In a first, Delhi Metro will build a single-tube tunnel which would pass under operational elevated tracks between the Kalkaji Mandir and Nehru Place stations as part of works for its third phase of construction.
DMRC had to shelve the conventional method of using Tunnel Boring Machines (TBM) for digging the 176-m tunnel so as to enable the existing overhead tracks to run smoothly.
The tunnel between the Kalkaji Mandir and Okhla Phase-III stations on the upcoming Janakpuri West-Botanical Garden corridor is being constructed by Delhi Metro Rail Corporation (DMRC) by way of manual excavation.
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DMRC chief spokesperson Anuj Dayal said the decision to go for manual excavation, also known as New Austrian Tunnelling Method (NATM), was prompted by the shortage of space and unfeasible local soil and rock structures which are unsuitable for insertion of a large TBM.
"We could not go for the TBM method or build twin tunnels like we did for the existing tunnels as the tunnel is passing through a narrow corridor between two pillars of the operational elevated track. The local rock and soil structures are not feasible either (for such tunnelling)," he said.
Instead of building two separate tubes, the NATM method involves construction of a single tube of around 10-m diameter in which the two metro tracks are laid.
Construction has been completed for a length of 158-m of the tunnel after the work was taken up in September last year. The work is expected to be completed by June of next year.
DMRC officials said that three more single-tube tunnels will be constructed at Azadpur, Netaji Subhash Place and Lajpat Nagar on the Majlis Park-Shiv Vihar section under the already operational elevated corridors of Line 2, 1 and 6, respectively.
Previously, the NATM method was used at Chawri Bazar, Budhha Jayanti Park and Saket.
The progress of work on the Phase-III network of DMRC, of which the under-construction tunnel is a part, has slowed down owing to difficulties in land acquisition.
Recently, DMRC Managing Director Mangu Singh wrote to Delhi Chief Secretary DM Spolia seeking his intervention in the matter which may result in DMRC missing the project deadline of 2016.
