Know about the technique that led to the fulfillment of India's first underwater Metro tunnel in Kolkata

Here's the buzz: Afcons Infrastructure has achieved a landmark by successfully boring 2.9-km twin tunnels below river Hooghly in the East West Metro Project, in Kolkata! One of the company's biggest achievements in the area of underground metro infrastructure has been the successful completion of tunnelling for India's first underwater Metro project. The tunnelling was completed in just 67 days against the budgeted 127 days, which attests to the company's meticulous planning and efficient execution. These tunnels connect two of the world's busiest railway stations, Howrah and Sealdah. Once operational, the service will revolutionise travel in Kolkata, the first city in India to have an underground metro. Taking stock While tunnelling has been done under rivers across the world, and some projects have met with success, Afcons was faced with several challenges when it took up a project of this magnitude. Detailed investigations, case studies, planning, design considerations and logistic arrangements were carried out before the tunnel-boring machines (TBMs) could plunge below the river. What made the East West Metro Project even tougher was the alignment. It passes through challenging topography, necessitating tunnelling in the vicinity or directly below dilapidated buildings, railway offices, a busy and packed railway yard, busy roads, bridges and flyovers and heritage monuments. The geology of Kolkata and Howrah is erratic, especially the topmost stratum, which is soft and weak, adding to the challenge. Precise planning The engineers focussed on selecting suitable designs and technologies for the successful delivery of the project. The designing of this project was entrusted to specialised world-class design companies such as Atkins, Systra and Tunnelconsult. Afcons chose Herrenkeneckht TBMs for this project, based on its track record in manufacturing machines tailor-made for specific project requirements. HK is also engaged in the maintenance of the two TBMs deployed for the Kolkata Metro project. While Kolkata Metro Rail Corporation (KMRCL) carried out soil investigation work at the pre-tender stage for feasibility study, Afcons carried out soil investigation work near the entire tunnel alignment, deploying floating barges and pontoons in the river portion. The company also conducted extensive surveys of the buildings in proximity to the alignment before carrying out tunnelling. Unique challenges While executing this project, the tunnel crew faced the unique problem of the TBMs moving through soft clay strata at a lesser depth over structures that are almost a century old. Some of them are ravaged to the extent of being dilapidated. Apart from the main Colvin Court building, the TBMs passed below the quarters for service staff, which are in poor shape. These structures house high-ranking Railways officials. Another issue of concern was mining below Howrah Railway Yard, one of the busiest railway yards in the country with over 670 running trains and a footfall of 10.5 lakh daily. Boring below these structures was a herculean task. Mining had to be done over 350 m below the yard for each tunnel. The TBMs had to negotiate a sharp curve of radius 228 m, plunging into a new geology (stiff clay) and passed by the Bankim Setu piers. There are only a few comparable examples of such a tight curve undertaken in India. One more curve of 226 m radius was achieved near Rajbhawan before Esplanade Station. Obtaining CRS approvals for the project was another gigantic task, and this was achieved after satisfactory closure of all requirements. The procedure was hectic, but it helped gain several insights into project management during the tunnelling operation. Key innovations Afcons deployed a highly experienced tunnel crew on board, should an entry into the cutting chamber be required under high hyperbaric pressures. In the remote possibility of river water ingress, the TBMs could shut down like a submarine for safe evacuation. A project of this scale involves a number of innovations and the engineers improved upon numerous processes while planning the project. Owing to site constraints, the TBMs had to be driven through Howrah station in advance. This was a delicate operation, where they could encounter difficulty 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-of-its-kind attempted in India by in-house design engineers. Relief measures Special efforts were made to ensure that there was no loss of life and property during the TBM operations. More than five teams were deployed to carry out extensive repairs in all critical buildings. There was no compromise on safety, with adequate traffic diversions, restricted pedestrian movements and timely evacuations. About 300 people were relocated to hotels during tunnelling operation near their premises, and were brought back safely to their homes after the task was completed. This gesture won Afcons accolades from the public, government authorities, and others, and set new standards in project management. Last-minute risks This project has been a learning curve for Afcons. Underground tunnelling is in itself a challenging task, but underwater tunnelling is far more complex and risky. The chances of water ingress are high and so are the possibilities of geological surprises owing to the impact of tunnelling. Once the tunnelling began under the riverbed, the team ensured it did not stop under any circumstances. In underwater tunnelling, the bore had to be dug much deeper owing to the possibility of water inflow. This required a high level of detailing, precision and preparation. The meticulous planning bore fruit with the TBMs crossing the river seamlessly covering a distance of 11-12 m daily. About the author: Virender Kaul is Project Manager of East West Metro Project, Kolkata. Share your views on this article at feedback@ConstructionWorld.in