Execution
A team of highly experienced tunnel experts was deployed, should an entry into the cutting chamber be required under high hyperbaric pressures river.
The tunnelling alignment was such that it passed through several heritage structures like Howrah Rail Yard, which is one of the busiest rail yards in the country with over 670 trains running daily and a footfall of over 10 lakh. Boring below these structures was precarious, and tunnelling was done more than 350m below the yard for each tunnel. The TBMs had to negotiate a sharp curve of radius 228m, plunging into clay surface and then pass by the piers of Bankim Setu. One more curve of 226m radius was achieved near Rajbhavan before Esplanade station. There are very few examples of such tight curves being undertaken in Metro tunnelling in the country. Despite the constraints, the tunnelling activity began as per schedule, with the two TBMs covering roughly 11 to 12 metres in a day.
Safety measures
Another key factor in this project was safety. Special efforts were undertaken to ensure there was no loss of life or property during tunnelling. The project team painstakingly obtained all the necessary CRS approvals after satisfactory closure of all requirements. While this was a tough task, it helped the team gain several insights into project management.
Adequate traffic diversions, timely evacuations and restricted pedestrian movements were undertaken. More than five teams were deployed to carry out extensive repairs in all critical buildings along the TBM alignment. More than 300 people were relocated in alternate accommodations during the operations and then safely brought to their homes setting new and humane standards in project management.
Afcons is now constructing India’s deepest Metro station at Howrah. At 30m below surface level, once ready, the multi-level Howrah Metro station will cover the area of over five lakh square feet. The diaphragm wall for the station, which at 1.5m, is the thickest to have ever been built in India. The depth of the D-wall is 46m, which is the deepest in the country so far. Recently, the base slab of the Howrah Metro Station was constructed, marking a major milestone in the project. At 30m deep, it is almost equivalent to a 10-storey building, below the historic Howrah Railway Station. The Metro station runs so deep because it is aligned with the underwater Metro tunnels. The tunnels are almost 26m below the Hooghly riverbed.
Salient Features
• The twin tunnels were built in just 67 days while the budgeted time for tunnelling the 2.9km stretch was 127 days.
• At 30m below surface level, once ready, the multi-level Howrah Metro station will cover the area of over five lakh square feet.
• Afcons designed a ‘soft eye’ of styrofoam replacing M-40 concrete, which worked extremely well for this project.
• At 30m deep, it is almost equivalent to a 10-storey building, below the historic Howrah Railway Station. The Metro station runs so deep because it is aligned with the underwater Metro tunnels. The tunnels are almost 26m below the Hooghly riverbed.
Photo credit: ‘Afcons Photos’
