Underpass construction
Contents[hide] |
[edit] Introduction
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.
[edit] Construction methods
There are three main methods for constructing underpasses:
- Precast concrete units.
- In situ concrete.
- Thrust-bored units.
Precast concrete units are often manufactured as standard units and can be provided to site as complete box-like open-ended sections, portal frame segments, or as separate wall and roof units.
Box units are typically jointed using a pre-formed sealant strip in a socket and spigot joint. Connection plates in the floor and roof are used to bolt together the units.
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.
In situ concrete underpasses are constructed using the same methods as any underground tunneling construction.
[edit] Waterproofing
Waterproofing underpasses is commonly achieved by applying one of the following:
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.
[edit] Lighting and drainage
To try and avoid vandalism occurring, underpass lighting should be made using toughened glass and recessed into the walls or roof units.
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.
Drainage comprises normal falls, screeded floor and gullies or continuous side channels which will flow into a storm sewer as and when required.
[edit] Find out more
[edit] Related articles on Designing Buildings Wiki
Featured articles and news
The increasing costs of repair and remediation
Highlighted by regulator of social housing, as acceleration plan continues.
Free topic guide on mould in buildings
The new TG 26/2024 published by BSRIA.
Greater control for LAs over private rental selective licensing
A brief explanation of changes with the NRLA response.
Practice costs for architectural technologists
Salary standards and working out what you’re worth.
The Health and Safety Executive at 50
And over 200 years of Operational Safety and Health.
Thermal imaging surveys a brief intro
Thermal Imaging of Buildings; a pocket guide BG 72/2017.
Internally insulating a historical building
An experimental DIY approach using mineral thermal lime plaster.
Tree species selection for green infrastructure: A guide for specifiers.
50 million new trees over 25 years.
Art of Building CIOB photographic competition public vote
The last week to vote for a winner until 10 January 2025.
The future of the Grenfell Tower site
Principles, promises, recommendations and a decision expected in February 2025.
20 years of the Chartered Environmentalist
If not now, when?
Journeys in Industrious England
Thomas Baskerville’s expeditions in the 1600s.
Top 25 Building Safety Wiki articles of 2024
Take a look what most people have been reading about.
Life and death at Highgate Cemetery
Balancing burials and tourism.
The 25 most read articles on DB for 2024
Design portion to procurement route and all between.
The act of preservation may sometimes be futile.
Twas the site before Christmas...
A rhyme for the industry and a thankyou to our supporters.
Plumbing and heating systems in schools
New apprentice pay rates coming into effect in the new year
Addressing the impact of recent national minimum wage changes.