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Flowing over the flood Huge concrete pours are a major feature of Mowlem's M4 underbridge construction contract west of London. Dave Parker reports on the project and the traffic diversion needed to k

Thursday 9 July saw a massive 650m3 of high strength concrete delivered to John Mowlem's site on the M4 near Maidenhead in less than 12 hours. Supplied by Ready Mixed Concrete (Transite) the C40 mix was placed and finished by specialist subcontractor Magnum Construction. This was the last of two bridge deck pours - the first had been successfully completed a week earlier - and its completion was a major milestone in a hectic programme.

Main contractor Mowlem is building the bridge to carry the M4 across the flood relief channel forming part of the Maidenhead, Windsor & Eton flood alleviation scheme. It has diverted motorway traffic on to a temporary embankment during bridge construction.

Mowlem won the £2.7M contract mainly because it undertook to complete the works faster than any of its competitors, explains Highways Agency project manager Stuart Brook. 'The maximum permitted time was 305 working days. Mowlem contracted to do it in 268.

'After winning the contract this was cut to 213, and then to 166. At the moment we are running about a week late because of bad weather, but this still means completion by the beginning of September.'

When the new bridge opens at the end of August it will cross a dip in the ground little more than 30m wide. Eventually this dip will be part of the 11.8km, £183M flood relief channel which will link the Thames above Maidenhead to the Thames below Windsor when it is completed at the end of 2001.

The channel has to pass under several obstacles, not least the main Paddington to West Country railway line. But the crossing with the greatest potential for disruption was the M4 between Junctions 7 and 8/9, a stretch notorious for congestion, with flows reaching 120,000 vehicles a day. Traffic management was always the key to the crossing project, as Hyder Consulting resident engineer Ray Postill confirms.

'The main factors were the sheer volumes of traffic, and the relationship between the channel route and Marsh Lane over bridge to the west,' he says. 'All possible geometries were considered, but with plenty of land available to the north we eventually went for a full diversion, a rarely available option. But the proximity of Marsh Lane meant curves had to be tight, making a 50mph limit unavoidable.'

Mowlem adopted Hyder's indicative diversion scheme almost without amendment. A borrow pit was opened on the line of the channel to win the 20,000m3 of Type 1 flood plain gravels needed to construct the 2.5m high temporary embankment. Work started in mid-February and was completed well ahead of programme, but not without problems.

'Mowlem originally hoped to get away without a capping layer to the fill, as the excavated material was so good,' Postill reports. 'But bad weather meant that a capping layer of crushed concrete was needed. Even so, most of the embankment was built without putting extra traffic onto public roads.'

Motorway traffic was fully diverted on to the new embankment by April.

The next challenge facing the contractor was the bridge foundations. Groundwater levels were only 500mm below ground level, and the bridge's two central piers were founded 8m deep. With most of the excavation in gravels overlaying clay, dewatering was never going to be easy.

However, the bridge design included permanent sheet steel pile anti- scour walls each side of the channel. Mowlem opted to extend these to form a central cofferdam, and installed well-points around it.

When pumping began another problem emerged. 'We couldn't get rid of the water fast enough,' says Postill. 'And not only were volumes too high to discharge into ditches, suspended solids were too high as well.

'Eventually the Environment Agency agreed to the construction of a temporary 100m by 50m holding lagoon on the channel route to the south of the bridge, which had already been stripped of topsoil.'

Water depth in the lagoon stabilised at around 1.5m until pumping stopped, after which it took two days for the water to drain away. Subsequent dewatering operations were on a smaller scale and ditch discharge was permitted.

Hyder went for a semi-integral bridge design for the 56m long, 35m wide structure. A central span of 18m is flanked by two 14m spans, with piers monolithic with the 800mm deep deck.

At the abutments the deck sits on elastomeric bearings, and each abutment features an internal walkway to allow easy inspection of the bearings and movement joints from below. These joints are of the asphaltic plug type, allowing 25mm of movement in each direction.

Postill says insitu construction was chosen because of the difficult access below the bridge. Magnum used two concrete pumps supplied by Cardiff- based Ascus to place the structural concrete in the deck. One of these, a Schwing 1200HDR with a 52m boom, is claimed to have the longest reach in Britain.

The pump mix design features a 30% cement replacement by ground granulated blast furnace slag and a Cormix water reducing admixture. Compaction was by BUNION, an air driven cylinder that spins against the direction of travel, screeding, compacting and finishing in one operation.

Temperature-matched curing, where electrodes cast into the pour measure internal temperatures and reproduce them in the test cube curing tank, is used to control falsework stripping times. 'We're air-curing cubes on the deck as well, and doing standard cube tests,' adds Postill.

'The seven day results from the first pour were interesting. Cubes from the deck reached nearly 39N/mm2, and temperature-matched cubes topped 40N/mm2. But standard cubes water-cured at 20C only reached 36N/mm2 on average.'

These results provided adequate assurance that the concrete in the deck had passed the 'three times working stress' limit, allowing falsework to be removed. Similar results are expected from the second pour, keeping the project on target for completion in early September.

Funding for the new bridge comes from the Environment Agency. Both Agencies involved in the project must be delighted by the comparative ease with which a major underbridge will have been slotted under one of the busiest motorways in the country. Later this year work will begin on the crossing of the main railway line, where ground freezing is already under way. Everyone involved will be hoping that project goes as smoothly as the one on the M4.

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