A boring machine started digging the Metro tunnel from Kidderpore to Victoria on Thursday morning, kick-starting the construction of the underground section of the long-delayed Joka-BBD Bag corridor (Purple Line).
P. Uday Kumar Reddy, the Metro general manager, pressed a button to mark the ceremonial launch of tunnel boring minutes after 10am, triggering cheers from the engineers and workers. “It is a historic day,” he said.
Two tunnel-boring machines (TBMs) have been lowered into a launching shaft at St Thomas’ Boys’ School in Kidderpore.
The TBMs arrived from Germany via Tamil Nadu. The TBMs are called Durga and Divya. Durga started boring on Thursday. Divya will start working after Durga has bored around 100m, said an official of RVNL, the implementing agency of the Purple Line.
The corridor is now functional on an 8km elevated stretch between Joka and Majerhat. The next stop, Mominpore, will be the last elevated station and Kidderpore the first underground station.
In the first phase, the TBMs will construct the twin tunnels to Victoria, a distance of around 1.7km. In the second phase, they will link Victoria and Park Street, a distance of around 900m.
“If things go according to plan, the TBMs should reach Victoria by May 2026 and Park Street by December 2026,” said Vipin Kumar, the RVNL’s chief project manager for the Purple Line.
Once the machines reach Victoria, they will be retrieved and relaunched to build the stretch till Park Street, he said.
The state government’s alleged refusal to allow construction at Alipore Bodyguard Lines had long stalled the construction of the Kidderpore station, one of the many roadblocks that have pushed the project back. After a series of negotiations, the clearance from the state finally came in the last week of June.
The RVNL was awarded the contract for Kidderpore, Victoria, Park Street and Esplanade stations in 2023. Larsen & Toubro was entrusted by the RVNL to build the underground section the same year.
The launching shaft St Thomas’ Boys’ School is 37m long, 22m wide and 17m deep. The TBMs have an outer diameter of 6.63m each. Each has a length of 95m and weighs around 600 tons. Precast concrete segments with a thickness of 275mm are being used to build the tunnel that will be having a diameter of 5.8m.
“Two TBMs were lowered in one month. It was challenging because of a severe space constraint. We are working in a congested area. The length of the TBMs are 95m. But we have lowered them in a shaft that is only 37m long. Parts of the machines have been lowered and the remaining parts are at the site. As the machines keep moving forward underground, the remaining parts will be lowered and connected by umbilical cords. We have innovated and improvised according to the requirement,” Kumar said at the site.
Tunnel-boring machines have not had a smooth run in Calcutta.
The construction of a 2.63km Esplanade-Sealdah section of the East-West Metro (Green Line) — more specifically, a 200m stretch in Bowbazar — has seen four incidents of water leakage and soil subsidence since 2019, delaying the project for years.
The stretch between Kidderpore and Park Street has multiple heritage buildings. Kumar said due precaution was being taken.
“We have already mapped all the important heritage buildings and taken all due care in terms of building condition survey and instrumentation to regularly monitor the effects of vibration and noise levels. All precautions have been taken to ensure that no untoward incident happens,” he said.