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Fitness regime

Bridges Engineers replacing a bridge bordering of London's proposed 2012 Olympic Games site are feeling the pressure, discovers Mark Hansford.

Temple Mills bridge is just a javelin throw from the site of the proposed Olympic Park in Stratford, east London, if the International Olympic Committee should name London as the venue for the 2012 Olympic Games next month.

The reinforced concrete bridge, built in 1963, carries one of the area's major routes over the River Lea but over the past 42 years has suffered from chloride attack, leaving it severely weakened and uneconomic to repair. Since May last year, therefore, contractor Norwest Holst has been engaged on a £2M, 18 month project to replace it.

The bridge comprises two nearly identical 50m long, five span structures each carrying three lanes of the A106, a footway/cycleway and a central paved reservation.

The decks, separated by a 3.66m wide gap, consist of precast, prestressed concrete beams with insitu concrete infill. They are supported on piled, reinforced concrete piers and abutments. Unusually, in an early attempt at a integral bridge, the piers are split in two half-way up where they are articulated by bearings.

Most of the problems stem from inadequate waterproofing and poor maintenance. The bridge was built just before the Highways Agency introduced specific requirements for waterproofing. The bitumen impregnated jute used failed to prevent water ingress - to the extent that stalactites formed on the underside of the deck.

Water ingress had also caused the sliding bearings of the abutments to seize to the point where they were beyond repair.

It got worse. Reinforcement corrosion progressed to the stage that the continuous deck could no longer sustain its hogging moment under vehicle loading. Fortunately the original contractor had designed beams to cater for simply supported loading, rather than attempt to refine the design to suit the complex continuous frame system it was tasked with building.

'So although the bridge might not be supporting loads in the way designed, reserve strength in the beams has saved it from collapsing into the river, ' explains Hyder project manager Javad Akhtar.

Hyder was brought in by Hackney Borough Council in August 2003 to make sense of two years of inspections undertaken after the A106 was detrunked and handed to the borough in 2000.

The consultant immediately instigated traffic, hydraulic and geotechnical studies while developing a number of scheme 'reference designs'.

An early favourite was to encapsulate the bridge using reinforced precast concrete arches sprung from the existing foundations. 'No traffic diversions, no service diversions, yet resulting in quite a nice arch bridge, ' says Akhtar. But it was ruled out because of worries over the hydraulic and ecological effects on the River Lea.

This left complete demolition and replacement with a semi-integral deck as the only practical alternative. Again the Environment Agency flexed its muscles: With refurbishment the Agency can only demand capacity not be reduced, but with reconstruction it can demand additional capacity.

Accordingly, the new bridge deck will be 500mm higher than the original, providing for a 1:100 year event and factoring in a 20% allowance for sea level rise resulting from global warming. The structure will also allow for easier passage of small mammals along the river banks.

The increased clearance ruled out reducing the number of spans to two, which would have demanded a deeper deck. Instead, Norwest Holst opted to keep to the five span arrangement and reuse the existing foundations and abutment supports.

This recycling has knocked £600,000 off the potential construction price.

Construction has been a rapid affair. 'Use of the NEC Form C contract enabled us to fast-track the tendering processes to meet Hackney's programme, ' says Akhtar. 'It allowed us to issue design and build tender documents within two months of appointment.

The results of the initial studies were disseminated during the early part of the tender period, leaving any follow-up work under the control of the contractor.' Norwest Holst was appointed in February 2004 and work began in May. To keep traffic moving throughout the works, one carriageway is being Around 100, 11m long, precast, prestressed, PFA concrete beams are needed for each deck.

tackled at a time, northern first, with a contraflow in operation on the remaining deck. Four lanes have been squeezed on to each section to minimise disruption.

Demolition and construction requires extreme care, with three 1m diameter sewage pipes running just below the river floor along the bridge's centre line. It is also imperative that demolition waste does not fall into the river. Pontoons have been used to create a platform from which to operate breakers and act as a dustpan for falling waste.

Piers supporting the northbound carriageway deck were demolished down to river foundation level. To construct the new piers, precast concrete rings were fi rst embedded around the existing 1.5m diameter pile caps. As well as creating enlarged pedestals they served as cofferdams during river-level works and provided shuttering moulds for the concrete pour of the new piers.

Abutments were then exposed, new faces cut and new bearings put on. Finally, in the last month, the deck has been added. Around 100, 11m long, precast, prestressed, PFA concrete beams are needed for each deck. Once the beams are dropped in, insitu concrete is poured, tying them together and making the structure continuous.

To provide long term durability this time, a ground granulated blast furnace slag concrete mix is being used.

This in turn is subject to a two part spray waterproofing.

Work on the north deck is now drawing to a close. Traffic will be diverted on to the new deck this month, allowing demolition of the south deck to begin. This operation will introduce the next big hurdle - diverting major electricity transmission cables that run through the deck.

Once the cables are shifted Akhtar is expecting plain sailing.

'We've been through a learning curve on the north deck, so the south deck should be straightforward.' The project is scheduled for completion in December.

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