This was the first time when the Metro Engineers have used the 'New Austrian Tunneling Method' (NATM) on "soft" soil as the technology was earlier used on hard rocks in Chawri Bazar and the Ridge area.

The Tunnel Boring Machine (TBM) was not used here as the distance was small and lowering the TBM near the ASI protected monument could have been difficult and very expensive, a Delhi Metro spokesperson said.

The 185 metres NATM tunnel breakthrough achieved at the South end of Saket Metro station is a part of 1.85 kms Qutub Minar-Saket stretch which also covers a cut-n-cover underground tunnel of 1,020 metres and 645 metres elevated section.

"The work in this area was difficult as the DMRC had to divert a 247 metres long, six meters wide and five meters deep old natural storm water drainage which was flowing over the alignment of NATM tunnel," the spokesperson said.