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A40, White City to Paddington Green, London, UK
associated engineer
G Maunsell & Partners
date  29th September 1966 - July 1970
UK era  Modern  |  category  Road Viaduct  |  reference  TQ234811
ICE reference number  HEW 2333
Westway was Europe’s longest elevated concrete roadway when it opened. It carries the A40 trunk road, which was formerly a motorway, from west to central London. Both prestressing and post-tensioning techniques were used in the structure, which was built to ease traffic congestion in Shepherd’s Bush. It remains in constant use.
Westway is 4km long and runs between the A40 Western Avenue at Wood Lane and the Harrow Road Flyover, designed by Sir Bruce White & Partners. Its elevated roundabout at White City forms an intersection with the A3220 West Cross Route. The enabling Act of Parliament was passed on 29th October 1962, site clearance began in 1964 and construction began at the west end on 29th September 1966. The work was divided into four sections, each with expansion joints at either end, and all including landscaping and tree planting.
The first 1.19km section finished at the St Mark’s Road abutment. It encompassed the roundabout with slip roads and flyover, plus a skew bridge over London Underground's Central Line track, a subway and a pair of slip roads. The 670m long second part ran from St Mark’s Road to Acklam Road, and included a service road from St Mark’s Road to Ladbroke Grove.
The third 1.16km section ran between Acklam Road to near Torquay Street. It crosses the main line railway near Westbourne Park Station using a continuous spine beam with steel comb bridge joints at either end, and included reinstatement of the railway land beneath the flyover.
The last section was 975m long. It had ground level connections via slip roads to the Ranelagh and Westbourne Terrace bridges, new eastbound and westbound stretches of Harrow Road, a double roundabout and three subways. Some of the works pass over Paddington Goods Depot and part of the Grand Union Canal.
Alignment criteria were: min. radius 460m, min. gradient 1 in 200, max. gradient 1 in 25 on main ramps and 1 in 20 on slip roads (though the underpass at Westbourne Terrace Bridge is 1 in 15). The designed-for speed for the main route was 80kph, with slip roads 48kph, and operating speed limit 97kph.
West of the White City roundabout and east of Porchester Road the flyover is dual two lanes, overall width 11m. The super-elevated highway over the roundabout is also dual two lanes, width 7.3m. Between Bramley Road and Porchester Road the road widens to dual three lanes, overall width 12.5m. Hard shoulders were constructed where clearance permitted — a design change added at tender stage. Span lengths vary. Where the width is constant and roadway parallel to the ground, it is 15.2m. Elsewhere, spans increase to 62m.
All superstructures, including abutments, are carried on large diameter cylindrical columns. They have concealed drainage and services, similar safety barriers and lighting, and all decks were covered with a waterproof membrane, a hot-rolled base and wearing courses of asphalt.
The whole scheme is underlain by firm to very stiff London clay, which led to foundations 1.1m in diameter above piled bases 3.2m in diameter carrying loads of 386-457 tonnes. The diameter was increased to 1.2m for foundations that carry loads up to 610 tonnes.
Wherever possible, precast concrete segments were used for the main roadway, prestressed span by span (a method pioneered on the Hammersmith Flyover, 1962). A set of torsion boxes 1.5m deep with cantilever units on each side was developed — the wider boxes had internal spines. Over the canal, the box girder increases to 1.8m deep. Transitions between carriageway widths, connecting deck slabs and diaphragms were cast in situ.
The elevated roundabout was also cast in situ. It uses a continuous spine box ring girder, internal diameter 121.9m, with the precast box girders of the slip roads attached to it at the appropriate places by structural cantilevers. The cellular ring girder has internal diaphragms over columns and was built in seven sequential concreting and prestressing operations, working clockwise. The roundabout can move radially, forming a proving ring to absorb the longitudinal forces in the flyover.
All slip roads have single cell box girders 1.5m deep with cantilever slabs and a single bearing at each column. Except for the pair of continuous two span bridges west of Wood Lane, all were erected using the span-by-span method and then post-tensioned longitudinally.
The Acklam Road to Torquay Street section is carried on 62m spans, each comprising 26 precast units and weighing up to 132 tonnes. To place each span accurately — total dead load 3,040 tonnes — it was supported on 2m deep temporary girders and trestles and lowered into position using sand jacks.
Between Westbourne Terrance and Porchester Road the two eastbound carriageways of Harrow Road lie north of Westway, while the two westbound carriageways run beneath, between its 'legs'. Both highways form a double-decked structure over railway maintenance facilities. The whole is carried by 15.2m span ‘top hat’ shear connected box beams supported by reinforced concrete portal frames, with additional steel portals at alternate spans.
The Police would not permit Westway to open until their traffic control and surveillance system was operational. Shortly before the planned date the system’s purpose-made coaxial cable was stolen. It was recovered but held as evidence. Eventually, the Assistant Commissioner of Police allowed detectives to keep 600mm of it and released the rest. The flyover opened in July 1970, with minor works completed a year later, by which time 47,000 vehicles were using the route daily. The project cost £36.5m, 25% from a government grant.
The project was let as a single contract, though sub-contracts for road heating, lighting, subway finishes, railings, signing, road markings, and traffic control were awarded later. Carriageways, ramps and the elevated roundabout — a total area of 35,285 sq m — are heated by 50V metal mats embedded in the surface.
Box beams were a common form of construction in the 1960s and 70s and offer instant access to the deck once in position. Though still popular with railway companies, they are used less by highway agencies as it is not possible to inspect their interior surfaces for corrosion.
The flyover cut a large swathe through north Kensington, and hundreds of houses were demolished. In particular it passes, very close to Acklam Road, overlooking many residents’ windows. At the time, compensation was not available to people living outside the highway limits, even though they may have been affected adversely (legislation changed in 1973) but Greater London Council purchased or soundproofed the affected Acklam Road properties.
Westway was demoted from motorway status in May 2000 following the formation of Transport for London, the organisation now responsible for its upkeep. It was originally part of a scheme for three concentric highways around London, an idea that was abandoned in the 1970s.
Over the years, the local area around the flyover deteriorated and was felt to be threatening and depressing. In September 2000 a pilot scheme of lighting, cleaning and painting began on the underside near Portobello Road. Maunsell Ltd’s maintenance and repair team provided funding (along with the government and the Highways Agency), advice and supervision.
Architect: William Holford & Partners
Project manager: Douglas Stanley Elbourne
Engineer: John Baxter
Engineer: David Lee
Contractor John Laing Construction Ltd
Research: ECPK
"Westway elevated roundabout" in Engineering,
Vol.205, pp.474-475, London, March 1968
"Discussion of Papers 7435 and 7469: Western Avenue Extension (Westway)" by J.W. Baxter et al, in ICE Proceedings, Vol.54, Issue 1, pp.111-113, London, February 1973
"The precast concrete bridge beam: the first 50 years" by Howard P.J. Taylor,
in The Structural Engineer, Vol.76, Issue 21, London, November 1998
"The Westway Project", press release, Maunsell Ltd, Beckenham, 2000, available at www.urbaneye.org.uk
reference sources   CEH London