New Delhi, Oct 22 (UNI) In a breakthrough in the ongoing construction of the Vishwavidyala - Jahangirpuri corridor of Phase-II of the Delhi Metro, the Delhi metro Rail Corporation today accomplished the construction of a tunnel below the bed of the Najafgarh Nala (Drain), a part of the 6.36 km corridor.
''This is the first time that the Delhi Metro is being constructed below a water body,''DMRC spokesperson Anuja Dyal said.
The final tunnel breakthrough in its construction which started in May, took place today when the last portion of soil that separated tunnels being dug from both sides was finally completed.
The top of the tunnel is about 0.5 metres and its base is 6.7 metres below the bed of the drain. The total length of the tunnel in the Vishwavidyalaya - Jahangirpuri section is one km of which 40 metres is directly below the drain.
The construction of the Delhi Metro below the Najafgarh drain, was a challenge, as the tunnel had to be constructed below the bed of the drain and a bridge above it, thus making it a three-tier crossing.
Two temporary steel bridges were built above the drain so that traffic would not be blocked. The flow of the drain was also not stopped at any time, as this would have caused inconvenience for the public of Delhi.
For this purpose, two-thirds of the drain was first diverted by sand filling. The Najafgarh Nala, a storm water drain, is 50-60 years old. As harmful gases such as methane gas are released, special precautions were taken for the safety of the workers.
After the diversion, side walls, technically called 'diaphragm walls', were built by excavating soil by a special 'grab machine' from Italy.
A roof slab was then constructed and waterproofing done by using PVC-based waterproofing membrane and reinforced cement.
Thereafter, construction of the tunnel was carried out by cut-and-cover method and the entire excavation of soil below the drain was completed in a record 30 days time.
The original flow of the drain has since been restored and the water now flows above the Metro tunnel. The original road above the drain will be restored within three months.
The 6.36 km Vishwavidyala - Jahangirpuri corridor is scheduled to be opened in October 2009 and construction is progressing as per schedule.