One of the feature points of the East-West Metro, that will connect Howrah with Kolkata, was the proposed tunnel beneath the Hooghly river. However, not many know that a tunnel already exists beneath the river for more than 80 years.
The tunnel is not used for transportation, though. Instead, it serves as a passage for electric cables. It was conceived by the British engineers of CESC to transport electricity to the budding industrial town of Howrah. Jute mills had started proliferating and a lot of power was needed to run the jute mills.
The idea of constructing a tunnel beneath a river was touted as a daring idea back then; it was even difficult to imagine. But British expertise ensured that it took just ten months, from 1930 to 1931, to build the 1,734-foot-long and 6-foot-wide tunnel, at a cost of Rs 54,17,928. And after eight decades, it still remains a rarity; it is the only such tunnel in India.
British engineers, though, had expertise in digging tunnels. They had constructed the underground railway system in London, commonly known as the ‘Tube’, and they used this expertise here. Only this time the tunnel was under a river, and narrower, being as it is for passing cables. Labourers from Punjab worked at the construction site.
Special measures had to be taken to construct the tunnel. The atmospheric pressure inside the tunnel is more than that outside. Hence, the workers had to acclimatise to the higher pressure. For that purpose, the workers, before entering the tunnel, spent some time inside a special chamber. The inner compartment of the tunnel was supported by iron rings, a mixture of cement and concrete was used to reinforce the tunnel. But the huge pressure of the water flowing above ensured that some water still seeped in. To carry this off, a channel was dug in the floor of the tunnel. The full tunnel is lighted to ensure workers can enter inside for maintenance or any other purpose.
Electric cables ran along the walls of the tunnel, six of 6 kV each, carrying electricity from the CESC power plants on the Kolkata side of the river to the industrial hub of Howrah. At that time, CESC had only three power stations, in Mulajor in northern Kolkata, in Kashipur in central Kolkata, and in Metiabruz in the southern part of the city. Howrah had a 165 KW generating power station, constructed in 1906. However, the growing number of jute mills of Howrah needed huge power supply and power generated at the Kolkata stations was decided to be fed to Howrah.
After the tunnel became operational in 1931, 15 MW power began to be transferred to Howrah. Today, the number can be up to 150 MW. In 1956, DVC began supplying electricity to Kolkata through cables of 33 kV passing through this tunnel.
It must be mentioned here that when this tunnel was conceived, the Howrah Bridge was yet to be planned. Else, perhaps, the cables could have passed along the underside of the bridge, a much more cost-effective method of transferring electricity.
The construction of the tunnel was a huge achievement back then. With much less technological help, engineers achieved something that was unheard of. It was a landmark in civil engineering history. It is commonly thought of now, in view of the digging of the tunnel for the East-West Metro, that the soft soil beneath the river is not ideal for this sort of activity. Perhaps the engineers should take some inspiration from a tunnel constructed so long ago, and with much less technological help.
