Boxes, each the size of three train coaches, are being pushed through the soil to create a Metro Rail tunnel at the city airport.
The 14 boxes constructed of concrete and reinforced cement are being used to build the 135-metre tunnel in the last leg of the New Garia-Airport Metro corridor (Orange Line).
Engineers involved in the project said the “box-pushing” method is being used for the first time to create a Metro tunnel in Calcutta.
This method is used to make small tunnels under railway tracks and busy roads without disrupting traffic on the ground.
“It entails the construction of 14 boxes at a point near the site. The boxes are being cast and then pushed forward with the help of hydraulic jacks. As the boxes move forward, a cutter, fixed at the head of each box, excavates soil to make way forward,” said an official of Rail Vikas Nigam Ltd, the implementing agency of the New Garia-Airport line.
Each box is 14.85m wide, 7.35m deep and 9.6m long or 1,047 cubic metres. The standard size of an LHB coach used in a long-distance train of Indian Railways is 308 cubic metres.
The tunnel is being built after the line enters the airport premises, turning right from VIP Road. It starts near the airport flyover and goes up to the Y-junction of the airport station near the post office on the approach road to the terminal building.
The Y-junction is an interface between the Orange Line and the Noapara-Airport corridor or the Yellow Line and is about 400m from the Biman Bandar station, said officials.
Metro officials said that until now, two tunnel construction methods have been followed in Calcutta.
The cut-and-cover method was first used to build the north-south line in the 1970s and 80s. It involved excavating a trench, constructing the tunnel structure and then covering the trench again.
Tunnel boring machines are now being used in several projects like the East-West and Joka-Esplanade Metros.
The method involves excavation for tunnels with a circular cross-section through various types of soil with the help of tunnel-boring machines. This process does not disrupt traffic on the ground.
“None of these two methods could be used in this stretch,” said an RVNL official.
He said vehicles cannot be stopped on the road because it is the main approach to Calcutta airport’s integrated terminal building. Also, tunnel boring machines cannot be used for such a short length of construction.
“Box-push tunnel construction was the only viable method here,” said the RVNL official.
A similar box-pushing method was used to construct a pedestrian underpass at New Town. The underpass was built at Nazrul Tirtha, said an official.
At the airport, the tunnel will be completed in 18 months. Construction started at the beginning of this year.
“The thrust bed created for the construction of the boxes is 50m long. We can cast four boxes at a time. Till now, four boxes have been cast. Casting the fifth is underway,” said the RVNL official.
It takes about 25 to 30 days to cast one box.
A shear key pocket supports the hydraulic jacks, giving them the thrust needed to push the boxes forward. Ramps in the jack protrude out and start pushing a box in the front.
Box-pushing method takes more time than tunnel-boring machines or the conventional cut-and-cover method does. But for short stretches where traffic cannot be disrupted, box-pushing is the ideal method, railway engineers said.
The technology was earlier been used in railway projects, including the Bengaluru Metro.
Trains in both directions, towards the airport and towards New Garia, will run through the same 135m tunnel once it is ready.
In the tunnels constructed with the help of TBMs, trains run through two circular tunnels, connected by cross passages at intervals.
“Here, there will be one tunnel for two trains,” said the official.
The 30km Orange Line between New Garia and airport was originally supposed to be wrapped up by 2016-17, but it got delayed because of land logjams at several places.
Now, most of the land problems have been resolved. The line is now functional on a 5.4km stretch between New Garia and Ruby.
Trial runs are underway in the next 4.4km stretch from Ruby to Beleghata.
Construction is in progress from Beleghata to the airport, a distance of over 19km. The current target to complete the project is March 2026, officials said.
The construction of the 4.46km Beleghata-Sector V section is almost complete except for the 366m gap in the viaduct on the Bypass at Chingrighata and a pedestrian crossover point at Beleghata station, said officials.