The bridge that will stand tall for 100 years

Mumbai: In one of the passages in the classic science fiction adventure novel ‘Twenty Thousand Leagues Under the Seas’, French writer Jules Verne talks about divers in search of giant squids. Almost a century and a half since it was written at least half a dozen divers – and this time for real – between 2016-18 jumped into various channels of the Thane creek surrounded by mudflats, as a precursor to the construction of Mumbai Trans Harbour Link (MTHL) -- India’s longest sea bridge.

Diving around 10-15 metres under the creek waters, the divers weren’t in search of multi-tentacle sea animals but pipelines supplying oil and gas to Mumbai. The outcome of this two-year-long exercise helped identify the impediments under water and ensure that the piers holding up the sea bridge are sturdy enough to sustain for over 100 years.

With nearly 1100 pillars erected, the Mumbai Metropolitan Region Development Authority (MMRDA) and their contractors from L&T and Tata Projects are in the last phase of constructing the ₹18,000-crore sea bridge by September, this year. By December, five years after work started, Mumbaikars will finally be able to cruise on this six-lane bridge connecting south Mumbai via Sewri with Nhava Sheva and the proposed Navi Mumbai Airport. Clearly, it was time to look beyond the sea to ease travel in a saturated Mumbai.

The state government expects at least 70,000 vehicles to use the sea bridge daily. It hopes that the bridge will help achieve $0.25 trillion economy for Mumbai Metropolitian Region (MMR). The MTHL is expected to invite commercial hubs and speed up economic activities on its far end.

As S V R Srinivas, Metropolitan Commissioner, MMRDA, said: “This is an engine for Mumbai and its metropolitan region’s economic growth. This six-lane access-controlled sea bridge is not just an engineering marvel but also an important connector between South Mumbai and Chirle, in Navi Mumbai – it will be the new gateway to the mainland. After this is completed, Mumbai will no longer be considered an island city.”

Rahul Shah – President and Chief Operating Officer - Buildings and Infrastructure, Tata Projects Limited said, “We are thrilled to announce that we have successfully developed a ground breaking solution to address the logistical challenges of supplying concrete onto the marine for the MTHL project. By constructing a cement batching plant in the middle of the sea, we were able to not only maintain the quality of the concrete, but also eliminate the need for ferrying it from the land, which used to take up to 8-9 hours of travel time. Thanks to this innovative approach, we can reach the site in just 15 minutes. This achievement showcases our commitment to finding creative and efficient solutions to complex engineering problems and we are excited to see the positive impact it has on the MTHL project."

S V Desai, Whole-Time Director and Senior EVP (Civil Infrastructure), Larsen & Toubro said, “Building India’s longest sea bridge has been challenging, which we have manoeuvred graciously with MMRDA. The iconic project will redefine connectivity between Mumbai and Navi Mumbai. Despite executing the project in a city with many logistical challenges, we have been able to move men, machinery, equipment and material. We adopted augmented technologies and automation to overcome the post-pandemic impact. In short, this project is a culmination of our project management competences and execution aptitudes.”

What lies beneath the creek?

Although MTHL is being touted as an “engineering marvel”, it all started from the bottom. The role of divers navigating through floating oil and gas pipelines located 10-15 metres under water in Thane creek was crucial. The creek spans for almost 60 kms from Sewri to Panvel.

“The pipelines are neither parallel nor perpendicular to the alignment of the bridge, neither are they fixed,” said an engineer.

They are around 700-800 metres long and around 200 mm thick, situated in the midst of the backwaters near Butcher Island, off the coast of Mumbai. “The pipelines that carry oil are a critical asset, which had to be protected while constructing the pillars in the middle of the sea. The smallest puncture in a pipeline would lead to an environment and energy disaster of enormous proportions,” said an engineer.

A sonar (sound navigation and ranging) study was commissioned for which highly sophisticated equipment were used to identify the location of the pipeline’s alignment which is not fixed on the seabed. “Ultimately, we had to engage divers who could swim to depths of up to 10-15 meters to map their location and tag them. We conducted this study through all seasons,” he added.

The divers had to face the forces of nature while on the job. Strong undercurrents, rapid tidal waves, which varied from 1 to 4.5mts, and mudflats made visibility under water extremely poor. “As it is a creek, the water mixed with muck meant zero visibility. We had to touch these pipelines inch-by-inch in order to place the buoys against them for mapping,” explained another engineer working on the 22-km long sea bridge.

These buoys not only determined how to position and erect the pillars --- to maintain the distance between each in the middle of the sea while bypassing the oil pipelines --- but also to ensure that the barges that would carry multi-tonne girders could be anchored for support. The underwater survey took nearly four months to cover the 700-800 meters long oil and gas pipeline bypassing the MTHL bridge.

Negotiating mangroves and flamingos

Once the underwater strategies were in place, the engineers turned their focus on the bridge above water. The task was divided into three components -- land, inter-tidal and marine.

Logistics to connect MTHL from Sewri in South Mumbai with the Eastern Freeway was the biggest concern. Lack of space to enter the sea and poor road quality for bringing the girders from the Sewri end, presence of underground utilities and the existence of migratory flamingos made it a tedious task. Lugging cement concrete and steel girders weighing up to 2600 tonne on top of the pillars, which would form the road surface on which vehicles will ply in future, was particularly challenging, said engineers. In February 2023, work on connecting the Eastern Freeway with MTHL began. The 40-mt long composite steel girder spans, each weighing about 130 metric tonne, will connect the two road bridges.

