Two Tunnel Boring Machines (TBMs), just arrived from China, will soon begin boring an underground path from Saidapet to Gemini.

For the average commuter in the city, much of the visible metro work largely tends to be the construction along the two elevated corridors – from Chennai Airport to Little Mount, and from St. Thomas Mount to Koyambedu. But, a whole network is simultaneously unfolding beneath Chennai’s surface, for 19 of the 32 metro stations coming up will be underground.

Officials with Chennai Metro Rail Limited (CMRL) said there has been considerable progress at the Nehru Park site. The first TBM, launched in July, has readied a nearly 100 metre-long tunnel and the second machine will be launched within a fortnight. A total of three TBMs are currently stationed at the May Day Park, Chintadripet. Very soon, the newly-arrived TBMs will be transported to Saidapet to commence work along the line connecting Saidapet, Chamiers Road, Teynampet and Gemini stations.

Underground construction work for metro rail, CMRL officials observe, is complex and time-consuming. A total of 11 TBMs have been commissioned to bore tunnels along the underground stretch, which CMRL, for convenience in construction, has divided into five packages.

Typically, the TBMs – giant machines each comprising three huge parts of cutter head, middle shield and tail skin – are transported to a construction site and are assembled there. At any given point, one TBM will have to be at least 100 metres from another that may also be boring its way into the earth.

The underground stations are being built at a depth of around 17 metres, with the exception of the Chennai Central metro station which would be 28 metres below the ground to accommodate two levels. The operation of the TBM may have to be tweaked to suit peculiar soil conditions – as is the case around the Cooum river in Chennai.

According to V. Somasundaram, chief general manager (construction), CMRL, construction in that area would have to be carried out at a depth of 20-25 metres below the river bed. “Below the Cooum river bed, the surface will be rocky. While the process might be time-consuming, it is absolutely safe and the method is a time-tested, straightforward one,” he said.

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