For the first time in the history of Kolkata Metro, a tunnel will have a slice of the tunnel boring machine buried inside it forever.
Even as the metro services along the entire EastWest Corridor from Howrah Maidan to Salt Lake Sector V might get started in a year or so, Bowbazar will have permanent marks of the tunnel boring machines-Urvi and Chandi.
In an unprecedented move, a part of the TBMs is to remain buried, below the site of the August 2019 mishap, for times immortal. Generally, when a TBM makes a breakthrough, its cutter head is brought out by cutting the wall and placing a cradle.
The TBM is then taken out by pushing it through a jack system. However, in the case of Bowbazar, which is not the end of the tunnel, there is no support available for the jack system at the site of the mishap. Moreover, the normal lifespan of the giant borers is over and the two TBMs have worked overtime. Considering the facts, jack and other parts have been taken out of the ground through the retrieval shaft measuring 38m long, 10m wide and 38m deep.
According to sources, the ring-shaped exterior of the TBM made of tough steel and known as ‘shield’ is to remain in, for providing support to the tunnel wall. The shield measuring about 8 to 10 metres will remain buried underneath, forever.
The buried shield will be kept behind the concrete rings of the tunnel that work as walls and will remain hidden. The task of the tunnel will be completed after the concrete rings of the wall are cast.
If everything goes according to the plan, the engineers involved are expecting to complete the task by December.