels are run along underneath the city streets – to avoid disturbing buildings – and constructed by the cut and cover technique. This consists simply of digging huge trenches down the middle of main roads which happen to run in the right direction, lining the floor and walls with brick or concrete to stabilize and strengthen them, and then putting girders from wall to wall over the top on which a new road foundation and surface can be laid. This can cause a great deal of road traffic disturbance while it is being done, but it is the cheapest method and often adopted for that reason. The rails and ancillary equipment are laid in the trench after the roof has been restored.
Where street disturbance must be reduced to aminimum an alternative approach is needed. Narrow trenches are dug on either side of the road to the depth of the tunnel. Headings are driven across between the trenches and supports inserted to carry the road, and the ground underneath is dug out to form a tunnel. In areas where the ground is rocky (as with the New York Subway), small charges of explosives are used to break the rock up without causing damage to adjoining buildings.
Special problems exist where the water table is close to the surface and the cuttings may become flooded. In some cases the water can be pumped out. A different approach was adopted during the construction of the subway system in the Dutch city of Rotterdam. Cuttings for the tunnels were dug in the normal manner and allowed to fill with water, with pilings at the sides to hold the ground back. The tunnels were manufactured as precast sections in dry docks and floated into position in the channels, where they were sunk onto piling foundations. The sections were then joined up and the cuttings refilled with earth. The same basic technique was used to carry the line across the Niewe Maas river – the sections were laid in a trench cut in the river bed. Similar methods have been used for a number of other systems, including the Trans-Bay tube of the San Francisco Bay Area Rapid Transit (BART) system.
For deeper lines, construction is carried out by tunneling from access shafts. In rock, which is self-supporting, this is a relatively straightfoward operation, but in softer soils, sand or gravel it is necessary to use a tunneling shield which supports the ground through which it is cutting.