India’s first under-river metro tunnel paves way for country’s first undersea rail 'chunnel'

The know-how and experience gained and perfected while digging through the alluvial soil in the Ganga river system’s bed has given Indian engineering an edge that it hopes to replicate in even more ambitious projects including some over the high seas. 

Railway experts say that the experience along with technical expertise will now come in handy for the upcoming first 7-km long undersea rail tunnel at Thane Creek in Maharashtra for the Mumbai-Ahmedabad bullet train project.This will be part of a total 21-km long underground tunnel between the Bandra-Kurla complex to Shilphata.

The possibilities are endless – ranging from under-river metro railway projects elsewhere in India and abroad to the even more ambitious 23 kilometre long Palk Street bridge and tunnel between Dhanushkodi in Tamil Nadu and Mannar island off the Lankan coast.

Running 13 meters below the bottom of the Hooghly River and 38 meters below the ground level, the 4.8-km-long tunnel linking Kolkata and Howrah, whose 520-meter stretch is just below the river water, faced innumerable technical and non-technical challenges making its completion and operationalisation a landmark achievement.

The Kolkata Metro Project besides having an under-river tunnel also has the distinction of having the deepest metro station at Howrah which is 30 meters below the ground.    

The First Tunnel Under The River 

It was more than 100 years ago in 1921 that a British engineer Sir Harley Dalrymple-Hay had for the first time proposed that the East and West sides of Kolkata should be connected through an underwater metro. 

However, the ambitious project did not see the light of day as the railways considered it too costly and difficult to handle given the alluvial soil that made up the Gangetic bed and pushed it into the background.  

However, Dalrymple-Hay was assigned by the Calcutta Electric Supply Corporation to build a smaller tunnel linking the two cities for electric cables. This tunnel, the first of its kind in India, was done by hand without any boring machines and completed in 1931.

Need For A Metro Tunnel

Though the Howrah Bridge over the Hooghly River provided the desired connectivity when it became operational in 1946, the huge flow of traffic in the later years kept city planners on their toes for alternate connectivity routes.

Due to densely populated areas on both sides of the river bank, another bridge was nearly impossible for lack of space and under-river connectivity was the only option.

“It was vital to provide seamless connectivity to Howrah and Sealdah, the World’s two busiest railway stations, separated by Hooghly, handling lakhs of passengers daily,” a senior official from the Kolkata Metro Rail Corporation Limited (KMRCL) said highlighting the necessity of the under-river tunnel.

He added, “As the city is bursting at the seam, Howrah Bridge is handling traffic much beyond its capacity and its maintenance burns a big hole in the Kolkata Port Trust’s pocket.”

In 1971, Kolkata’s master plan once again emphasized the need for such a project but both the state as well as the Centre were hesitant as they lacked the confidence to go in for such a mammoth exercise.

Project experts say that technological advancement in Russia and many European countries made it possible to have under-river tunnels for inter-city connectivity and this gave a cue to Indian engineers to deploy a similar technology here for an under-river project.

Indigenous & Global Help In Realising Dream Project 

After years of delay and deliberations, when the Indian Railways, in 2010, handed over the contract for this project to the construction firm AIL through international bidding, the underground soil condition of Kolkata posed the first big challenge.

“In June last year, we were awarded a contract from the National High-Speed Rail Corporation Limited (NHSRCL) to construct the undersea tunnel for the bullet train project. The successful completion of the under-river tunnel earned us this contract,” Tamal Biswas, Project Manager, Afcons Infrastructure Limited, said.

“The underground soil is alluvial, which is loose and muddy. Besides, water channels with connectivity to the main river are running underground everywhere in the city,” Biswas said. “You never know when an aquifer will come in the way of the tunnel and flood it. We collaborated with Russian, South African and European engineers to guide us through the challenging task.”

The project experts had to carry out extensive geotechnical studies and hydrographic surveys to identify aquifers and understand the ever-changing soil parameters.

After studying the soil condition, they bought two customized tunnel boring machines (TBMs) from Germany with some peculiar specifications to suit the project's needs.

“Knowing that the underground areas where the TBMs were supposed to move were vulnerable, we adopted soil stabilization techniques and the moment the TBMs moved ahead, we applied this technique as an additional support measure to avoid subsidence,” another expert, who was part of the project said.  

Technological Maturity 

Civil Engineers engaged in various tunnel projects say that wide-ranging tunnel projects in different geo-technical conditions in the country have given them enough experience and maturity to analyse all types of soil strata and today they are head to head with any Southeast Asian country in under-water tunnel technology that one can think of.

“Let’s take the case of Kolkata and Chennai. While hydrogeological conditions are very high in Kolkata, Chennai has more of a sandy surface due to the sea. While constructing an underground tunnel for the Chennai metro, we adopted a completely different strategy than that of Kolkata,” a Metro official, associated with the Chennai metro project, said.   

Besides having a better understanding of various soil conditions, Indian engineers have also become well-versed in using TBMs under different hydrogeological conditions and customizing them to suit the demands of a particular project.

According to Biswas, the German TBMs that they used in Kolkata had special features to get converted into submarines in case of flooding of the tunnel. 

“At present, we are better prepared to navigate through the sea when we have taken up the Bullet train project because we have the technical know-how due to a lot of experimentation that we have already gone through,” he said.

In February this year, the inauguration of the longest intercity undersea twin tunnels known as the Mumbai Coastal Road Project (MCRP) has put India ahead of many countries in achieving a significant milestone in modern urban transport.

Constructed by Larsen & Toubro, the twin tunnel is an engineering marvel under the Arabian Sea giving a huge benefit in decongesting the Mumbai traffic problems.

Undersea rail tunnel for the Bullet train is going to the next milestone. Engineers associated with the Kolkata metro project carried out extensive research starting from the English Channel to similar projects in Singapore, China, and other countries, said a KMRCL official. 

“It has given us a wealth of experience to handle soil conditions as well as TBMs. It has taught us in real terms how underwater tunneling is completely different and far more challenging than that of underground tunneling,” the official said. 

“In underground tunneling, you can afford to stop or postpone the work if there is any resource that you need to build up. But in underwater tunneling you have to stick to the schedule so you have to foresee a lot of things including possible obstacles in advance.”