The Delhi Metro is set for the final breakthrough in tunnelling work at Lajpat Nagar on the Central Secretariat- Badarpur line. On Monday the last pre-cast segments (rings) of the underground stretch of Phase-II were erected, following which the final breakthrough will be achieved.

“The rings were erected by the last Tunnel Boring Machine (TBM) between Jangpura and Lajpat Nagar, a distance of about half a kilometre, and the TBM is now poised for the final breakthrough of Phase-II at Lajpat Nagar on the Central Secretariat-Badarpur line,” said a Delhi Metro Rail Corporation spokesperson.

Work involving New Austrian Tunnelling Method (NATM) is already over and only small cut-and-cover portions remain which is expected to be completed in two months. For Phase-II, DMRC used 14 TBMs. “The TBMs conducted 30 tunnelling drives in record time and were used on three corridors: Central Secretariat-Qutub Minar, Airport Express Line and Central Secretariat-Badarpur,” the spokesperson said.

The Central Secretariat-Qutub Minar corridor comprising 12.53 km is almost fully underground and TBMs were used to build 7.15 km of tunnels. Of this, 4.01 km of the Udyog Bhawan to Green Park stretch was built by TBMs.

In the Green Park-Saket stretch of the Central Secretariat – Qutub Minar corridor, two Earth Pressure Balance Machines (EPBMs) were used that completed 3.14 km of tunnel. The average daily rate of tunnelling in this stretch was 14.4 metres.

The 22.7 km long Airport Express Line, of which 15.7 km is underground, consists of 3.8 km built by TBMs. Three TBMs were used to build 2.2 km of tunnel from New Delhi to RML Hospital Circle. One of these TBMs was earlier used on the Green Park- Saket stretch also.

Two more TBMs were used on the Airport Express Line to build 1.6 km of tunnel from Dwarka Sector 21 to Indira Gandhi International Airport. Both the machines were EPBMs and manufactured by Okumura, Japan.

On the 20-km-long Central Secretariat-Badarpur line, the TBM tunnelling was done for a length of 4.2 km. Four EPBMs were employed in different stretches of the corridor from September 2008 to October 2009.

According to the spokesperson, DMRC also employed the New Austrian Tunnelling Method (NATM) at two places during Phase-II. The technique was used to build a 2.85 km-long tunnel between Talkotara Garden and Buddha Jayanti Park on the Airport Express Line, which is India’s longest tunnel in an urban area built by NATM. The ground profile comprised of weathered quartzite and schists.

The other place where NATM was used was for a short distance of 185 m. near Qutub Minar in the construction of the Central Secretariat-Qutub Minar corridor, where the technology was used in soft soil conditions for the first time by DMRC.

“DMRC’s most challenging TBM operation during Phase-II was tunnelling from INA to Jor Bagh as hard unforeseen rock was encountered that resulted in severe damage to some of the cutting tools and caused the cutter wheel to stall in September 2008. After exploring various options, it was decided that replacement of the soft ground cutting wheel of the TBM with a suitable rock cutting wheel was the best option. The work was delayed by about six months but the original tunnel alignment was maintained and the tunnel completed as designed and within tolerance,” the spokesperson said.

During Phase-II, DMRC constructed its deepest tunnel so far which passed below the existing tunnel of Central Secretariat – Vishwavidyalaya line at Rajiv Chowk. The depth of the new tunnel, which is part of the Airport Express Line, is 45 metres and it is passes 20 to 25 metres below the old tunnel.

About 28 per cent of Delhi Metro’s Phase-II is underground, i.e., 35 km out of 125 km, up from 20 per cent in Phase-I in which 13.17 km out of 65.1 km was underground.

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