The bridge weighs the equivalent of 50,000 African elephants, and the wires used are equivalent to the circumference of the Earth, writes Trevor English.

The Bandra-Worli Sea Link is one of India's most significant marvels of engineering. It was the first bridge to be constructed in open sea conditions in the region, marking the beginning of a period of engineering achievement for India. 

During peak construction, about 4,000 workers and 150 engineers were involved in the building of this bridge. Between 2001 and 2009, the main years of the bridge's construction, about 25.7 million work hours were put into building the eight-lane highway and cable-stayed bridge.

Prior to the bridge's construction, the Mahim causeway was the only road that connected Bandra and the western suburbs of Mumbai to Worli and central Mumbai.

The bridge weights the equivalent of 50,000 African elephants, and the wires used are equivalent to the circumference of the earth. Ameya Ad/Wikimedia Commons

This meant that all traffic across the Bandra channel had to use the congested Mahim causeway, which became a bottleneck for more than 140,000 commuters each day. This resulted in terrible congestion and it could take more than an hour to cover the 4.9 miles (8km) distance.

The proposed solution was an integrated highway and bridge project called the West Island Freeway system, an integrated highway and bridge project.

The first step was the Bandra-Worli Sea Link, which was commissioned for construction by the Maharashtra State Road Development Corporation Limited. 

Engineering of bridge

The first foundation stone was laid in 1999, with construction beginning in 2000 and lasting roughly nine years, ending in 2009. 

Originally, the bridge was estimated to cost about $93 million and be completed in five years, but like every huge engineering project, there were delays and cost overruns.

On May 27, 2008, a major milestone was achieved when the cable-stayed bridge was connected to the approach spans to give a completed link.

The construction of the bridge was delayed by payment disputes and protests by fishermen and was fully completed and opened to the public on June 30, 2009. The final bridge ended up costing $220 million.

The bridge is a cable-stayed design, which means that there are central towers that have cables running from it to different segments of the roadway. This bridge was also the first cable-stayed bridge in India to be constructed in open seas.

A depiction of the Bandra-Worli Sea Link on a map, Source: Abhijitsathe/Wikimedia Commons

The chairman of the construction company responsible for the bridge had this to say about the project: "Construction of the Bandra-Worli Sea Link has been one of the most challenging infrastructure projects undertaken in recent times in India.

"We took on this project with the quest to set new benchmarks in precision engineering and prove India's infrastructure development capabilities.

"Reflecting on the hard work of our engineers and more than 3,000 workers who have raised this Sea Link in rough open sea conditions, I am proud to say that we have truly built a monument to human skills, enterprise, and determination."

Planning and construction

The construction of the sea link took place in five specific stages: construction of the Worli junction; construction of the cloverleaf interchange in Bandra; construction of an approach road with toll plaza in Bandra; construction of the central cable-stayed spans and roadway; and finally, improvements to the main road.

The construction of the spans and roadway, part four, took the longest out of all of the segments and took place largely out in the sea. As for planning all this construction, crews took detailed surveys of the seabed throughout the channel to determine how they were going to develop the foundations.

Surveys found that the underwater terrain varied widely, from extremely soft broken rock and sand to extremely hard rock. Since the survey gave engineers an accurate landscape of the underground substructures, they were able to plan accordingly for each section. 

As for the roadway construction, it was precast in segments, which were then carried using an overhead gantry along the already constructed road. The pieces were laid one by one. 

Notably, the bridge was the first structure in India to use seismic arresters, which allow the massive structure to withstand earthquakes of up to 7.0 on the Richter scale. 

Drawing back to the foundation that makes all of this possible though, the massive breadth of the structure and the varying geotechnical conditions made the engineering rather complex. Complications included the presence of a variable intertidal zone, exposing parts of the foundation at low tide.

Like many structures built on highly variable uneven ground, the foundations were constructed using large concrete reinforced piles. These are essentially long columns that jut into the ground.

They take advantage of the friction forces between the soil and the side of the column to remain steady, rather than supporting themselves on solid subsurface rock. This is similar to the technique used in constructing the Burj Khalifa. 

The foundation of the cable-stayed bridge is composed of 120 reinforced piles of two metres (6.6ft) in diameter each. The foundations of the viaducts are made up of 484 total piles each measuring 1.5 metres (4.9ft) in diameter. 

As for the height of the bridge, the tallest pylon on the bridge stretches 128 metres (420ft) high. It is diamond-shaped, with lower legs that flare outwards. As the pylon towers get taller, their cross-section slowly decreases.

Challenge of construction

The Bandra-Worli Sea link is made up of twin continuous concrete box girder sections, which for much of the span are supported by piers spaced apart at roughly 164ft (50 metres). The resultant roadway can support eight lanes of traffic (four in each direction) along with a pedestrian passage on one side. 

The whole of the sea link actually has two bridges. The larger of the bridges has a span of 1,640ft (500 metres) and the smaller has a span of 492ft (150 metres).

The longer spanned bridge has a height of 413ft (126 metres), stretching 43 stories high above the coastline. The bridges also had to be of sufficient height to allow a large amount of maritime traffic underneath, primarily large fishing vessels and barges. 

The bridge structure encompasses two main channels: the Bandra channel and the Worli channel, thus the bridge's name. The Bandra channel span is 1,968ft (600 metres) whereas the Worli channel section is only 1,148ft (350 metres) long.

The head engineer of the project management team said that a "method of balanced cantilever was adopted for the construction of the deck for the Bandra cable-stayed bridge. 

"This is considered to be one of the biggest cable-stayed bridges in the world with concrete deck built with this method of construction. It was therefore a real challenge to monitor the behaviour of the structure during all erection stages and to compare the same with the behaviour of the corresponding theoretical computer model of the bridge, to ensure achievement of the desired geometry at the end [of construction].

"It was also essential to control the stresses in the critical bridge elements during every single construction stage to ensure the adequacy of the bridge elements all the time".

Technological and security aspects of bridge

The bridge today is filled with CCTV cameras and different security devices at the hands of the Mumbai Police force. These security measures are extra steps taken by the local government to monitor traffic flow and ensure that no damage is done to the bridge, either accidentally or intentionally. The cameras also monitor the boat traffic below the bridge as well. 

As for powering the bridge, there is a significant amount of cabling in the structure to ensure that the lights on the cable-stayed portion of the bridge can be lit up at night. There are also diesel generators in case the local electrical network fails, to ensure that the street lights and security cameras stay functioning. 

All of this lighting protection is especially necessary for a bridge with such a long span. It would be severely dangerous for cars to get trapped on the main span of the bridge when the power went out. 

Interesting facts about Bandra-Worli Sea Link

• The bridge is the equivalent weight of 50,000 African elephants. 
• The height of the Bandra cable-stayed bridge is 63 times the height of one of the tallest ancient structures in Delhi. In total, 90,000 tons of concrete was used during the construction process and there was extensive lighting installed along the bridge. The bridge consumes roughly 1MW of electricity per day.
• There is also a 16-lane automated toll plaza that charges drivers before they cross the sea link. Though the toll is quite worth it. Previous to the bridge, the journey from Worli to Bandra took about 60 to 90 minutes. Today, the trip takes about 10 minutes.

This article  was written by Trevor English and first appeared in Interesting Engineering.