TBMs take up deepest under surface digging in Mumbai

The twin tunnel under Malabar Hill will perhaps be the most onerous to construct as connecting the starting point of the Coastal Road on Marine Drive to Breach Candy where the underground tunnel portion would be 1.9km, including the entry and exit stretches where the length would be 3. 4km. As it is going to be a set of two tunnels, the effective tunnel length would be 6.8km, with a cumulative 3.8km being underground. Among the deepest under surface structures in the city, the tunnels will be 20-25 metres below Girgaum Chowpatty and 70-75 metres below Malabar Hill. An interesting point is that the Malabar Hill depth is relative to the height of the hillock. The correct depth parameter of any structure is with respect to mean sea level, or MSL. Thus, throughout the stretch, the tunnel will maintain a below-MSL depth of 6-13 metres. It is very challenging constructing the tunnels as at most locations through there is hard rock. Tunneling will include use of TBMs and the cut-and cover method, which involves digging a trench with excavators, constructing the structure, and covering the trench.