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Working on water

Bridges Bangladesh

After years of depending on ferries, Bangladesh is in the middle of a bridge building boom. Alan Sparks reports from one British led project nearing completion on the Meghna River.

Communications in Bangladesh have been crippled for centuries by the three mighty rivers that dissect the country. A bridge has already tamed the Jamuna (NCE 15 June 1998) and now a host of improved crossings are planned for the other major waterways.

Opening its toll barriers next month to the myriad of ramshackle vehicles that crowd Bangladesh's roads will be the British built Bhairab Bridge, a dual two lane 1.2km long crossing over the Meghna River, 90km north east of the capital, Dhaka.

Fast flowing waters in the monsoon season swell the river depth by 7m to a maximum 34m. But the riverbed at Bhairab is made up of silty sand and alluvial deposits up to 40m deep. Advanced pile testing techniques (see box) and an innovative jack-down pile cap construction sequence were devised to tackle the heart of this foundation dilemma.

Edmund Nuttall is the design and build contractor for the £68M bridge and approach works, with JacobGibb as river training works designer and, in conjunction with local consultant DDC, also overall designer.

The main bridge structure was designed by Robert Benaim Associates, with hydraulic modelling and studies performed by HR Wallingford. On behalf of the client, Bangladesh Roads & Highways Department, Halcrow in association with local consultants BCL and ESL is providing technical and contractual assistance as well as construction supervision.

'Taking on a 33 month programme for such a testing project would make me nervous even in the UK, ' says Nuttall project director Pat Swift.

'But out here all the added constraints and extra planning required make this an immense management and engineering challenge.'

Designed for seismic events and ship impact, the bridge consists of seven 110m spans and two 79.5m approach spans.

The post-tensioned concrete box girder bridge was constructed in insitu segmental balanced cantilevers and will be topped with asphalt. Elastomeric bearings transfer the load through twin walled reinforced concrete piers onto pile caps 2.5m deep. These then spread the load into six 2m diameter piles - some as deep as 80m founding in the underlying silty sands.

Conventional circular sheet pile cofferdams were used to construct five of the shallower pilecaps and piers. 'For the other three deepwater piers, the pressure of the flow and the threat of ship impact on a cofferdam would be too great, ' says Swift.

'So we elected to cast the 2500t pile cap and pier at high level, above the river, and then jack the completed work down to the correct level underwater, supported from temporary extensions to the permanent steel pile casings, ' says Swift.

This meant that individual 4.6m diameter casings had to be fitted around each pile together with specially crafted seals which enabled the structure to be jacked down over each pile, yet hold back the water from each of the six critical pile-to-pilecap connections.

Once in position, inflatable seals ensured complete water tightness before any seepage was pumped away and workers could complete structural connections within the temporary casing.

Each of the 48 piles required approximately 200m 3of C40 concrete which, although heavily retarded, still had to provide early strength. The 28mm thick 2m diameter permanent steel pile casings - used to protect against scour - were driven by a large steam hammer before the full length piles were bored. Each pour lasted up to 12 hours with each pile constructed in two day cycles.

Such was the temperature and humidity that controlling the heat of hydration was crucial.

'As well as using ice and chilled mixing water, all concrete aggregates were kept in specially built shaded bays and sprayed with chilled water, ' reports Swift.

'Also, concrete mixer truck drums were wrapped in damp hessian (made from local jute) to keep the concrete input temperature as low as possible.'

Since there is no stone or rock in southern Bangladesh, boulders and aggregate were brought by barge and truck from Sylhet near the Indian border.

Modelling the concrete strength gain allowed engineers to programme the period of maximum heat of hydration for the coolest part of the day - between four and six o'clock in the morning. 'If we were not operating a 24 hour site then this would not have been possible, ' explains Swift.

Pile integrity was checked using sonic logging tubes that stretched from the toe to the surface. These were also used to pump grout straight down to the pile base where a tube Ó manchette arrangement forced high strength grout below the pile toe to enhance the base capacity.

The sleek sculptured lines of the bridge deck were formed in 4.5m insitu segments by an adjustable travelling shuttering system. These were cast using C50 concrete in four day cycles, with each segment stressed once a concrete strength of 27.5N/mm 2was reached - usually after 14 hours.

To protect the river banks from scour and to safeguard the slopes against a seismic event, extensive design and modelling work was undertaken. The solution adopted involved construction of 1km of stable slopes to a smooth river profile protected by boulders on geotextile, laid over the reprofiled riverbanks.

The geotextile was sewn up into large sheets, some as big as 50m square, using a bamboo grid to provide stability and manoeuvrability. These were floated and sunk into position with a prescribed overlap using a specially adapted barge. The overlying boulder protection was then placed by a fleet of small vessels, explains Swift.

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