Kolkata, located on the bank of River Ganges in India, is considered to be one of the most densely populated cities in the world. It has an underground metro rail connection cutting across the city running predominantly in the north-south direction. Another underground metro rail connection is being built in the east-west direction connecting the heart of the city to the railway stations and the airport. As a part of the project, twin tunnels of diameter 6.1 m and 15 m apart are being constructed below the Ganges and through the most congested part of the city. The tunnels are located at a depth between 17 m and 24 m below the ground surface. The soil layers in Kolkata essentially consists of soft clays (clayey silt to silty clay) deposited by the river with layers of dense sand at much greater depth. Several old heritage structures have been identified along the route of the E-W metro. These structures are located between 19 m and 33 m from the centerline of the tunnels. Damage to these heritage buildings during the construction of the tunnels due to excessive settlement and/or vibration is a concern. Both static and dynamic finite element analyses have been performed to estimate the settlement and the vibration at those heritage structures due to the tunnel constructions. The results of the analyses are compared with the reported case histories of tunnel constructions worldwide on soft soils and with the empirical methods proposed by Mair et al. and FHWA to estimate the ground settlement due to the proposed tunnel construction.
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