A worker stands inside one of the Metro tunnels under construction in New Delhi, India, in preparation for the Commonwealth Games this October. To overcome the challenges of a tight three-and-a-half-year schedule and construction underneath a densely populated city, engineers used 14 tunnel-boring machines (TBMs) to dig the underground thoroughfare.
One of these machines completed the final subterranean stretch last fall, covering roughly 1,640 feet between two stations on the Central Secretariat-Badarpur line. The TBMs averaged nearly 27 feet a day as they cut through the Delhi silt; while they tunneled, the machines simultaneously installed four-foot-long precast rings that were bolted together to prevent the walls from collapsing.
Before the tunnel can be used, workers will fill and flatten the bottom of the tunnel to lay tracks later on and install electrical connections and ventilation shafts that will extend to the surface.