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 recently 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’. The Mumbai Metropolitan Region Development Authority (MMRDA) was the implementing body of the six-lane elevated bridge, which originates from Sewri and terminates at Nhava Sheva in Uran taluka (Raigad district). The bridge, which can withstand earthquakes up to 6.5 magnitude, has a life span of 100 years and connects Mumbai with its satellite city, Navi Mumbai. Larsen & Toubro (L&T), Tata Projects, Japan’s IHI Corporation and Daewoo Engineering were the companies that executed the project with many technological features; from the two construction contracts, one went to L&T and the other to Tata Projects. “The MTHL-1 project began on March 23, 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. “This accomplishment of timely completion of such a mega project has been achieved by L&T despite challenges like adverse climatic conditions and the COVID-19 pandemic, which disrupted our global supply chain and availability of workforce. Since inception, the project team kept challenging itself and achieved significant milestones. Starting with the pile foundation works in December 2018, the team began the first superstructure concrete span erection works in February 2020.” Orthotropic steel deck 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. The Japan International Cooperation Agency (JICA) was the principal financier of the project. According to JICA, OSD 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 and ecosystem. Special steel decks enable the bridge to have longer spans than possible with regular girders. The OSDs were manufactured by Japan’s IHI Infrastructure System, under IHI Corporation, a heavy-industry manufacturer, and assembled at the project site with maximum lengths of 180 m and a maximum weight of around 2,800 metric tonne (mt). 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. The shortest steel span on the MTHL is 110 m. 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 in the area. “A first-of-its-kind engineering feat in India, OSD span erection works started in January 2022 and about 38 OSD spans of lengths varying from 85 to 180 m were erected within a challenging 15 months, including three months of non-working phase during the monsoon,” says Patil. “Finally, the bridge, now officially named Atal Setu, was inaugurated and opened to traffic by the Prime Minister of India on January 12, 2024.” OSD features:OSDs allow for efficient use of materials, optimising distribution of steel in the structure. They have very low self-weight compared to concrete and composite structures of similar requirement with better resistance to seismic and wind forces. These modules can be fabricated and assembled in a controlled environment – assuring safety during construction and adherence to quality requirements. They eliminate the requirement to construct high pylons, reducing the risk of working at extreme heights. They safeguard the flight path of flamingos, ensuring minimal disruption to their natural patterns. 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. It is an engineering design and construction marvel and we are proud to have been part of this iconic project.” 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. “India’s longest sea-bridge used a range of noteworthy construction materials, techniques and technologies,” shares Patil. These include: 51,444 mt of high tensile strength steel conforming to Japanese standards used in OSD fabrication 95,000 mt fusion-bonded epoxy-coated rebar and rebar with Fe500 and Fe550D grade as reinforcement in structures for concrete reinforcement 10,000 mt zinc-coated wax-filled HDPE co-extruded strands and flow-filled epoxy-coated HT strands for pre-stressing of concrete spans High-strength concrete with a chloride migration co-efficient value of < 2 x 10-12 m2/s, providing durability and resistance to environmental factors 660 Rm of longitudinal expansion joints connect with the existing bridge span of the Eastern Freeway 608 elastomeric bearings for concrete spans with large size (up to 1,150 × 1150 × 341mm) 459 spherical bearings for approach spans 92 pot bridge bearings for controlled movement and flexibility in the bridge structure, adapting to various loads and dynamic condition. 108 tune mass dampeners (tmd) for OSD bridge girders Many technologies were implemented for the first time in the country, OSD being one of the most crucial. L&T partnered with Japanese experts for design and fabrication of structure; deployed an OSD expert onsite for knowledge transfer at every stage; developed inhouse methodologies for assembly and erection activities and engaged expertise from other arms of the company. 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. A programmed self-propelled transporter was used to load the decks onto the barge and computer-controlled jacks were used to install them on the bridge within a precision band of 5 mm. The barge used to transport the decks was specifically built for the MTHL project by L&T at its Kattupalli Shipyard. The concrete section of the bridge comprises over 12,000 concrete boxes that were precast in a casting yard. The concrete segments were joined to form 60-m-long spans, which were then installed on the bridge's pillars using a mobile gantry. The foundations for the MTHL are 47 m at its deepest points to support the weight of the bridge. An automated girder launching system was used to lay the bridge's foundation, marking the first time the system was used in India. As for the human element, on average, over 14,000 skilled workers and 5,000 workers and engineers worked round the clock in three shifts for completion of the project. Expert consultants from 10 different countries, including Brazil, Denmark, Japan, South Korea, Switzerland, the UK, US and some Middle Eastern countries, were involved in various aspects of the project. As for the cost, according to information obtained through a Right to Information (RTI) query by activist Anil Galgali, it saw an escalation of 14.9 per cent, or Rs 21.92 billion. The original cost of about Rs 147 billion increased to around Rs 180 billion owing to COVID-induced lockdown delays. Endurance, sustainability and longevity “The enduring structural integrity of the bridge is ensured through a robust foundation designed to withstand the test of time,” says