Delhi Metro to build tunnel between two pillars

NEW DELHI: For Metro commuters on the Mandi House-Badarpur corridor (Violet line), intense activity right under their train may come as a surprise. Delhi Metro, as part of the ongoing construction of the Janakpuri-Botanical Garden corridor of phase-III, is building a tunnel between two pillars of the Violet line.

The tunnelling of 176m of this underground tunnel is being done without a tunnel boring machine this time. Also, there won't be two tunnels for up and down tracks but only a single tube tunnel through which both the tracks will pass.

The tunnel is being built using a new technology-new Austrian tunnelling method (NATM)-that excavates tunnels in tight spaces. "The tunnel between Kalkaji Mandir and Okhla Phase III passes below piers 135 and 136 of the violet line. The distance between the two piers is 28m. It would have been impossible to construct twin tunnels within such a short space. Therefore, a decision was taken to construct a single tube twin tunnel using the NATM technology," Delhi Metro Rail Corporation ( DMRC) spokesman Anuj Dayal said. While NATM has earlier been used in the construction of Phase II (Saket) and the Airport Metro line, it's the first time that the technology is being used in Phase III construction.

The up and down lines of the tunnel will be divided by a wall, which will have emergency openings. After the Kalkaji stretch, the underground tunnel of the Janakpuri-Botanical Garden corridor will turn to the regular tunnels. "Tunnelling was required for a short stretch of 176m. It would have been inconvenient and time-consuming to use a tunnel boring machine as separate shafts for insertion and retrieval of the machine would have to be constructed," added Dayal.

The average depth of the tunnel is 20.5m from the ground level and the diameter will be 10.7m. "The tunnel crown is about 9.4m below the base of the piers. For passing below the piers, custom-made bolts are being installed which hold the soil," said Dayal.

Round-the-clock monitoring of the displacement of soil strata is also being done, said DMRC officials. The construction of the tunnel started in September, 2013 and is expected to be completed by June next year. At present, construction of 158m of the 176m has been completed.