The Delhi Metro Rail Corporation on Wednesday achieved the final breakthrough in the construction of the country’s longest tunnel in an urban area from Talkatora Garden to Buddha Jayanti Park here using the New Austrian Tunnelling Method (NATM).
With this breakthrough, the DMRC is now nearing completion of tunnelling work for the ongoing Phase-II construction as less than 2 km of 35 km is left to be covered.
The 2.85 km-long tunnel from Talkatora Garden to Buddha Jayanti Park is part of the 22.7 km Airport Express Line which will connect the city centre to the international airport.
“The second phase of the Delhi Metro comprises 125 km, of which 35 km are underground. The DMRC used 14 tunnel boring machines (TBMs) which have so far conducted 30 tunnelling drives, besides NATM and the traditional cut-and-cover technique for the underground construction,” said a DMRC spokesperson.
He said that with Wednesday’s breakthrough at Buddha Jayanti Park, the NATM work for Phase-II is now complete. Earlier NATM technology was used was for a short distance of 185 m near Qutub Minar on the Central Secretariat- Gurgaon corridor.
“Only five drives by TBMs now remain in Phase-II to cover 1.3 km, of which 675 m is on the Central Secretariat- Badarpur line and 600 m on the Airport Express Line. Besides this, about 550 m needs to be constructed by cut-and-cover method remain on the Airport Express Line,” the spokesperson said.
The NATM technology was used for the Talkatora Garden - Buddha Jayanti Park stretch as it would have been difficult to use TBMs in the area with the ground comprising of weathered quartzite and schists. NATM technology has been used to build long tunnels in non-urban areas, including for Konkan Railway, but the 2.85 km tunnel is the longest so far in an Indian city.
Unlike other metro tunnelling work where a pair of tunnels is built, the NATM tunnel consists of a single oval shaped bore of 10 m diameter in which the twin metro tracks will be laid later after creating a central wall for track separation. The tunnelling work began in December 2007 with the sinking of three access shafts (deep holes), one at each end of the tunnel and one in the middle.
“After that, controlled explosions were carried out at several locations to break the rock and the pieces were taken out through the shafts using excavating machinery. The tunnelling was extremely challenging as soft soil was encountered in certain sections in between rocky patches. As this increased the possibility of loose earth caving in, extra precautions were taken and labourers were specially trained for the purpose,” the spokesperson said.
It took a total of 5.2 million man-hours, including 0.7 million of engineers and technical staff to complete the tunnelling.
The work was executed by a consortium of an Austrian company Alpine and Hindustan Construction Company of India. In Phase I, NATM technology was used in Chawri Bazaar.