Patil. “Seismic (earthquake) loading is fully considered in the design. A rigorous multimodal dynamic seismic analysis using a site-specific response spectrum has been carried out for design of the substructure. In-depth underwater surveys were conducted to ascertain that the foundations do not interfere with the existing underground utilities. Based on survey findings, foundation designs were adjusted or modified to ensure compatibility and avoid any adverse impact on existing infrastructure. The foundation piles of 2.2-m diameter were designed to carry their load by friction and end-bearing and are founded in the hard rock, reaching up to 47 m below water level. The piles are anchored strongly into the hard rock with anchoring of six times the diameter of piles, i.e., 13.2 m. High-strength M45 and M55 grade concrete was used to ensure durability and strength of the foundation, providing a resilient support system for the bridge. Integrity of all the piles was ensured through cross-hole sonic logging.” “Building a marvel like this in a very eco-sensitive zone took a lot of care and design,” elaborates Pai. “The ground strata was studied and the bridge foundation designed for the intertidal and marine section. Pile foundations were used for the marine section and land and interchanged; these are open foundations. The pile foundations incorporated state-of-the-art, reverse circulating drilling technology chosen for its capacity to maximise vibrations, eliminate noise pollution and efficiently dispose of excavated material. This approach safeguards the surrounding environment from potential damage during foundation construction.” In terms of the green element and sustainability, estimates suggest that using the bridge would lead to a savings of 10 million litre of fuel annually, apart from a reduction of CO2 emissions by over 25,000 mt. MTHL is expected to see 39,300 vehicles daily between the Sewri and Shivaji Nagar interchange while 9,800 vehicles will run between the Shivaji Nagar and Chirle interchange. The bridge, designed to carry about 70,000 vehicles daily, according to MMRDA estimates, will save a significant amount of commute time. 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 high resistance to temperature variations. They also demonstrate better adhesion between stone aggregates and bitumen, reduce traffic noise, improve wet-weather skid resistance and enhance safety, longevity and visibility of road surface marking. Motorists can expect a pothole-free ride, irrespective of weather conditions. 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 by a few hundred thousand tonne as vehicles will cross the bridge at a maximum speed limit of 100 kmph. 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 most challenging segments of the project was where it needed to be built at a height of 15 m above sea level. Another challenge lay in the marine portion, where engineers and workers had to dig nearly 47 m deep into the sea bed. As the bridge was situated near sensitive establishments like ONGC, JNPA and BARC, construction in these segments became more difficult with engineers having to ensure they did not damage any underwater pipelines or communication cables. Elaborating upon the engineering and marine construction challenges, Patil says, “This part of the Mumbai coast is also home to some fishing villages, a power plant, oil refineries and other sensitive installations. There is also a dedicated shipping channel to Nava Sheva port. The engineering, therefore, had to take into consideration construction that would least disturb subsea facilities like gas pipelines, HT cables, loading jetty, coal handling jetty and other facilities. These constraints imposed restrictions on span configuration and bridge deck types in design. The project involved many international engineering organisations and efficient coordination between them resulted in effective design implementation on the site.” He lists some marine construction challenges: Logistics: “A major challenge in marine construction was travel time in boats of about three to four hours on a daily basis to reach the work location, which disrupted productive time available, and also ensuring labour welfare and medical assistance on call to create a healthy and safe working environment. We had to identify a nearby jetty for marine operations and control to reduce logistical challenge. To ensure round-the-clock productive work, the shifts were increased from two to three.” Severe climatic conditions: “Working in a marine environment came with a challenge of managing work while tackling high winds, tidal variations, sea-sickness, extended monsoons and the threat of cyclones. A dedicated weather intelligence agency was deployed for precise weather forecasting, enabling meticulous planning of activities and movement in the sea.” Skill of workforce: “We had workmen having limited or no experience of working in marine conditions at heights up to 27 m above water level. We had to set up multiple training centres within the project site to train and enhance the skills of staff and workmen with domain experts. Up to 120 workmen were trained per day. The project has developed a competent workforce for marine projects of similar requirements.”  Mitigating 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 highly sensitive area, from the MTHL while the noise barriers are meant to minimise disturbance to flamingos and migratory birds caused by moving traffic. Similarly, on the 7.807 km of stretch over the sea, over 900 m of noise barrier and 1.2 km of vision barrier have been installed. Further, several measures have been implemented with a commitment to environmental sustainability, as Patil shares. “Efficient design and a unique concrete mix design optimised consumption of natural resources and reduced carbon emissions. Situated in the Sewri mudflats, an eco-sensitive zone with mangrove trees and a habitat for numerous birds, the project recognised the significance of preserving this delicate ecosystem.” Significant measures taken include: 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: The project implemented use of DRDO-patented bio-toilets to reduce solid waste generation and ensure eco-friendly discharge. Muck disposal: Generated muck from piling activities was disposed of in designated locations, preventing adverse effects on nearby ecosystems. Special warm white lights: During construction at night, special warm white lights were used to minimise disruption to local flora and fauna. MMRDA said it would declare nearly 2 km on the Sewri side as a ‘silent zone’, as well as near schools and other sensitive areas on the Navi Mumbai side. Even the construction equipment used during the project was fitted with silencers to reduce the potential impact of noise on migratory birds. The project resorted to reverse circulation drilling (RCD) methodology, which helps reduce noise levels and speed up construction in marine areas. This construction technique was employed for the first time in India. Piling activities used RCD machines, which produce significantly lower noise and vibrations, thereby mitigating the impact on flora and fauna. 