In metropolitan cities, underground railway lines of Mass Rapid Transit Systems are the lifeline to the daily commuters. However, these underground lines cause vibrations while trains move. This ground-borne vibrations may cause damage to heritage buildings and fa & ccedil;ade elements. Humans can feel this vibration, and the comfort of people living nearby is compromised if vibrations cross threshold limit. In the current study, a two-stage coupled analysis is conducted to assess ground-borne vibrations in the free field generated by moving trains in a circular shaped tunnel. Two sub-models are generated-(a) train-track sub-model and (b) tunnel-soil coupling sub-model. The preceding model is a closed-form analytical solution which calculates the quasi-static effect of dynamic interactions between the train wheel and the railway track. The follower model is a 2D FE model to calculate the transfer of dynamic forces from track-tunnel interface to the ground surface through the soil medium. It is found that the computed results fairly match with experimental results for both amplitude and frequency content of the vibration. It is observed that ground vibrations reduce with distance from tunnel and any structure or residents staying beyond 30 m distance would not be affected by vibration as only 25% of vibration is present at this distance. The vibration is found to increase with velocity of train and at soft ground conditions to limit vibration, the velocity of train can be restricted. It is found that the frequency content of vibration is in interference range of human life and critical zone of frequency of structures. Therefore, careful assessment of vibration is required during finalization of the metro project particularly if the ground has shear wave velocity of less than 400 m/s.

Underground train-induced vibrations can cause nearby residents discomfort, damage to buildings, and disturbance for equipment. One of the most effective ways to reduce vibrations is using wave barriers along the propagation path of the waves. Many parameters are involved in determining the efficiency of these barriers: the barrier's dimension, distance from the source of vibration, and material property, to name a few. Simultaneous study of these parameters is complex since numerical analysis of alternatives is time-consuming. Therefore, in this study, by coupling the three-dimensional finite element method and an optimization algorithm, an attempt is made to provide a comprehensive solution to find the optimal wave barriers for Tehran metro line 4 as a case study. The current study evaluates two strategies: using in-filled trenches and topology-optimized barriers. In the first strategy, results show that soft-material trenches with maximum depth close to the observation point have the best performance. Further investigations on jet grout trenches show better performance in stiffer soil and lower train speed. Using dual trenches improves performance only up to 2%, so it does not provide a suitable option. For various practical reasons, there may be no tendency to use soft-material trenches, which perform well in vibration reduction. Therefore, in the second strategy, the improvement of a hard trench (jet grout) performance by topology optimization is investigated. According to this study, topology optimization is an effective method for improving barrier performance.