NEW DELHI: Bringing the city closer to Delhi Metro's second Phase of operations, Delhi Metro Rail Corporation (DMRC) on Wednesday completed construction of the largest tunnel in any urban city in India under the Central Ridge. In order to preserve the rare and diverse flora and fauna of the protected forest, the 2.85km-long tunnel from Talkatora to Buddha Jayanti Park was made using the New Austrian Tunnelling Method (NATM). With this tunnel breakthrough, only 2km of tunnelling is left to be completed for Phase II, in which 35km of the Metro is underground.

The alignment of Metro line for the Airport Express Line goes under the ridge, which has several rare species of trees and animals. To ensure these are preserved, DMRC decided to adopt the NATM method in which a tunnel is constructed by controlled blasting under the earth's surface.

Construction of the tunnel began in December 2007, when three access shafts were dug up one each at the start and end of the tunnel and one in the middle.

Controlled explosions were carried out at several locations to break the rock and the pieces taken out through the shafts. Once the muck was cleared, the tunnel was given a concrete lining. The process was extremely challenging for DMRC as soil conditions were mixed and unknown, and all through the process, the ridge couldn't be disturbed. To ensure safety and keep the earth from caving in, engineers say they moved very slowly and created just about 1.5 metres of the tunnel daily. All through construction, equipment such as 3D targets, extensometers and inclinometers were used to monitor settlements.

"A total of 5.2 million manhours, including 0.7 million of engineers and technical staff, were utilised during the tunnelling,'' said a DMRC official.

The other option available was the cut-and-cover technique, but that would have required clearing the entire surface over the tunnel alignment to dig and create it. This would have destroyed the forest. Due to time constraint, a tunnel boring machine couldn't have been used either as the tunnel had to be created in a record two years, in time for the Commonwealth Games.

Unlike other underground Delhi Metro lines which have two tunnels, one for the train running in either direction, here, there is a single oval-shaped bore in which both the Metro tracks will be laid after creating a central wall for track separation. This large bore makes it the largest tunnel created in any urban city in India.

Delhi Metro's Phase II spans across 125km, of which 35km is underground. DMRC used three techniques for underground stretches tunnel boring machines, NATM and the traditional cut-and-cover method. Now, only five drives by TBMs remain to cover about 1.3km, of which 675m is on the Central Secretariat-Badarpur line and 600m on the Airport Express Line. Besides this, about 550m to be constructed by cut-and-cover method remains on the Airport Express Line.

In Phase-I, NATM technology was used in Chawri Bazaar. The technology originated between 1957 and 1965 in Austria and has been used in many places across the world.