Shouta Nomoto
Study on vibration-isolating walls to reduce the ground vibration cased by Shinkansen
Hirofumi Toyota
Although theoretical considerations and analytical study such as FEM have revealed that vibration isolating ditch and wall have some effect to reduce the vibration cased by train, the previous study have not reached the perfect elucidation of transmission mechanism of vibration or most effective material of isolating wall. Therefore, fundamental and experimental data for designing vibration-isolating wall was collected to achieve the purpose of reducing the vibration from Shinkansen. Three types of tests, those are field measurement, element test and model test using soil container, were conducted in the study.
In the field measurement, the peak frequency of vibration could be confirmed and classified during passing of Shinkansen. The frequency band was 5 types: Super low range (4 ~ 5 Hz), Low range (8 ~ 20 Hz), Middle range (20 ~ 30 Hz), High range (40 ~ 80 Hz), and Super high range (100 ~ 200 Hz). In addition, peak frequency depends on the train organization and the measurement location (pier or ground). Moreover, when the train organization is similar, the frequency characteristic depends on the train types.
In the element test, materials of vibration-isolating wall, which are urethane foam, sponge, brass, aluminium, acrylic plate, foaming lightweight mortar and kaolin, were examined. In the conclusion, the vibration damping effect has similar trend with the theory based on the impedance ratio between soil and the material. The urethane is the most effective in the tested material for reducing the vibration.
In the model test, frequency characteristics vary according to the type of foundation and measuring depth. Furthermore, ideal distance attenuation of vibration was not sometimes appeared and the acceleration amplified with distance in z-direction (vertical) vibration. In those cases showing such trend, there is a possibility that surface waves such as Rayleigh waves are generated because acceleration obit shows loops in vertical and horizontal directions. The effectiveness of isolating wall was demonstrated in only limited frequency. In the case of pile foundation, deeper isolating wall showed a higher vibration damping effect in spite of no effect of depth of the wall in footing foundation. This means that when the vibration point is located under the ground, the high reduction effect is obtained by the setting of deeper wall than vibration point.
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