Keywords: ballasted track, measurement, spectral analysis, natural frequency.

Dynamic loads acting on a ballasted track under traffic loads were directly measured at 10 kHz sampling intervals. Spectral analysis is performed to determine the dynamic characteristics and the natural mode of the ballast layer. The results indicated that broadband vibration components act on the ballast layer, which exhibits two different types of behaviour depending on the frequency. For vibration components in the high-frequency region, the layer has high rigidity, resists dynamic loading sufficiently and absorbs impact loading adequately. However, in the lowfrequency range, the ballast layer easily non-elastically deforms and is hard to absorb load components. The acceleration curve regarding the ballast dynamic response indicates the first-order elastic resonance mode of the ballast aggregate at the high-frequency domain of several hundred Hertz at which the whole ballast aggregate repeats vertical expansion and shrinkage elastically. The compliance curve identifies the rigid body vibrational mode at the low-frequency region beneath 100 Hz 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 stiffness of the ballast layer. Finite element transient response analysis using a fine ballast aggregate model reveals that the dynamic stress acts and concentrates only on an angularity part of the ballast gravel. The stress acting on an angularity part is inferred to be about 1100 times greater than the average stress on the ballast surface.

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