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Missing link

The A12 Hackney to M11 link road should soon bring to an end almost a century of traffic jams in the East End. Matthew Jones reports to start our highway management special feature. Photographs by David Jones.

Slithering its way through a narrow corridor bordered by railway lines, ancient forest and residential streets, the A12 Hackney to M11 link road is approaching the end of an odyssey.

Its history must rank among the most chequered of any piece of infrastructure built in Britain. A new road along similar lines was first proposed as far back as 1903 by the Royal Commission on London Traffic, but it wasn't until September 1961 that the first public local inquiry was held.

That scheme was dropped following construction of the M11, and would have to wait a further 32 years and pass the test of three public inquiries, a Parliamentary Bill and a High Court challenge to be finally given the go ahead .

WS Atkins project director John Heather has been with the scheme since 1973. He remembers with frustration the further aggravation brought to the project by squatters and the arrival of road protesters from Twyford Down in 1993.

'They clearly had an agenda to cause delay to the scheme but the number of locals who were protesting was actually only a small minority,' he says.

Sure enough, the project was delayed and is now about 12 months behind schedule. But ironically it has been the politics of the last Government, not the tree hugging eco-warriors, that have caused the final hold up.

All four of the 6.4km road's contracts were originally programmed to finish together. But perversely contracts one and three - both vital interchanges - were put on hold to reduce government spending. The near-complete contract two is now having to be gated off for a year to prevent speed-hungry youths racing motorbikes on the dual three lane carriageway.

The delays combined with technical difficulties on the tunnel sections have taken their toll. Last April the Government confirmed that the link is running 97M over budget - more than 40% of the original contract value.

Yet despite all the problems there is much to be celebrated on the scheme. Much of the road runs in a cutting to minimise the impact on the community, or is bordered by carefully designed noise reduction walls. Finishes to the structures are attractive, and wherever possible spare land has been planted or put to good use for the community.

But most importantly the road should have completed its aim - to improve dramatically the traffic-clogged residential streets through Wanstead and Leyton.

Contract 1

The last girders were lifted into place this week for the 173m long Lea viaduct which forms the last stitch to the A102M at the western end of the link.

Joint venture contractor Fitzpatrick/Sir Robert McAlpine managed to build the structure without getting its feet wet - despite the fact that one of its reinforced concrete piers is now in the middle of the river.

'The river is on a bend here so we were able to build one pier on the western bank before straightening the course out and building the other pier on the newly formed eastern bank,' explains commercial manager Malcolm Fisher.

An innovative technique was also used to construct the approaches to the viaduct. Rather than replacing the poor ground with imported material the contractor opted to for a system of ground improvement. Concrete columns were installed through the sub-strata and a mattress of imported material interlaced with geogrid laid over the top.

'That saved us excavating 50,000m3 of material and meant we didn't have to add to the congestion on the roads,' says Fisher.

Contract 2

While most of us were partying, WS Atkins resident engineer Peter Mann and Kvaerner Construction roads agent Leo Martin spent their New Year's Eve installing a new bridge over the link road in Leyton.

To avoid disrupting traffic during normal working time, the two span 6,000t structure was built 40m off line and then pushed into place with hydraulic jacks.

'The Christmas week was the only one that the local authority would allow us to do it in,' says Martin.

The contract was awarded substantial completion last week, and has been one of the least troublesome parts of the project.

Kvaerner made things easier for itself by proposing an alternative form of construction for the retaining walls using a mixture of hard and soft piles instead of interlocking hard-hard piles. The option also shaved 2.5M off the scheme value.

Contract 3

Pedestrians, equestrians, cyclists and cows posed one of the biggest challenges for designer WS Atkins at the Green Man interchange.

The roundabout is bordered by Epping Forest and Wanstead Flats and is a popular local leisure area. It is also used for grazing cattle, which are allowed to roam free in summer under an ancient bylaw.

To get around this, WS Atkins incorporated cattle grids into the sliproads and designed special bridges and subways to allow users of the forest to cross the interchange.

Further down the route. the link road passes into a 175m long twin tunnel. The reinforced concrete cut and cover structure was value engineered by joint venture contractor Norwest Holst/Nuttall after the contract award to remove the need for drainage beneath the tunnel base.

'We built the tunnel as a continuous box instead of having joints which reduced the need for drainage and enabled us to reduce the tonnage of reinforcement,' says project manager Andrew Verness.

Contract 4

Planning construction of the George Green tunnel took up to two years, and it is not hard to see why. The 300m long twin-cell structure runs just 3m above the Underground's Central Line for much of its length, and avoiding ground movements during construction was paramount.

'When you excavate above a Tube line, the ground beneath it tends to heave and that to the sides tends to move inwards,' says WS Atkins project director John Heather.

To get around this a controlled rate of excavation was specified and, unusually, the tunnel was constructed by cover and cut.

'We installed the secant piles which form the walls then cast the roof slab straight on to formwork laid on the ground,' says Norwest Holst contracts manager Chris Joselin. This allowed traffic to be diverted on to the roof of the tunnel during construction.

Once the roof was in place, the tunnel face was excavated in two phases, taking it first down half way and installing a tubular steel prop to stop the piles toeing inwards, and then continuing down to base level.

NEW CIVIL ENGINEER 12 NOVEMBER 1998

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