- Project plans
- Project activities
- Legislation and standards
- Industry context
Last edited 27 Sep 2017
An underpass, or subway, is a tunnel containing a road or pedestrian passageway running underneath a road or railway. Underpasses can also be constructed to allow wildlife to pass safely under a transport corridor.
There are three main methods for constructing underpasses:
Portal frame units, which are pre-stressed, require the lower waterproofing membrane to be placed on a concrete slab, with continuous concrete bearing pads (usually 300 mm wide x 25 mm deep) are laid on top. The units are then placed in position, with lubrication applied to reduce stress-induced friction.
Wall and roof unit systems comprise precast units that are placed in position with the floor laid in situ using the units as shuttering. The roof units are then placed and the in situ loading slab poured, with loading requirements determining the thickness.
Thrust-bored units require a suspension of bentonite as lubrication. As a means of transmitting the thrust load, the units must have direct edge contact rather than the pre-formed sealing strip. This jointing method should allow edge contact for jacking, but be capable of receiving a sealing compound from the inner face. This is possible by forming a rebated joint filled with mortar prior to applying the sealant.
Waterproofing underpasses is commonly achieved by applying one of the following:
- Mastic asphalt.
- Bituminous or neoprene sheeting.
- Painted membrane.
Two coats of mastic asphalt should be laid, with joints staggered at least 150 mm. Bituminous sheeting should be applied in 2-3 layers, with hot bitumen fully bedding each layer. A self-adhesive bituminous polythene can be used and have the advantage of being easier and quicker to lie in place.
Once in place, the waterproof membrane must be protected by constructing a concrete-block skin against the membrane before backfilling with granular material. This helps to avoid the membrane being punctured.
The locality and use of the underpass will determine the finishes. Urban underpasses are often painted in bright colours or to suit the finish of its immediate surroundings, using mosaics, tiling and rendering. For underpasses where aesthetics are not as important, a plain concrete finish will often be chosen.
 Find out more
 Related articles on Designing Buildings Wiki
Featured articles and news
Do you know your Rococo from your De Stijl, your Gothic from your Post-modernist?
May outlines a new funding strategy for housing associations and says the 'stigma' of social housing needs to end.
RIBA launches a consultation on a new Plan of Work for Fire Safety.
This article offers some basic rules to follow when writing your next specification.
The iconic Mackintosh Building will definitely be rebuilt, board chairwoman confirms.
The machinery used to fashion stone has changed dramatically - and so have the products.
This type of pile provides support to the building, as well as acting as a heat source and a heat sink.
Why investors are adopting the SDGs and why civil engineering could be crucial for delivering them.
Read about all the winners from the London ceremony of CIAT's 2018 Architectural Technology Awards.
How do you find the right stone to conserve historic buildings?
Appointment agreements often include a ‘scope of services’ setting out the consultant's performance on a project.