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Foundations - Contractor May Gurney is minimising risk by beefing up reinforcement on a piling job in Middlesbrough.Bernadette Redfern visited.

Driving along the A66 in Middlesbrough it is impossible to miss the Riverside Stadium.

Unfortunately for the residents of Middlehaven, where it stands, people travelling to it can make traffic a nightmare, especially on match day. The only point of access is from the Cargo Fleet Roundabout on the main A66 carriageway.

To make access easier, Middlesbrough Council with Tees Valley Regeneration and development agency One NorthEast is creating a new £7.6M grade separated junction as part of the Middlehaven area regeneration project.

'The separated junction improvement project will increase capacity on the A66 and provide direct access into Middlehaven, ' says the client group project manager Ian Harvey.

Balfour Beatty secured the main contract and brought in foundations specialist May Gurney Geotechnical to drive the two rows of 24, 900mm diameter piles needed to support the new A66 flyover.

'We are using CFA piles on this job, which not many contractors would do, ' says May Gurney area manager Dean Gibson. CFA piles can be driven much faster than their rotary bored counterparts but they are also more risky.

Unlike rotary bored piles, the concrete is pumped into the shaft through the centre of the auger, before the steel reinforcement cage is dropped in. 'You just don't know how long it will take to get the cage in and this can end up causing a major delay, ' says Gibson. 'This is the bit that we worried about at tender stage.'

Piling teams have to rely on the self-weight of the steel reinforcement to sink it through up to 19m of concrete. If there are any problems with concrete quality or if there is a delay during the concrete pour and it starts curing before the rebar cage is ready to go in, pushing the steel through can become difficult.

To minimise the risk, May Gurney has maximised the steel. 'We have bulked up the reinforcement to make the cage more rigid, so that it slides in better, ' says site foreman Darren Daniels.

The team replaced the eight T32 steel bars in each pile with T40s, increasing the weight of each cage to nearly 3t. The result is that cages slide into the concrete like a knife through butter.

'We also have a vibrating table on site just in case, but we haven't needed it so far, ' says Daniels.

The six strong pile driving team was getting into its stride by the 14th pile, which incredibly only took half an hour to install.

Typically, excavation through boulder clay down to underlying mudstone takes only 15 minutes, though it can take up to an hour to drill through hard rock.

Pouring the concrete takes 10 minutes and the steel cage glides through the concrete in just two.

'If these were rotary bored we would only be doing two or three a day, ' says Daniels.

Once a pile is complete 1.6m of reinforcement is still visible and will eventually be cast into the superstructure. The connection will be subject to large moments of almost 1,000kNm. 'This is why they are so big, ' says designer Halcrow's site rep Beth Robertson. 'There is full transfer of moment into the foundations.'

Although work is moving fast the team lost a day when the concrete supplier's plant broke down. 'It a special mix so we couldn't get it from anywhere else, ' says Gibson. The site is in a heavily industrialised area of the North East, with aggressive ground conditions. Halcrow specified a C32/40 Class 4 sulphate resistant mix, which is being used across the entire site.

Piling is still expected to take just under three weeks to complete.

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