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Ground-borne vibrations generated by underground railways can impact nearby structures, residents, and passengers. Sometimes, resulting vibrations can be the subject of legal complaints from the owners of buildings in the immediate vicinity, which can be a serious concern. Therefore, with execution of any underground railway project, it is equally important to ensure its sustainable and harmonious coexistence with surroundings. Wave barrier, through absorption and dispersion of wave energy, can be an effective measure to confront such kind of challenges. Wave barriers have been used as an engineered solution for mitigating vibrations from surface railway. However, its potential for the case of underground railway needs to be established.
In this research, two-dimensional numerical studies of vibrations generated by underground railway have been conducted using Finite Element software PLAXIS. Results have shown that ground surface vibrations for underground railways can exceed the residential tolerance limit in the frequency range of 30-70 Hz at close distances. Extensive parametric studies have been conducted to explore the performance of different wave barriers made of geofoam (EPS15), rubber chips, soil bentonite, and concrete. Vibrations are presented in decibels (dB) in one-third octave frequency bands. Influence of barrier dimensions and locations as well as train speed, tunnel depth, soil properties have been considered.
Geofoam barriers have emerged as the most promising solution showing substantial vibration reduction of 8-10 dB in soft soil and 5-6 dB in stiffer soil within the frequency range of 30-70 Hz. Barrier width of 0.5 m or more is adequate, while barrier depth varying from dt-2m to dt+5m is found to be effective where dt is the depth of tunnel bottom. Barriers need to be located at a distance of 7 m from the tunnel edge or more. Increased soil damping is also beneficial. However, for rubber chips and soil bentonite barriers, effectiveness is found only for very few cases. Concrete barriers were found to be effective only in very soft soil requiring wider and deeper barriers. Multiple barrier arrangement for these barriers enhances their performance.
As a case study, vibration analysis at a particular site near proposed MRT Line 1 in Dhaka city was carried out. It has been observed that geofoam barriers may be effective in reducing vibrations in stratified soil. This study would be a significant contribution to available literature on subway induced vibrations and its mitigation, bearing local significance in the context of upcoming underground metro in Dhaka city. |
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