The Chennai Metro Rail will soon be constructing a new underground route, with a tunnel to be built 20 metres below the bed of the Adyar River.

This will be Chennai's first metro tunnel to cross the Adyar River.

The tunnel-boring machine (TBM), called Kaveri, has already begun work from the Greenways metro station site in RA Puram but has only managed to complete 60 metres of tunnelling so far.

The TBM began tunnelling in February. It is currently around 300 metres from the river, with the 1.2 km stretch of the line passing under the river and extending from Greenways Road metro station to Adyar Junction metro station. It will cross the Adyar near Thiru Vi Ka Bridge.

The stretch is part of the 45.8 km Corridor-3 from Madhavaram Milk Colony to Siruseri Sipcot and will require careful consideration of water pressure levels in order to construct a robust tunnel to withstand the surrounding conditions.

To ensure the tunnel's strength, plans are in place to construct the tunnel wall with a thickness of 35cm. If the pressure under the river is higher, the tunnel's thickness may be increased to 40 or 45 cm.

The tunnel boring machines are also equipped to seal gaps between two tunnel segments and rings to prevent leakage. Despite these measures, experts predict that the tunnelling process will be somewhat slow at first.

The machine is currently only partially buried underground, which is slowing down the process. However, once the machine is completely underground, it is expected to pick up speed.

According to CMRL project director T Archunan, the crown (top) of the tunnel will be located seven metres below the river bed. The TBM will tunnel through the Durgabai Deshmukh Road from Greenways Road, and cross the Adyar near Thiru Vi Ka Bridge before reaching Adyar junction, where a station will be built.

According to authorities of Chennai Metro, the TBM should complete the river bed tunnelling by July and arrive at Adyar junction by November.

To retrieve the tunnelling machine Kaveri once it reaches the junction, a portion of the road near Adyar junction has been barricaded, and a shaft will be constructed underground. The trenching process for the shaft construction is scheduled to begin at the site next week, with machinery being set up for the work.

This marks a significant milestone for the Chennai Metro Rail, as the Adyar River section is the first such stretch to be built as part of the phase-2 network.

A team of geologists have conducted soil tests in the Adyar several months prior, and officials have stated that while sand is prevalent under the river, building tunnels underwater may still prove difficult due to the pressure.

Chennai Metro Phase II

The expansion of Metro Phase II is planned with a network length of 118.9 km with 128 stations.

It consists of three corridors, namely Corridor-3 from Madhavaram to SIPCOT (45.8 km), Corridor-4 from Lighthouse to Poonamalle Bypass (26.1 km), Corridor-5 from Madhavaram to Sholinganallur (47 km).

The estimated cost of the Phase II project is‌‌ Rs 63,246 crore.

According to the present route, corridor 3, Purple Line, will feature 44 stations. About 26.72 km will be underground and 19 km will be elevated.

Madhavaram-Sipcot Metro Line

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