The underwater tunnel below the bed of Hooghly River connect two iconic Indian Railways stations – Howrah Junction and Sealdah which is a result of precise planning and meticulous execution. The twin tunnels were built in just 67 days while the budgeted time for tunnelling the 2.9km stretch was 127 days. Once operational, the service will revolutionise travel in Kolkata.
Underwater tunnelling has been done across the world, and with considerable success. IIndia will soon get its first underwater metro rail tunnel by 2021! Kolkata Metro- India’s oldest metro rail network, will soon have an European-style underwater tunnel as part of its East-West Metro project. The project is overseen by Kolkata Metro Rail Corporation Ltd (a government of India entity) and executed by Afcons Infrastructure Ltd.
There were several challenges when the work for a project of this magnitude had been taken up. Detailed investigations, case studies, planning, design considerations and logistic arrangements had to be worked out before the tunnel boring machines (TBMs) could plunge below the river. Tunnelling, be it underground or underwater it is very challenging due to the risk of water ingress and impact of topography. The bore has to be dug deeper due to possibility of water inflow which requires high level of detailing and preparation.
However, it is the alignment of the project which pass through a precarious topography necessitating tunnel in the vicinity. The geology of Kolkata and Howrah is erratic, especially the topmost soil is soft and weak. The settlement prone sensitive topography made the team face extreme engineering challenges.
At the outset, suitable designs and technologies had to be selected for a successful delivery of the project. The designing was entrusted to specialised world-class design companies such as Atkins, Systra and Tunnelconsult. The team chose Herrenkeneckht (HK) Tunnel Boring Machines (TBMs) for this project based on their track record in manufacturing machines which are tailor-made for specific project requirements. HK is also engaged in maintenance of the two TBMs deployed for the Kolkata Metro project.
KMRCL carried out soil investigation work at the pre-tender stage for feasibility study. In addition to this, Afcons also carried out extensive soil investigation work for the entire tunnel alignment, deploying floating barges and pontoons in the river portion. The company also conducted extensive condition survey of the buildings in proximity before carrying out tunnelling to ensure there were no untoward surprises during tunnelling as well as to map out the safest layer below the riverbed for tunnelling.
The Project incorporated the world’s latest and best practices in design and implementation processes thereby saving in time, cost and space.
Afcons engineers improved upon numerous processes while planning the project. Due to site constraints, the TBMs had to be driven through Howrah station in advance. This was a delicate operation, where they encountered difficulties in crossing the diaphragm walls. Afcons designed a ‘soft eye’ of styrofoam replacing M-40 concrete, which worked extremely well for this project. This is a first design-of-its kind attempted in India by in-house design engineers. Certain criteria in the segmental lining were adapted and additional operational precautions and checks were implemented to prevent any untoward incidents.
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.
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.
• 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’