Besides using Tunnel Boring Machines (TBMs) to excavate & build tunnels for running metro trains, civil contractors in all metro projects across the world also build cross-passage tunnels for providing passengers with an emergency escape route to the parallel tunnel and for performing maintenance work. These cross-passages are reinforced concrete structures and are typically found at regular intervals of every 250-400 meters depending upon the length of the tunnel & soil characteristics.
The upcoming 38.235 Magenta metro line in Delhi has about 19.7 km of twin tunnels running through west and south Delhi, and as per my estimates, roughly 50 cross-passages are in various stages of completion right now. Usually their construction is taken up only after the tunnels are built to ease working conditions and increase efficiency, but there have been times when work has commenced while tunneling is still in progress. Tunneling is always unpredictable and the time for building these really depends on the soil, water & gas conditions, and sometimes on irate property owners who get concerned about the impact to their buildings above.
Multiple methods exist to build them. Here’s a short informative video of one of them:
Here are some images of one such cross-passage tunnel being built between the Panchsheel Park & Chirag Delhi stations:
For more images & updates on the Phase 3 project, visit the Delhi section of The Metro Rail Guy!