Keywords: ballasted track, spectral analysis, field measurement, finite element method, elastic vibration mode, rigid-body vibration mode.

Spectral analysis, based on train-related dynamic loads acting on an existing ballast layer which were directly measured at 10 kHz sampling intervals, revealed that dynamic response loads acting on a ballast layer consist of vibration components with a very wide frequency range. The accelerating curve regarding ballast dynamic response indicated that the first-order elastic resonance mode of the ballast aggregate around 650-700Hz in which the whole ballast aggregate repeats the vertical expansion and shrinkage elastically. Finite Element transient response analysis using the ballast aggregate model revealed that the natural frequency of the first-order natural mode which represents the vertical motion of the entire aggregate is around 730Hz in the case of a 40cm thick ballast layer. This analytical result coincides substantially with the measured one. The compliance curve identified the rigid body vibrational mode at around 40Hz at which a mass of the track structure with an additional mass given by a train vibrates simultaneously up and down as a result of the spring stiffness of the ballast layer. A weak roadbed and excessive overburden mass theoretically cause the reduction of the frequency in the rigid body mode. Therefore, it can be inferred that the occurrence of ballast resonance with the passing-axle frequency causes the shift and the flow of ballast gravel.

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