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Can do canoe

Building the Olympic-standard Cardiff Canoe Slalom Centre presented the project team with a concrete forming job with a difference.

An Olympic-standard slalom canoeing course requires hundreds of jagged edges, undulations and inserts in the concrete course to replicate the bubbling turbulence of a real white water environment. Sophisticated hydraulic modelling by client Hydrostadium and consultant Hyder Consulting was used to design a concrete channel with curves and bumps set out to the last millimetre to create the swells and surges of water to test the elite canoeists.

This made no ordinary construction contract for Dean and Dyball, which is building the 254m long slalom channel on disused dockland within the Cardiff Bay Barrage under a £12.4M design and build contract.

When NCE visited the site in early March, six months into a 68 week construction contract, work was continuing apace on forming the U-shaped concrete channel, which will be typically 13m wide. The surface of the concrete base slab has about 30 recessed holes of different sizes – like jagged shapes cut out of a layer of pastry.

Into these recessed areas plastic inserts, known as caillebotis, will be cast. These are the fixings onto which omni-floats can be secured. The omni-floats resist the flow of water and create the extra turbulence the canoeists require.

“The client and its designer have spent so much time doing the hydraulic modelling that the setting out of these caillebotis units has got to be absolutely spot on,” says Dean and Dyball project manager Greg Phillips. Once the 270mm thick base slab of the slalom course is in place, a 200mm thick reinforced concrete wall is formed at a right angle and is buttressed behind with block work and infill.

The curving U-shape and other undulations in the wall will be formed by a 200mm layer of sprayed concrete that has been traditionally used to stabilise rock faces. The spray concrete is reinforced with plastic fibres, rather than wire or mesh, to achieve an even smoother finish. The random curvature of the walls adds to the difficulty of negotiating the course. As a result, individual coordinates for the concrete walls had to be set out every 500mm.

“Forming the walls takes a lot longer than it normally does,” says Phillips. “Usually we would be setting out the coordinates every 10m.” Forming the steel reinforced concrete base slab is also a far more intricate process than is usual. The course drops in level by 4.5m over its length, with an average gradient of 1.5%. However, the level of the surface jags up and down by between 10mm and 15mm every 2m to add to the turbulence. This made the screeding of the concrete surface very intricate and time consuming. “You would usually form the surface of a concrete slab with a screed rail every 20m but here we were screeding it every 3m to 4m,” says Phillips.

All in all, 2,500m3 of concrete was required, using a standard C25 mix to form the course that winds up and down a rectangular plot of land that once housed Europe’s largest coal dock, and required substantial decontamination by client Cardiff City Council after it acquired the land in 2000.

Remediation of the land was completed in 2005 and the slalom course will form the latest addition to the Cardiff International Sports Village, which already boasts an Olympicstandard swimming pool. The course includes a concrete chamber containing four pumps that will keep the water flowing through the slalom at 4m3/s.

Work on the concrete chamber started after completion of a cofferdam with a combi-system steel tubular and sheet pile retaining wall. Having started on site last September with enabling works including sewer diversions, Dean & Dyball began excavating the slalom course in December, removing the silts and clays to enable it to cast the base slab on top of a 200mm stone aggregate foundation and a 75mm binding layer. Excavated earth has been retained for other projects at the Sports Village site.

Most of the concrete for the basement slab and concrete wall has been poured, but there are still missing pieces of the jigsaw. This is because of the need to maintain access for plant to move off the site once its work is done. The largest piece of plant is a 165t crawler crane out on a floating pontoon in the bay, which is driving the 200m long combi-system piles that will form a retaining water pool next to the course. The combi-system comprises 1.6m diameter steel tubular piles and sheet piles, driven a maximum depth of 20m into silt, clay and mudstone.

You would usually form the concrete slab surface with a screed rail every 20m. Here we were screeding it every 3m to 4m

Greg Phillips, Dean & Dyball

The large tubular piles help achieve the stability needed without ground anchors, for which there was no room because of the concrete course. Once the retaining structures in the water are completed at the end of this month, the rig will be dragged back from the pontoon and through the site, and work can start on filling in the missing links in the course. This is typical of some of the difficulties encountered in keeping access open at such a tight rectangular site, mostly surrounded by water.

All available space was at a premium and even the concrete, batched nearby at a plant in Cardiff Docks, was piped in through a 30m pump from a wagon parked nearby. “We have three cranes on site, and with the wagons as well it makes life very difficult,” says Philips. “It takes a lot of coordination every day with the piling manager based on site. “You would normally have a meeting every few days and then get on with it but we are meeting every morning and afternoon.”

Concrete works are expected to be complete by October with the forming of the curving walls with shotcrete. This will free up the last few months of the contract for commissioning and testing the four pumps and other mechanical and electrical systems – such as the conveyor belt that will lift the canoeists more than 4.5m from the retaining water pool to the starter pool – before the facility is opened in March 2010.  

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