When working under pneumatic pressure, it is possible to create a dry closed off workarea below the water. The waterlevel is temporarily lowered using a compressed air system. Depending on the scale of the project and the available space, (electrical) machines may be used. Materials and staff access and leave the workspace through separate air locks. This technology allows underground excavation and pouring of concrete floors. After the floor has hardened, the overpressure can be removed and finishing work performed.
This technology is for instance used a wall-roof method, in which the construction pit walls and roof are built first and subsequent work takes place below ground level under air pressure to prevent water ingress. In certain situations, a floor is constructed first in the dry below ground level, with further work carried out below that floor under air pressure. For deep underground car parks, this method enables both top-down and bottom-up work from ground level.
This technology was used on a large scale for the Tramtunnel in The Hague, where traditional construction was not possible due to the problems created by incoming groundwater. This technology is also used for the deeper subwaystations (up to 30 metres below ground level!) of the Noord-Zuidlijn in Amsterdam.