In a landmark day for Mumbai, India's longest (22 km) sea-bridge and the world's 12th longest, the Atal Setu or Mumbai Trans Harbour Sealink (MTHL) project, constructed at a cost of Rs.180 billion, was inaugurated. In a view shared by millions of Mumbaikars, Anand Mahindra, CEO, Mahindra Group, shared a video of the bridge on X, saying he couldn’t wait to drive down this ‘golden ribbon’. Mumbai Metropolitan Region Development Authority (MMRDA) was the implementing body of the six-lane elevated bridge which has an estimated 100 years life span and connects Mumbai with its satellite city, Navi Mumbai. Larsen & Toubro (L&T), Tata Projects, Japan’s IHI Corporation and Daewoo Engineering were some companies that executed the project with many technological features and from the two construction contracts, one went to L&T and another to Tata Projects. “The MTHL-1 project began on 23 March 2018 with the signing of the contract between MMRDA and L&T-IHI and was completed within a record duration of 69 months,” says SD Patil, Vice President and Deputy Project Director, MTHL Project, L&T. Orthotropic steel deck (OSD) technology One of the most complex mega infrastructure projects undertaken in the country saw the incorporation of Japanese construction technology and the first use of orthotropic steel deck (OSD) technology in India. These bridges have a lower self-weight and stronger structure than concrete or composite girder bridges and require fewer and smaller piers, leading to a reduction of construction time and a reduced impact on the environment. Special steel decks enable the bridge to have longer spans with regular girders. OSDs were manufactured by Japan’s IHI Infrastructure and assembled at the project site. A total of 70 orthotropic decks were used on the bridge, requiring about 96,250 tonne of steel. The steel spans weigh up to 2,600 tonne and make up 4.7 km of the bridge length while the rest of the bridge is made up of 60-m-long concrete spans weighing up to 130 tonne each. This 4.7-km section sits at the highest point of the MTHL and includes a 180-m steel span, the longest steel span in India. MMRDA chose to use steel spans in these sections to eliminate the need to construct pillars to support the bridge, which would hinder the movement of ships. Commenting on the project, Vinayak Pai, Managing Director, Tata Projects, says, “The MTHL Package 2, which Tata Projects undertook along with our partner Daewoo E&C (South Korea), had 0.3.6 million cu m of concrete, almost 65,000 MT of reinforcement steel and 208 spans of 60 m each, formed from 3,100 precast segments and it needed almost 6 km of post-tension strands to hold it all together.” Materials, techniques and technologies The bridge includes a 16.5-km sea link and 5.5-km viaducts on land on either end of the bridge. Each segment of the viaduct weighs 75 tonne and over 10,000 such segments have been cast at two precast segment yards, one on the Mumbai side and another on the Navi Mumbai side. Its sheer magnitude can be gauged from the materials that went into its making: 177,903 MT of steel and 504,253 MT of cement. To put it in perspective, the amount of steel used by the megastructure in its construction is equivalent to the weight of 500 Boeing airplanes or 17 times that of the Eiffel Tower's weight. The structural steel used in the bridge is enough to support the erection of four Howrah Bridges and the concrete used to cast the structures of the bridge is six times more than the quantity used for building the Statue of Unity. The bridge has been constructed using corrosion-resistant materials to withstand weather variations and high wind speeds and tides, along with natural phenomena like earthquakes and cyclones. The process The bridge was constructed in three sections. It required the use of 165,000 t of reinforcement steel, 96,250 t of structural steel and 830,000 cu m of concrete. Only about 3 per cent of the bridge structure was built onsite including the foundations and pouring cement over the metal girders. The rest of the bridge was prefabricated and transported to the site for installation. The parts for the orthotropic decks were manufactured in Japan, Myanmar, Taiwan and Vietnam and shipped to Karanja Port in Uran for assembly. The decks were carried to the construction site by barge and each deck required about three days to install in place. The barge was built by L&T at its Kattupalli Shipyard. An automated girder launching system was used to lay the foundation, marking the first time that the system was used in India. Endurance, sustainability and