The Bandra-Worli Sea Link

The Mumbai Sea Link is India’s first bridge that has been constructed in open sea conditions.

Begun in 2000. Completed in 2009. The Bandra-Worli Sea Link (BWSL) has come a long way to become an outstanding example of precision engineering. The bridge has brought respite to over 100,000 commuters who used to get stuck in traffic congestion daily for over an hour to travel a distance of 8 km, hardly a 15-minute ride otherwise.

The BWSL is part of a larger proposal to upgrade the road transportation network of greater Mumbai. In the subsequent project plan, the Sea Link has to be extended to Haji Ali and then to Nariman Point. Commissioned by the Maharashtra State Road Development Corporation Limited (MSRDC) and the Maharashtra Government, the bridge has been constructed by Hindustan Construction Company (HCC). For the next five years, the defect liability of the BWSL lies with HCC.

TECHNOLOGY
India’s first cable-stay, open sea bridge spans 600 m in length and 126 m in height (equivalent to a 43 storied building). Material, components, technology, and manpower: everything has been used in enormous amount to build the BWSL amidst the waves of the Arabian Sea.

The BWSL project involved over 3,000 workers. HCC also hired experts from China, Egypt, Canada, Switzerland, Britain, Serbia, Australia, Singapore, Thailand, Hong Kong, Indonesia, and Philippines to build the 270,000 ton bridge. The mega structure is made of 2,342 pre-cast segments, 40,000 metric ton (MT) of reinforced concrete, and 5,400 MT of iron bars. The bridge comprises 230,000 cubic meter of concrete and a cable-stay system made of 2,250 km of high strength galvanized steel wires which support the bridge.

It was a challenge to build the bridge on a sea bed that primarily consists of sand, weathered rocks and residual soil. To deal with it, HCC carried out an extensive subsurface exploration and drilling program, digging 191 bores inside the sea, to define the subsurface stratigraphy and determine the rock types and material proppublicationerties to optimize the foundation design. Due to a highly variable geology, the design calculations were carefully applied pier by pier. (Pier is the exposed portion of the pillar, above the sea surface.)

Since the superstructure rests on the piers, the Osterberg cell (O-cell) technology has been used for the first time in India to check pillar strength up to 9600 MT. The O-cell technology is best suited to test large capacity foundation elements. The Polytron discs used in pier bearings has also been used for the first time in India. The discs are supposed to be non-corrosive and therefore, long lasting.

The BWSL consists of twin carriageways of four lanes each for traffic in each direction. It is complete with concrete barriers and service side-walks on each side. Each carriageway, except at the cable-stayed portion, is supported on pillars spaced at 50 m. The bridge has been built using precast, post-tensioned, segmental concrete sections. Each segment weighs between 110 ton and 140 ton and the length varies between 3000 mm and 3200 mm. An overhead crane, called the launching truss, with self-launching capability was custom-built by HCC to lift 15 segments at a time and lay the superstructure.

Constructing 126 m tall central column (pylon) of the main cable-stay bridge, which supports the 250 m superstructure on either side, required the highest degree of precision engineering. The central pylon is supported by 52 piles and a base as big as half the football field. Each pile, the portion of the pillar below the sea surface, goes around 45 m deep into the sea bed. The arms of the pylon had to be tilted in various phases, requiring accuracy of plus/minus 1 mm per 3 m tall concreting. The overall bridge has been built with tolerance of plus/minus 5 mm. Each of the 424 cables used in the BWSL is made-to-order as the width and length of each cable varies. Undoubtedly, a lot of meticulous planning and minute calculations have gone into building the mammoth structure.

While constructing BWSL, the eco-friendly aspect has also been taken care of. Fly ash, a waste product extracted from thermal power plants, was used in concrete to make the construction durable as well as environment friendly.

FEATURES
The 8-lane, cable-stayed BWSL has been built to reduce commuters’ travel time from 60-75 minutes to 7 minutes. The 25 m high pier in open sea also gives ample headroom to marine traffic.

Some of the most modern security systems, including electronic eyes on the top and underneath, have been set up at the BWSL. It is also equipped with a modern, 16 lanes toll plaza with state-of-the-art toll collection systems. The collection system provides option to pay the toll manually by cash, through a smart card, or electronically through on-board units mounted on the vehicles.

The control center located near the toll plaza is housed with the electronic tolling controls. The transmission system comprises fiber-optic cable housed in PVC conduits, running parallel to the Bandra-Worli corridor.

The Sea Link is also equipped with advanced traffic monitoring, surveillance, information and control systems. It has CCTVs, traffic counting and vehicle classification system, variable message signs, remote weather information system, and emergency telephones. In addition, pull out locations are also provided to allow enforcement officers to safely pull out of traffic.

The BWSL is also likely to be well lit as a reliable and dependable power supply has been arranged for the bridge. There are house diesel generator sets and auto -mains failure panels to cater to critical load such as the monitoring, surveillance, communication equipment and aviation obstruction lights.

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The majestic BWSL is a beautiful example of perfection. It is likely to change the way bridges are constructed in India. If the Sea Link, which is only the first phase of the Western Freeway Sea Project, succeeds in easing the congestion on Bandra-Worli route, the Government may think of proceeding with the remaining phases faster.

Vantika Dixit is Senior Assistant Editor of Technology Review India.