Chennai Metro Corridor 3: Adyar River Tunnel Work Hampered By Geological Setbacks

Chennai Metro Rail Limited (CMRL) is experiencing significant delays in its Adyar River tunnel project, a critical segment of Corridor-3, due to challenging geological conditions.

The tunnel boring machine (TBM) 'Kauvery,' which recently tunnelled beneath the Adyar River, has encountered mixed ground conditions consisting of both hard rock and earth, significantly slowing down the construction progress.

CMRL has deployed multiple TBMs across various parts of the city to construct the underground sections of the Phase II Project. For the stretch between Greenways Road and Adyar, CMRL engaged two TBMs, ‘Kauvery’ and ‘Adyar.’ These machines were launched from Greenways Road towards Adyar early last year.

Recently, ‘Kauvery’ successfully tunnelled through the Adyar riverbed and continued drilling towards the under-construction Adyar Metro station near The Theosophical Society.

CMRL officials reported that ‘Kauvery’ has encountered ‘mixed ground conditions’ containing both earth and hard rock, specifically charnockite (a type of metamorphic rock).

"The machine has to drill through hard rock. If it was only rock, the intervention required would be less. But the main issue is that it’s drilling through both hard rock and earth, which has been challenging as we have to constantly change the TBM’s digging tools," an official explained.

During the soil investigation, officials anticipated encountering hard rock but not mixed ground conditions at this location. The machine is currently drilling about 1.5 metres per day, with workers frequently changing the TBM's tools, according to The Hindu report.

"Usually, we do not have to repair the disc cutter often. But in this case, we have had to do it multiple times and use other special types of cutting tools," another official added.

Meanwhile, the second TBM, ‘Adyar,’ is still tunnelling underneath the river and is expected to cross it later this month.

During the planning stage, the water pressure in the river is taken into consideration. The tunnel wall will have a thickness of about 35 cm, but if the pressure beneath the river is higher, the thickness may be increased to 40 cm or 45 cm.

To prevent water seepage, the tunnel boring machines also waterproof or seal the gaps between segments and rings in the tunnel, as explained by R Ramanathan, a former official of Chennai Metro Rail Limited (CMRL).

Chennai Metro Phase II

Phase II comprises three corridors: Madhavaram to SIPCOT (Corridor-3), Light House to Poonamallee (Corridor-4), and Madhavaram to Sholinganallur (Corridor-5).

According to M A Siddique, the managing director of CMRL, Phase II, covering a distance of 116.1km, is expected to be opened to the public in stages between 2025 and 2028. The estimated cost of the Phase II project is‌‌ Rs 63,246 crore.

Three-and-a-half years after the commencement of construction on the Chennai Metro Rail's Phase-II project, only 20 kilometres of the network have now been completed.

This includes a 10.77 km tunnel and a 10 km elevated section, linking at least 25 localities across the city and suburbs.

Phase II of the metro project will include the construction of tunnels not only under the Adyar River but also under the Chetpet Lake.