Bridge over sea waters: Overcoming five challenges to construct the MTHL | Mumbai news - Hindustan Times
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Bridge over sea waters: Overcoming five challenges to construct the MTHL

Jan 11, 2024 09:42 PM IST

From keeping migratory flamingos safe to protecting the underground pipelines— here’s what the engineers of Atal Setu solved to make India’s longest sea bridge

The country's longest sea bridge — the Mumbai Trans Harbour Link (MTHL) or the Atal Bihari Vajpayee Sewri-Nhava Sheva Atal Setu — connecting Sewri in south Mumbai to Nhava Sheva in Navi Mumbai will do more than any bridge in the city. The 21.8 km-long bridge (16.5 km of which is over the Thane creek) will cut travel time between south Mumbai and Navi Mumbai (Nhava Sheva) from two hours to 15-20 minutes. Motorists will have to pay a one-way toll of 250, and the speed restriction is set at 100 kmph.

Mumbai, India - Jan. 11, 2024: A night view of the MTHL- Mumbai Trans Harbour Link Road on the eve of the PM Narendra Modi to inaugurate MTHL in Mumbai, India, on Thursday, January 11, 2024. (Photo by Satish Bate/ Hindustan Times)(Hindustan Times) PREMIUM
Mumbai, India - Jan. 11, 2024: A night view of the MTHL- Mumbai Trans Harbour Link Road on the eve of the PM Narendra Modi to inaugurate MTHL in Mumbai, India, on Thursday, January 11, 2024. (Photo by Satish Bate/ Hindustan Times)(Hindustan Times)

The country's longest sea bridge — the Mumbai Trans Harbour Link (MTHL) or the Atal Bihari Vajpayee Sewri-Nhava Sheva Atal Setu — connecting Sewri in south Mumbai to Nhava Sheva in Navi Mumbai will do more than any bridge in the city. The 21.8 km-long bridge (16.5 km of which is over the Thane creek) will cut travel time between south Mumbai and Navi Mumbai (Nhava Sheva) from two hours to 15-20 minutes. Motorists will have to pay a one-way toll of 250, and the speed restriction is set at 100 kmph.

First proposed in 1963, work on it only began in 2018 after all permissions and clearances were secured. Built at a cost of 17,843 crore, the bridge has been built using enhanced equipment and construction techniques and has CCTVs, streetlights and state-of-the-art safety mechanisms.

Building it has not been easy for the state — from Covid-19 to equipment and environment clearances to protect mangroves and the migratory flamingos — and HT Premium takes a look at some of the main challenges that its engineers and planners have faced.

Underwater pipelines

 

The Thane creek, an inlet of the Arabian Sea, transports “black gold” — oil and gas — through underwater pipelines located 10-15 metres below. For this bridge to come up, the engineers had to commission a sonar (sound navigation and ranging) study to identify the location of the pipelines’ alignment on the seabed. Situated amid the backwaters near Butcher Island, off the coast of Mumbai, these pipelines are a critical asset that needs to be protected at all costs. The smallest puncture could lead to an environmental and energy disaster of enormous proportions.

The project authorities also appointed professional sea divers to identify the pipelines. These divers faced strong undercurrents, rapid tidal waves varying from 1 to 4.5 mts, and mudflats that made visibility extremely poor. They had to touch these pipelines inch-by-inch to place the buoys against them for mapping. These buoys helped determine how to position the bridge’s pillars — and maintain the distance between each while bypassing the pipelines— and also ensured that the barges carrying the multi-tonne girders could be anchored for support.

This underwater survey took nearly four months to complete. It covered the 700-800 metre-long oil and gas pipelines. Work on erecting the pillars for MTHL began after this was done. The other challenge that these pipelines posed was that they were neither parallel nor perpendicular to the bridge's alignment or in a fixed condition.

Negotiating mangroves and flamingos

 

Besides underground utilities, the migratory flamingos needed some taking care of too. The engineers needed to figure out ways to lug concrete and steel girders weighing up to 2,600 tonnes on top of the pillars without upsetting the flamingos and other birds. A majority of these flamingos are found on the eco-sensitive south Mumbai side close to Sewri and Trombay on mudflats covered by mangroves. In February 2023, work on connecting the Eastern Freeway with the MTHL began — this required 40-mt long composite steel girder spans, each weighing about 130 metric tonnes, to be carried.

