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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.
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