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 testing and certification procedure for road restraint systems. MMRDA originally planned to install a 5-ft solid wall as the crash barrier but decided against it to avoid blocking the view from the sea link and creating a tunnel effect. “During the construction phase, several measures were taken to ensure the safety of the workforce employed,” says Patil. These include: Workforce training: 55,000 + persons were trained and nearly 4,000 training sessions conducted to educate and train the workforce for skill-based training and awareness. Tool box talks: Daily safety tool box talks were conducted, about 190,000+ since inception, before the start of each shift at the work location to reinforce safe work practices pertaining to the activities planned. Pink helmet system: New colour coding of a pink helmet was implemented for identification of newly inducted workers with limited experience to ensure their deployment in non-critical tasks in the initial period and close supervision at the workplace. Buddy system: Experienced workers paired with new workers for on-the-job support. Tracking: Digital online tracking (GPS tracker) of equipment was used to track location and ensure safety. Cameras were also installed on the project site to monitor and ensure a safe working environment. Third-party inspection: Equipment at site was monitored with a barcoding system and timely inspections were carried out. L&T-IHI Heroes: A monthly recognition system for safety-conscious workmen was adopted with wide publicity for their good work. Construction pack: All relevant documents like safe work procedures, hazard identification and risk assessment of various activities were made available to workmen supervisors. A pre-opening road safety audit was conducted by independent auditors to ensure the safety of the bridge for commuters. Other features for commuter well-being are as follows: The bridge has two emergency rescue bays for immediate rescue support in case of any incidents. Road signages and pavement markings have been included as per applicable standards and guidelines to ensure traffic safety. Vehicular crash barriers have been constructed on both sides of the bridge carriageway designed to withstand maximum potential collision force as per applicable standards. Median crash barrier openings with movable steel barriers have been provided at fixed intervals to facilitate diversion of traffic from one viaduct to other in case of emergencies. ITMS has been provided at regular intervals for bridge monitoring; VMS gantry provided for instruction or message to commuters; and an emergency lane (2.5 m shoulder) on the left side of carriageway has been provided for emergency vehicles like ambulance, fire engines, etc. Crash impact attenuators have been installed to absorb and dissipate kinetic energy in the event of collisions, minimising potential damage. Speed calming measures, like raised bar markings and reflective road studs, have been incorporated. Anti-glare panels have been installed in the median to ensure driving comfort at night. Other noteworthy features To ensure marine life was not disturbed at night, engineers installed 1,212 special lighting poles that illuminate only the carriageway without any luminous spill onto the sea. These eco-friendly and sustainability measures earned the bridge and MMRDA a sustainability certificate from the Bombay Natural History Society (BNHS). Further, the specially designed lighting poles can withstand high-velocity winds during monsoons and are equipped with safeguards against potential damage caused by lightning. What’s more, to eliminate the need for boom barriers and eradicate long queues at toll booths, MTHL has an 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. Also, the comprehensive surveillance system (CSS) uses a network of high-definition CCTV cameras. The speed violation detection system (SVDS) enforces speed limits to ensure enhanced road safety. A video incident detection system (VIDS) detects stalled vehicles, unauthorised pedestrian movement, wrong-side driving, fires, etc, and responds to incidents and mishaps. Emergency calling boxes (ECBs) provide commuters with a direct link to emergency services and variable message display (VMD) systems provide motorists with real-time information regarding traffic congestion and alerts in case of accidents or emergencies. The way ahead “Regarding expansion and plans for MTHL, several key developments are in progress,” shares Patil. “On the Mumbai side, the Sewri interchange already connects to the existing Eastern Freeway and Messant Road at Sewri. Additionally, two ramps are under construction to provide connectivity to Worli via the Sewri Worli elevated corridor. The upcoming Orange Gate underground tunnel project of MMRDA, awarded to L&T, will further enhance connectivity of MTHL-Eastern Freeway to the Mumbai Coastal Road. On the Navi Mumbai side, the proposed Chirle-Palaspe elevated corridor further improves connectivity to the Mumbai-Pune Expressway, bringing Goa and Pune closer for motorists. The Shivajinagar interchange is strategically positioned to link MTHL to the under-construction Navi Mumbai International Airport and Jawaharlal Nehru Port Trust (JNPT), besides local roads.” The project’s full potential will be realised upon the completion of additional projects including the Sewri-Worli elevated corridor, the Eastern Freeway-Marine Drive tunnel, the proposed Chirle-Palaspe connection linking with the Mumbai-Pune Expressway and the international airport in Navi Mumbai. That said, 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