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  Case Study on Metro Tunnel below the river Ganges in Kolkata, India

  The Ganges is considered as one of the most influential rivers in Indian peninsular and some great Indian cities are historically built in its bank. In line with the ever growing trend of global urbanization, population in Indian cities like Kolkata on the bank of river Ganges are increasing in rapid proportion and it becomes essential to develop modern mass transportation facilities in these cities. Twin bored tunnels of Kolkata East West Metro Corridor has been built below the river Ganges to connect Kolkata with the city of Howrah on other bank of the river at a depth of approximately 13m below river bed. The 5.55m internal diameter bored tunnels with RCC segmental liner has been constructed using Earth pressure balancing tunnel boring machines. This is the first transportation tunnel project in India under any mighty river. The river crossing width is 520m and the depth of water at average tide is about 13m. This ambitious initiative involved meticulous planning and investigation well in advance in view of prevalent juvenile alluvial geology in this Ganga- Brahmaputra deltaic region. The sub-soil geology in tunneling horizon as well as face pressure requirement below the river dictated proper choice of tunnel boring machine. An elaborate field instrumentation program was established for monitoring of cardinal tunneling parameters . Protocol for sufficient and full proof precautionary measures were ensured before launching of the TBMs inside the river bed. The tunneling parameters achieved during this uninterrupted successful tunneling below the river is compared with design parameters. This paper covers Geomorphology of Kolkata & , Geotechnical investigation below the river, hydrological data collection & its impact on alignment of the tunnel under river and choice of tunnel boring machine, Water proofing requirement of tunnel in construction and O&M Stage, Precautions taken during tunneling below river, Instrumentation for monitoring of important tunneling parameters and a comparison of theoretical and actual tunneling parameters during tunneling below river.

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