Less than 2 km of 35 km left after the final breakthrough was achieved today in the construction of India's longest tunnel in an urban area.
DMRC is nearing completion of tunnelling work for Phase-II, with less than 2 km of 35 km left after the final breakthrough was achieved today in the construction of India's longest tunnel in an urban area.
The 2.85 km-long tunnel, built using an innovative technology called New Austrian Tunneling Method (NATM), 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, DMRC spokesman Anuj Dayal said.
The second phase of the Delhi Metro comprises 125 km, of which 35 km is underground. DMRC used 14 tunnel boring machines (TBMs) which have so far conducted 30 tunnelling drives, using NATM and the traditional cut-and-cover technique for the underground passage construction.
The other place where the cutting edge technology was used was for a short distance of 185 mts near Qutub Minar in the construction of the Central Secretariat Gurgaon corridor.
"Only five drives by TBMs now remain in Phase-II to cover about 1.3 km, of which 675 mts is on the Central Secretariat Badarpur line and 600 mts on the Airport Express Line," Dayal said.
DMRC decided to use NATM technology for the Talkatora Garden-Buddha Jayanti Park stretch as employing TBMs would have been difficult because of the rocky profile of 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 10m 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, the DMRC official said.
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 task.
Of the total length of 2.85 km, tunnelling of a small distance of 180 mts was done by traditional cut-and-cover method in which the entire surface is cleared and a cavity created from the ground level itself.
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. The technology was developed between 1957 and 1965 in Austria and has been used in many places across the world.
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