KOLKATA: His eyes glistened at the technological marvel when the computer operated tunnel boring machine (TBM) nosed under the Hooghly to carve out two tunnels last Friday. "Technology has made it easier and safer," said 75-year old construction man Biswanath Bhunia. A chip fitted in front of the TBM will measure the earth pressure and find its way under the river bed with zero possibility of soil subsidence.Sitting at the Afcons Salt Lake office, the former Metro Tunnel Project manager of the country's first North-South Metro project in Kolkata narrated his tryst with such tunnelling under Circular Canal in 1979-84. The challenge was to join the Shyambazar five-point crossing with Bhupen Bose Avenue in north Kolkata. "It was a nightmare. The rest of the Metro alignment from Tollygunge was more or less a cut-and-cover job. But this was ruled out for Shyambazar because of the huge traffic, old buildings and above all the 300-tonne Netaji Statue at the five-point crossing. It took us years to do the up and down tunnelling work under compressed air pressure for 24 hours," Bhunia said.Working under compressed air has taken a toll on his body balance. "There is always a droning sound around the right ear. I have to bear with it as long as I live. It has restricted my outdoor activities. Some labourers from Malda and Murshidabad had a similar plight. Sometimes blood used to ooze out from their ears and nose. We had a decompression medical unit as part of the project to take care," he said.With the kind of technology at hand -shield tunnelling-and technical collaboration with Hungary's NIKEX, a bunch of dedicated engineers began the work with mechanically operated TBMs. They couldn't go below 15 metres underneath the canal, unlike now with computer operated TBMs doing the work 35 metres down the river bed.The first hurdle was the soppy soil in and around the Circular Canal with high possibility of water seepage. "There was a huge canal flowing above with tidal fluctuations with hundreds working below under airlock conditions. There was no key to unlock the chamber other than by compression and decompression. Any air leakage from the canal bed would have led to flooding of the tunnel underneath and rendering the airlock system inoperative. It was a high risk job," Bhunia recalled.The next hurdle was the 72-inch filter water pipeline from the Tallah Waterworks, and the large brick sewer network built to carry water to Circular Canal.All of them had to remain where they are.The project was held up for two years after a TBM got buried under earth due to collapse of tunnel face. Tripura governor Tathagata Roy, and deputy chief engineer of the Kolkata Metro then, said: "It was not an easy job in a controlled economy to terminate contracts and get new global consultants as it's now. We had to wait for months for getting spares of the TBMs" He recalled the Tollygune Rail Bridge near Charu Market had also posed a challenge for construction of Rabindra Sarobar station below. But tunnelling at north Kolkata part was the most challenging. "There was every chance of subsidence resulting in damage of the Netaji Statute," Bhunia said.

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