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Flying start for Stratford Box

Skanska Cementation Foundations is gearing up for massive diaphragm walling work on the Stratford Box, a vital element in section two of the Channel Tunnel Rail Link.

Construction of the Stratford Box is a key element of the second phase of the Channel Tunnel Rail Link (CTRL). The massive underground structure is for the new Stratford International Station in east London, which will be the gateway to the capital for passengers arriving from continental Europe.

Foundation contractor Cementation Foundations is working with main contractor and parent company Skanska UK Civil Engineering to install the diaphragm walls for the 1.2km long,50m wide and 25m deep excavation on the £105M Contract 230.Design of the structure is by Skanska Technology.

Cementation's contract is valued at about £30M.This is at least as big as the last major piling project of its kind, the Limehouse Link Tunnel in east London, says the firm's head of major projects, Julian Crawley, who worked on the Limehouse job in the early 1990s.

With diaphragm walling starting in May and due to be wrapped up by July next year, Crawley describes the project as a 'fast start, fast-track, all-out' programme. In the initial stages the emphasis will be on early completion of launch chambers for the neighbouring Contract 220 and 240 tunnels. Skanska is building both the tunnels in joint venture contracts worth a further £265M.

Cementation Foundations managing director Mike Putnam believes the firm's recent work on phase one of CTRL at Ashford in Kent (GE, February 2001), helped to win the work.

'While we are always looking at alternative solutions, with so much to do in such a short space of time diaphragm walling is the only real choice here, ' Crawley says.

Bid manager Mike Coles confirms that the varying ground conditions and the fact that tunnels have to be launched from both ends means the box represents a significant geotechnical challenge. The reinforced concrete diaphragm wall is a permanent feature of the station structure and will be 2,434m long, reaching depths of between 22m and 30m, equating to some 56,000m 2ofwall.

Crawley points out that with timing and dimensions on this scale - not to mention a design requirement calling for substantial quantities of 1.2m and 1.5m thick diaphragm wall - the firm cannot afford half-measures when it comes to specifying plant. It is wasting no time in assembling a monster fleet of rigs from the UK, the rest of Europe and Asia.

Plant director John Rooke is busy marshalling resources. A fleet of six rope-operated cranes equipped with hydraulic grabs is being summoned to excavate through the soft upper layers of Thames Gravels, and the mixed clays and silty sands of the Lambeth Group.

Following soft excavation, three Bauer reverse-circulation hydromills will be employed to grind through the dense Thanet Sands below.

Among the specialised gear being imported is a hydro-cutter and reel-handling unit on its way from Hong Kong-based sister firm Gammon Construction. One lowheadroom hydromill rig will work in a headroom of only 10m below high voltage cables at the west end of the box.

As the grabs and hydromills dig down, bentonite will be pumped into the excavation to stabilise the open panels of the box walls.'Over the years various polymers have been tried as alternatives, but bentonite is still the best thixotropic suspension for this work, ' Crawley explains.

Excavation equipment will be served by three desanding units each capable of cleaning and recycling up to 500m 3of bentonite an hour.

The diaphragm wall panels, between 2.6m and 7.4m long, will be cast using 40MN/m 2grade high-slump concrete placed at the bottom of the steel reinforcing cages through 250mm diameter tremmie pipes, displacing the bentonite.Panel volumes will be typically 200-250m 3, with a total volume of concrete of 75,000m 3.Temporary steel stop ends will be used at each construction joint to allow a 150mm water bar to be placed across the joints. Cementation Foundations' Dr Peter Shotton has designed the movement joint to meet the client, Union Railway's, requirement to allow flexibility of the wall when it is in service.

While the centre section of the box is designed as a straight cantilever structure, permanent long props will be built over the new line at each end to support the retaining walls and maintain the integrity of the box.The load-bearing steel columns supporting these props will be plunged into fresh concrete, using Cementation's patented Cemloc system that will ensure accurate positioning.

Seventy 1.5m diameter plunge piles will be installed down to 12m below surface, and with cement/bentonite plastic slurry cut-off walls at either end, at least 20 crawler cranes will be needed to support diaphragm walling and piling operations.

Work for the 1km long east and west bridges, which will carry existing rail services over the box, will be done during two marathon rail blockades - with up to four weeks of 24-hour working on the western side and two weeks on the eastern side.

Crawley says the project marks the continuation of the contractor's partnering philosophy applied on CTRL Section 1. He is convinced that the 'transparency and openness of partnering really works'He adds that Union Railways operates a streamlined management structure and is looking to Cementation Foundations to deliver its own quality control and self-certification to the highest levels.

'Quite apart from its size, what makes this job so challenging is the combination of piling techniques which will be required, ' he says.

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