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Sitting on the dock of the bay

Bridges Mumbai

As soon as this year's monsoon ends construction of a landmark bridge in Mumbai will get going in earnest. Bernadette Redfern reports from India.

When monsoon wind and rain hits India's west coast in July, contractors have little option but to rein back construction activities, batten down the hatches, and sit the bad weather out. Accordingly, the landmark £168M sea bridge project in Mumbai (formerly Bombay), already running five years late and 50% over budget, is in a state of limbo until the skies clear again in September.

The project on which this temporary halt has been called is a 5.6km long, eight lane road bridge that will slice across Mumbai's Mahim Bay, connecting the districts of Bandra and Worli.

Unexpectedly to the outsider, Mumbai has some of the highest land and real estate prices anywhere in the world. To solve an increasingly acute traffic congestion problem in the Mahim Bay area, it has proved easier and cheaper to build an offshore causeway than to attempt new road construction through and around the city's dense tangle of existing buildings and streets.

Consultant Dar Al-Handasah has designed the crossing as a pair of identical structures, consisting of precast segmental concrete viaducts with two sets of twin cable stayed bridges clearing the main navigation channels. Each structure will carry four lanes of traffic. While contractor Hindustan Construction Corporation (HCC) is not due to complete work until 2008, it is hoped the first four lane structure will be carrying traffic by 2006, says client Maharashatra State Road Development Corporation (MSRDC) vice president Anil Kumar Lakhina.

This will allow the state government to start collecting revenue from tolls to pay back the banks.

It should also offer cost savings: Formwork will be reused on the second structure, and HCC will be able to apply learning gained on the first crossing.

Viaducts will be supported at 50m intervals on cast insitu reinforced concrete piers, which already stride across the choppy waters of the bay.

Either side of their cable stay towers, the main bridges will provide spans of 232m.

The bridges are founded on 6m deep pile caps, gathering together 52, 2m diameter reinforced concrete piles. Four cast insitu reinforced concrete tower legs will diverge like the corners of an inverted pyramid from each pile cap to the outer edge of the deck. Above deck level they will then converge again, meeting 70m up to create a single tower section, from which 132 stay cables will fan out. Total tower height will be 120m.

Post tensioned transverse and longitudinal bars will be cast in to the tower head to resist local cable forces.

The deck for viaduct and bridge sections alike will be made up of 120t, reinforced C60 concrete segments that are match cast in 3m lengths on the project site at Bandra. Measuring 18m wide by 3m deep, they will have a curved 'fish belly' underside.

From the casting yard they will be transported by barge into position below a huge lifting gantry. This steel colossus will span between pier heads, enabling segments to be hoisted into place. Elements will be glued and stressed together with cables as each is added. Once a complete span has been assembled the whole thing will be post-tensioned. Expansion joints at each pier will allow for 140mm of movement.

For the bridges, deck elements will be added two at a time, one either side of the towers. After they have been glued and stressed and cable stays have been attached and tensioned, cranes will be advanced to the outer ends of the deck to lift the next segment into place.

Design changes

The history of Mumbai's causeway has been complex and messy. A five year delay, complete redesign and a change of consultant in 2003 have all pushed up costs by an extra 50% to £168M.

'We had to change the design to accommodate the demands of both the public and fishermen. The design and alignment of the bridge both changed, ' says MSRDC vice president Anil Kumar Lakhina.

Fishermen insisted that they needed longer navigation spans across the harbour than the original design provided. And for the first time in 20 years, 5,000 flamingos arrived in Mahim Bay.

'We had to move the bridge out further and in deeper sea and costs went haywire, ' Lakhina says. 'Also, the contractor under-bid, so as time went on and the changes emerged, the blame game began.'

This game continues, with HCC and MSRDC yet to come to an agreement on the extra costs. They hope to have negotiated a new contract before construction takes off again in earnest in September.

Originally the bridge section of the crossing was to have been built with pairs of 180m tall towers flanking the deck.

By reducing tower height by 60m HCC has been able to simplify construction.

'Dar's design was a scaled down version of that of the previous consultant, Sverdrup.

The 180m towers were pretty ambitious and would have tested the contractor's capabilities, ' says Lakhina.

'The new design is simple and elegant.'

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