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World in union

Built on the site of the home of Welsh Rugby, Cardiff Arms Park, the Welsh capital's new Millennium Stadium formed the centrepiece of the 1999 Rugby World Cup.

The 72,500 seater Cardiff Millennium Stadium cost £114M to build, with a £46M contribution from the Millennium Commission. It boasts Britain's first retractable stadium roof and this all-weather capability is just one of the features thought to have been key to Wales' successful bid for last year's Rugby World Cup finals.

The stadium sits on the banks of the River Taff in the centre of the Cardiff. Designed and built by main contractor Laing, it is 222m long and 181m wide and sits at 90degrees to the position of Cardiff Arms Park, with the pitch running north-south. Soaring 33m above the playing surface, the roof is supported by four, 90m high masts. The retractable section comprises two 55m by 76m panels that can be moved together to give a weatherproof covering.

Ground conditions are mixed fill over gravels, with Mercia Mudstone at depth. The water table is high, only a few metres below ground level, with a high tidal range. Dewatering was needed to allow excavation of a 5m deep basement under the stadium. Subsequent closing of the Cardiff Bay Barrage has raised and stabilised the water table, but this was allowed for in design.

Cardiff Arms Park was built on pads founded in the gravel layer while the new stadium is piled to minimise any movements that could affect the sliding roof. Foundation installation had to be sequenced around phased demolition of the old stadium, which meant parts of the site were not available until as late as August 1998. At one stage the roof of the north stand was being built as foundation work started in the south.

Piles had to satisfy a high performance specification over the structure's 120 year design life, including individual pile loads of 6,000kN. While the original design called for some 1000 rotary bored piles penetrating deep into the underlying Mercia Mudstone, foundation contractor Kvaerner Cementation Foundations suggested using the more economical option of continuous flight auger piling.

This method's viability was proved after two test piles were installed on site and a total of 1150, 600mm and 750mm diameter, 10m long CFA piles were installed for the stands. The change in method resulted in cost savings of about 30% on the £2M, ten month contract, according to Kvaerner Cementation.

The four corner masts supporting the stadium's complex sliding roof impose massive vertical and horizontal loads. Kvaerner Cementation and contract engineer WS Atkins carried out rigorous analyses to come up with the best foundation design to ensure strain compatibility across the structure was maintained. The optimum solution, in terms of design, cost and buildability, comprised 192, 900mm diameter heavily reinforced rotary bored piles up to 20m deep, some of which are raked at 1:6 to cope with horizontal forces.

Foundations for the North Stand were more complicated, as this part of the stadium backs on to the Cardiff Athletics Stadium which is an integral part of the new facility. Here minipiling was used to cope with restricted headroom and working conditions, and 70, 190mm diameter minipiles were installed. Compression and tension loads of 675kN and 400kN respectively meant 5m of rock penetration was needed. Central T50 reinforcing bars were installed with double corrosion protection over the top 6m for long- term durability.

Despite the well publicised losses incurred by Laing through design changes and delays during the two years of construction, the stadium was ready for the opening game of the World Cup tournament at the beginning of last October.

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