Mangroves and flamingos nestled in the mudflats surrounding the construction, makes this an eco-sensitive site. “We could not design this bridge as cable-stayed as it would have affected the flight path of the flamingos. So we used the method of laying orthotropic steel decks – India’s first – which are long stretches of steel spans that form a girder in the middle of this creek. To avoid disturbance of the birds, we used special devices to reduce noise levels from the machines and equipment and arranged special lights,” said an engineer representing one of the contractors. The lights were blanketed to curb light pollution in the surrounding areas. Likewise, the machines used for drilling and other activities had lower decibel levels.

The excavated soil and debris from under water were dumped deeper into the sea along this 60-km long creek, and not along the bay. Under normal conditions of bridge-building, soil investigations are carried out after every 50-150 mts depending on the strata. The approach for this project was different – engineers mapped soil conditions around every pillar that was to be set up. At both Sewri and Chirle ends of the project, engineers dug deep into the soil to gauge the depth of the seabed, and had to innovate digging methods by using special drills.

“If we wanted to make room for 10mx10m piers on the mudflats, we had to start digging 40mts ahead of this space from either side in the shape of a staircase. With the risk of soil collapsing and our inability to cut vertically through the mudflats, this technique ensured stability,” said an engineer.

Mapping every tiny detail on the entire length of the bridge was vital since the engineers couldn’t risk assuming the depth and rock conditions underwater.

“Conventionally, for a regular bridge, we cut 1.5 times deep into the rock. For MTHL, however, we cut six times deeper into the bedrock for strengthening the pillars. The rocks were so tough to drill that cutters of the special drilling/ boring machine were getting chipped off,” recounted another engineer.

Depending on whether the rock was soft, weathered or hard, the deepest that the pillars have gone into earth’s belly is 45mts towards the Sewri end and 36mts on Nhava Sheva end of the bridge.

A rust-coloured temporary bridge was constructed on both ends for carrying labourers, equipment, parking barges, delivering food for labourers and other activities. They will be removed once MTHL is ready and running.  

Managing time

“We reverse engineered various works based on the tide levels. Since shallow water levels and low tides are synonymous with a creek, ferrying multi-tonne steel and concrete girders was a challenge and movement of barges had to be timed,” said a contractor.

Contractors were tasked with receiving a week’s weather forecast from a Dubai-based agency. Tide levels, lunar cycles, wind speed, rain prediction, swell of water and variation in temperatures are monitored, and information disseminated. Thereafter, schedules were prepared for transporting girders that weighed anywhere between 700 metric tonne (MT) to 2600 MT that had to be launched on the piers.

“We used 100-year data on wind speed and earthquakes. We even experienced cyclones thrice during the construction period. Thanks to this information, we were able to protect the large cranes and gantries, secure barges and equipment and shift manpower four days before the date of cyclones,” said a senior engineer.

Project proponents also undertook a site-specific seismic study with predictions of earthquakes for the next 100 years. Although Mumbai is not prone to earthquakes, this bridge is meant to withstand 7.5 on Richter scale. With rapidly changing climatic conditions, MTHL is also being poised as a first to withstand the vagaries of the weather through the use of a Japanese technology that strengthens the cement concrete through epoxy coating. This arrests corrosion, protects the bridge from penetration of chlorine and salinity and even controls the expansion and contraction owing to temperature variations, thereby enhancing its durability.

The minimum length of the concrete girders is 60 meters weighing 700MT, divided into 15-16 segments. There are 180-meter long orthotropic steel decks (OSDs) -- which are steel girders -- weighing upto 2600MT. There are more than 50 OSDs on the 22-km stretch. The entire span forming a girder is then placed on top of the piers or pillars completing one girder. The smaller OSDs were 65 meters in length.

Transporting OSDs from the Sewri end was very challenging. Special piers of 2-3 kms into the sea and barges with the capacity to lug and carry up to 32,000 tonne were created. Thus, these OSDs from Sewri travelled eight nautical miles into the sea, all the way via Gateway of India. It took anywhere between 8-10 hours. They departed in shallow barges with appropriate carrying capacity to reach the middle of the sea at a time when there were high tides. These could also be in the middle of night between 1am-5am as well.

These OSDs were designed and manufactured in Japan, South Korea, China, Taiwan, Vietnam and Myanmar by the partner companies of both L&T and Tata Projects. They were pieced together on-site.

Once assembled, it took 8-10 days to erect the OSD on the pillars. A specialised barge – the size of a football field -- with a carrying capacity of 32,000 tonnes was developed on-site to carry these large OSDs.

If transportation of these girders weren’t enough challenge, the mobility of labour force to the site into the sea was equally daunting.

“Our whole intention was to minimise the trouble in the movement of manpower, machinery and materials. This is a backwater and there was no access from Sewri to ferry our men and materials. So we had to make arrangements from Navi Mumbai. Every day at least 1,000 labourers travelled by sea for 2.5-3 hours back and forth to the site at Sewri,” explained another engineer at the site. The entire project had close to 9,000 odd labourers and engineers working on this mega infrastructure project.

Standing in the middle of the sea, this steel-cum-concrete bridge looks like a wave, designed primarily by MMRDA and honed by Larsen & Toubro and Tata Projects Limited. Its height rises and drops though it is not steep. Asphalting of roads has begun along with painting of the pillars in shades of blue and grey. Work on placing noise and crash barriers and lighting is likely to begin after September once heavy-duty work is complete.

Last month, the president of Japan International Cooperation Agency (JICA) Dr Akihiko Tanaka visited MTHL for a review. The project cost is about ₹17,843 crore and JICA is the funding agency. JICA released the third tranche of the ₹1,927 crore for the under-construction MTHL early March. The first and second tranches were signed on March 31, 2017, and March 27, 2020, respectively. The ambitious project is expected to be ready for public use by December.