Ten piles up to 2.4 metres wide will be driven 25m - about the height of a seven-storey building - into the ground when work begins on the new Ferrymead bridge.

The piling work is expected to take 15 months and is the first stage of a 2 year project that will cost nearly $35 million.

"This is a huge construction project. We are building the Ferrymead bridge to the very highest engineering standards," Christchurch City Council city and environment general manager Jane Parfitt said.

The existing bridge, which carries about 30,000 vehicles a day, has long been identified as a weak link in Christchurch's transport network because the soil on which it sits is prone to liquefaction and because its piles are not founded to bedrock.

The bridge was being strengthened and widened when the February 2011 earthquake hit, causing structural damage.

The new bridge will have 10 piles, each weighing about 50 tonnes and measuring between 1.1m and 2.4m in diameter. They will be driven through sand and mud and 5m to 8m of bedrock so the bridge meets new safety requirements.

The bridge will have about 300 tonnes of reinforcing steel and 26 beams each weighing 45 tonnes. Once the piling work is completed, it is expected construction of the rest of the bridge will take about a year.

Temporary bridges have been erected to carry one-lane traffic in both directions while the new bridge is under construction. Demolition of the existing bridge is about to start and will take three months.

Jim Harland, southern regional director of the NZ Transport Agency (NZTA), which is providing $22m toward the cost of the project, said the new bridge would improve transport links to the eastern hill suburbs by providing additional capacity for buses, cyclists and pedestrians.

"The NZTA's investment in this project will ensure residents have a new bridge that is 100 per cent of the new building code and will provide an improved certainty of continued access if there is another significant event," he said.

The city council and the NZTA say disruption to traffic while the bridge is constructed will be minimal, but they warn the demolition and piling work will be noisy and likely to cause vibrations.

Work will be carried out from 7am to 6pm on weekdays and from 7am to noon on Saturdays, with shorter hours in winter.

Parfitt said the piling work would cause some shaking to nearby properties but it was likely to be a "nuisance rather than damaging".

"It will be nothing like the shakes they have had in the earthquakes," Parfitt said.

- © Fairfax NZ News

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