longevity In terms of sustainability, estimates suggest that using the bridge would lead to a savings of 10 million litres of fuel annually, apart from a reduction of CO2 emissions by over 25,000 MT. To put together a special recyclable road surface that offers a smooth ride, roads on both sides of the bridge have been paved with stone mastic asphalt (SMA), which has a high percentage of crushed, durable stone aggregates and a specialised asphalt binder. SMA roads promise 20-30 per cent increase in pavement life and reduce traffic noise, improve wet-weather skid resistance and enhance safety, longevity and visibility of road surface marking. The best part is that SMA pavements are 100 per cent recyclable at the end of their service life. The carbon footprint from vehicular emissions will also significantly reduce. Major challenges Apart from the factors mentioned above, the project has been given due respect in view of the multiple challenges faced during construction. One of the challenges was due to its location near sensitive establishments like ONGC, JNPA and BARC. Construction became difficult with engineers having to ensure that they did not damage any underwater pipelines. Mitigating the environmental impact MMRDA has installed noise (for 8.5 km) barriers and vision (for 6 km) barriers. The vision barriers are intended to block the view of the Bhabha Atomic Research Centre (BARC), which is a sensitive area, from MTHL while noise barriers are meant to minimise the disturbance to flamingos from moving traffic. Similarly, on the 7.807 km stretch over the sea, over 900 m of noise barriers and 1.2 km of vision barriers have been installed. Patil shared some measures: Temporary access bridge: In mudflat zones, a temporary access bridge was strategically used to minimise the ecological impact by eliminating dredging or backfilling. DRDO-patented bio-toilets: To reduce solid waste and ensure eco-friendly discharge. Muck disposal: Muck from piling activities was disposed of in designated locations. Special warm white lights: During construction at night, special warm white lights were used to minimise disruption to local flora and fauna. Even the equipment used during the project was fitted with silencers to reduce the impact of noise on migratory birds. The project resorted to reverse circulation drilling (RCD) methodology, to reduce noise levels and speed up construction. This technique was employed for the first time in India. Safety first The safety of travellers has been ensured with see-through (to not block the sea view) crash barriers with an overall height of 1,550 mm that have been tested to meet international standards. The barriers are a combination of concrete and metal with a 90-cm-high concrete section and the remaining 65 cm metal rail, complying with International Standard EN 1317, a common certification procedure for road restraint systems. “Several measures were taken to ensure the safety of the workforce,” says Patil, which include: Tool box talks: About 190,000+ since inception and before each shift for safe work practices. Pink helmet system: New colour coding was implemented to identify new workers with limited experience to ensure their deployment in non-critical tasks. Buddy system: Experienced workers were paired with new workers for on-the-job support. L&T-IHI Heroes: A monthly recognition system for safety conscious workmen. Features for commuter well-being are: The bridge has two emergency rescue bays. Median crash barrier openings with movable steel barriers at fixed intervals facilitate traffic diversion from one viaduct to another in an emergency. Crash impact attenuators have been installed to absorb and dissipate kinetic energy in the event of collisions, minimising potential damage. Anti-glare panels have been installed in the median to ensure driving comfort at night. Other noteworthy features To eradicate long queues at toll booths, in the open road tolling (ORT) system, scanners and cameras identify the FASTAG and electronically collect tolls so that vehicles need not halt for toll payments, leading to a better commuter experience, fuel efficiency, safety and cost-effectiveness. Future Watch It is interesting to note that the Atal Setu is the most expensive toll road linked to Mumbai – the cost for cars is Rs.250 one way and Rs.375 for a round trip in the initial year. (In comparison, the Bandra-Worli Sea Link costs Rs.85 one way and Rs.127 return.) A fabulous joy ride, to be sure, but considering the high cost, only time will tell how feasible it will be for daily commuters. - R SRINIVASAN