Estimation of ground vibration and settlement during underground tunneling in Kolkata, India

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A second underground metro line consisting of two parallel tunnels of diameter 6.1 m and 15 m apart is under construction through the busiest commercial areas of Kolkata, India. The crest of the tunnels is located at an average depth of 17.6 m below the ground. The Kolkata subsoil consists of mainly soft silty clays. Three old heritage buildings on raft foundations are located within the near proximity of the metro tunnels. This paper quantifies the possibility of damages to these old heritage buildings due to settlement and/or vibration during the subsurface tunnel construction based on a static and dynamic finite element analysis to satisfy the administration and the people of Kolkata regarding the safety of the surrounding structures during the tunneling and to obtain the required permissions for the work. The results of the static finite element analyses are compared with some well accepted empirical methods to quantify ground settlements due to tunneling of the E-W metro. The numerical results are also compared with the field instrumentation data recently made available. The numerical analyses and the two empirical methods show reasonable match for the settlements near the centerlines of the tunnels. But the empirical methods start to under-predict settlements with distance. Around 20 m from the centerline of 2nd tunnel, the numerical predictions match reasonably well with the measured values of the settlements. However, at 30 m distance, the numerical analyses somewhat under-predict the settlements. Even though it may have some limitations, a proper finite element analysis is strongly recommended over other empirical methods to estimate deformations due to underground tunneling. The settlements and the angular distortions in the three heritage buildings are found to be within tolerable limits. The peak particle velocity at the ground surface, obtained from the dynamic finite element analyses, is found to be about 0.003 mm/s due to the vibration during tunnel construction. This value is significantly less than the allowable value of 1 mm/s. No adverse effect due to the twin tunnel construction has been reported so far.