India’s deepest Metro vent shaft ready in Kolkata; stunning pictures show 15-storey depth

The deepest Metro ventilation shaft in India was completed in Kolkata, on Monday. The base slab of the escape shaft is 43.5m below ground level, equivalent to a 15-storey building; is being termed another 'milestone' for Kolkata’s East-West Metro project.  

The evacuation shaft is located at Strand Road near river Hooghly and is situated between the two tunnels. At Howrah Metro Station, from where the tunnels start entering the river, the tunnel invert is 30m deep. From there, the tunnels go deeper till 37m invert level while crossing the riverbed. Consequently, the Diaphragm Walls had to be constructed 56m deep from ground level as per base demand of the geometric profile of tunnel alignment. This necessitated the ventilation shaft to go as deep as 43.5m. The shaft has a 10.3m inner diameter circular lining wall of minimum 500mm thickness, and of concrete grade minimum M40 circumscribed by 1m thick Diaphragm Walls all around.  

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"The Metro shaft has been constructed adopting innovative engineering techniques and methodology to navigate geological challenges and overcome any impact on Kolkata’s circular railway track along river Hooghly," said Satya Narayan Kunwar, Project Manager, Afcons, the construction company which is developing the project.

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As the shaft is located very close to Hooghly river, there were multiple surprises due to ancient alluvial deposition, river course transformations, and varying foundation data of old buildings at the location. A detailed study was conducted for soil mapping. A large number of bore-hole investigation data was utilised and analysed for generating soil profiles in the Gangetic plain.  

Kolkata Metro Line 2 or East-West Metro Corridor is a rapid transit line of the Kolkata Metro. It currently consists of a small operational section within Salt Lake and will eventually connect Teghoria with Howrah by going underneath the Hooghly River. It would consist of 17 stations from Teghoria in the east to Howrah Maidan in the west, of which 11 would be elevated and 6 would be underground, with a total distance of 22 km.

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