- Joined
- Jan 27, 2012
- Messages
- 12,835
- Likes
- 7,762
CHENNAI: You take a train from Chennai Central to airport, a commute of 20 minutes. Within seconds, you will be darting at 80kmph, about 30 metres under the bed of the historic Cooum.
Welcome to metro rail of 2015, when commuters will plunge into the depths of Chennai, literally, untouched by the traffic over their heads. Till then, witness this unprecedented engineering feat - the making of twin tunnels on two corridors.
In a few months, giant tunnel boring machines will be quietly burrowing underneath the Cooum. "Chennai will soon see some engineering that has been done in a few cities abroad," said V Somasundaram, chief general manager, construction, Chennai Metro Rail Ltd. He was speaking at a function in SRM University, Kattankulathur.
The underground line from Washermanpet to Saidapet will be at its deepest point of 30.8 metres as it passes under the Cooum and the Beach-Egmore railway line opposite Ripon Buildings.
The line that comes from the high court area reaches metro's Central station in front of Ripon Buildings, turns left, crosses the railway line and the river and passes underneath the neighbourhoods of Chintadripet to reach the underground station at the new secretariat on Anna Salai.
Metro tunnels are made without cutting open the ground. Giant machines will bore the tunnels as the river flows above and suburban trains continue to operate above the surface. Traffic along Anna Salai and Poonamallee High Road will not be diverted during the boring. Two tunnels come one above the other near Central railway station. The Central-Anna Nagar route tunnel will be 23 metres below the surface, while the Washermanpet-airport route tunnel will run 28 metres under the ground.
Custom-made tunnel boring machines costing 60 crore each have been imported from Germany for the purpose. These machines have different types of cutting head suitable for sandy, slushy and rocky soil. Metro will use closed-face tunnelling machines because of the city's peculiar soil and water table.
"Rocks are found about 20 metres below Poonamallee High Road and the Cooum near the Central station. Machines suitable to cut across the rocks will be used to bore tunnels under the river," said the official.
Boring without cutting open the surface is going to be an engineering challenge because on some stretches the tunnels run under crowded neighbourhoods close to the foundations of highrises like LIC, and flyovers.
As the machines bore, air pressure and slurry generated will shore up the earth from collapsing. A sensor in front of the machine will continuously monitor soil and water pressure. As the earth above the tunnels is going to be balanced on the strength of air pressure, the tunnel boring machine is also called earth pressure balancing machine.
After drilling, the machine will install a ring made of reinforced concrete. But this will not prevent seepage. "Chennai has a water table at two metres for 60% of the year. So we plan to collect the seepage and pump it out of the tunnels. The alignment of the tunnel will dip a little to make it concave so that water will collect at one point on some stretches," said the official.
Metro rail plans to import 11 tunnel boring machines for drilling twin tunnels on two corridors. Boring will start at Washermanpet and the machine will proceed towards Manali, High Court, Central and then to Anna Salai. Another set of machines will be lowered at Shenoy Nagar. These will bore underneath Poonamallee High Road towards Tirumangalam.
"The Little Mount-Airport stretch will be completed by 2014, while the rest of the corridor will be completed by 2015," said Somasundaram.
Metro dips under historic Cooum - The Times of India