The engineers could not design this bridge as cable-stayed (similar to the Bandra Worli Sea Link) as it would have affected the flight path of the flamingos. The alternative was to lay orthotropic steel decks. These long stretches of steel spans, of 180 metres and more, formed into a girder in the middle of this creek. To avoid disturbing these birds, special devices were used to reduce noise from the machines and equipment. The engineers also arranged for special halogen tower lights with lowered luminosity while working at night. The lights were blanketed to curb light pollution in the surrounding areas.

Making sturdy pillars

 

The muddy waters of Thane creek made the task hard. When the work to construct the tall pillars that support the girders began, the primary aim was to ensure that the mud excavated did not return to the site. The excavated soil and debris from underwater 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 every 50-150 mts depending on the strata. However, in a novel approach, the engineers mapped soil conditions around every pillar to be set up. At both Sewri and Chirle's ends of the project, they dug deep into the seabed to gauge its depth. They also used special drilling equipment and methods.

Traditionally, workers cut 1.5 times deep into the rock, however for MTHL, the cut was six times deeper into the bedrock to safeguard the holding capacity and strength of these pillars. The rocks were so tough that the cutters of the drilling/boring machine were getting chipped off. To make room for the piers on a 10mx10m area of the mudflat, workers had to start digging 40m ahead of this area from either side — in the shape of a staircase. This was done to minimise the risk of soil collapsing. This technique also offered stability as workers were unable to cut vertically through the mudflats and the soft soil. The engineers couldn’t risk assuming the depth and rock conditions underwater, so they mapped the entire length of the bridge's alignment. The underwater rocks were either soft, weathered or hard — which is why, in some areas towards the Sewri end, the pillars are 45m deep and towards the Nhava Sheva end of the bridge, 36m deep.

Transporting men in the middle of the sea

 

One of the biggest challenges was finding men who agreed to work in the deep seas. The labour force was a critical component of this mega-project that was constantly monitored at various levels. More than 10,000 unskilled and semi-skilled labourers toiled for five years. The labour force was sceptical as the work involved going into the sea and working there night and day in unknown conditions. The weather was either windy or sunny, but the labourers had to bear it all. During festivals, the workforce would take leave and there was little guarantee of their return. The authorities had workers from various parts of the country — from Punjab, Haryana, Delhi, Uttar Pradesh, Bihar and Maharashtra to Andhra Pradesh and Karnataka — for this project.

Given that the bridge is built on a backwater with no access from Sewri to ferry the men and materials, the engineers had to make arrangements to ferry equipment and girders 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. The authorities prepared region-specific food and cuisines for the workers to keep the workers’ morale high. The workforce was ferried in barges and speed boats through multiple rounds so that the labourers reached the site safely. A rust-coloured temporary bridge was also constructed on both ends for carrying labourers, equipment, parking barges, delivering food for labourers and other logistics.

Carrying gigantic girders from land to sea

 

The most critical and crucial of all challenges was transporting girders that ranged from 700 to 2,600 metric tonnes in weight. A cement batching plant was constructed in the middle of the sea, another major innovation in this construction. The engineers were able to not only maintain the quality of the concrete but also eliminate the need for ferrying it from the land, which took up to 8-9 hours of travel time. This approach helped carry several batches of cement concrete to the site within minutes — an efficient solution to a complex engineering problem. They reverse-engineered various works based on the tide levels and relied on a calendar of weather forecasts. Since shallow water levels and low tides are synonymous with a creek, ferrying multi-tonne steel and concrete girders was a challenge and the movement of barges — each the size of a football field — had to be timed to perfection.

Contractors received a week’s weather forecast; tide levels, lunar cycles, wind speed, rain prediction, swell of water and variation in temperatures were monitored; and the information was disseminated. Thereafter, schedules were prepared for transporting these girders to be launched on the piers. They also used 100-year data on wind speed and earthquakes and even managed to tackle cyclones thrice during the construction period. Thanks to this information, they were able to protect the large cranes and gantries, secure barges and equipment, and girders and shift manpower four days before the date of cyclones.

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