There will be a total of 73 tunnels that will be built along the 35km stretch underground metro line. The tunnelling work between Central Secretariat and ITO on the 9.37 km long Central Secretariat-Kashmere Gate corridor has been completed.
There are other areas that are part of this project. From the South Extension there is going to be one to Lajpat Nagar and from Sarojini Nagar there is going to be one to INA. Apart from this there are going to be two more launched from Bhikaji Cama Place to Sarojini Nagar.
See the stretch between Bhikaji Cama Place and Lajpat Nagar which will be an underground stretch will have a total of eight Tunnel Boring Machine which includes the two each from INA to South Extension.
Now the exact full specifications are not available, however, it is said that the DMRC will install 6145 precast rings in the full length of the tunnelling drive which will be for 4.299 km. And yes, the internal diameter of the tunnel will be of 5.8meters and the width at 1.4meters. Hope this answers your question.
I hear that the tunnel will cover a distance of 924m and it has been started at INA with the first Tunnel Boring Machine. The first drive once the machine is installed is expected to start in November.
The work has started catching momentum for the Delhi Metro phase III. Recently the DMRC announced that the underground tunnelling has been started for the Metro between INA and South Extension. This is on the Mukundpur and Shiv Vihar corridor.