Keywords: ballasted track, field measurement, impact load, wave propagation, attenuation, spectrum analysis.

A ballast structure consists of compacted ballast rocks on a subgrade. The ballast layer has the important functions of distributing the immense load from the train traffic throughout the sleepers and of damping the dynamic loads. These important functions are accomplished through contact mechanisms that take place among respective ballast rocks. Because the frequent passage of trains loosens and deforms the ballast and the supporting bed through track deterioration, the ballast requires periodic maintenance, which is an important subject of technical research. As described in this paper, the author performed a fundamental examination of the propagation characteristic and the attenuation characteristic of ballast aggregate for traffic impact loads based on in-situ field measurements.

Ballast grains transmit an impact load to a roadbed through multi-contact loading conditions in the boundary layers separating individual ballasts. In a ballasted track, the occurrence of plastic deformation that progresses by the movement and wear of ballast grains is affected by the transmission characteristics of the impact load within the ballast grain aggregate. According to in-situ field measurements using the sensing sleeper and sensing stones, a fundamental examination was performed of the propagation characteristics and the attenuation characteristics of ballast aggregates for traffic impact loads. Application of spectrum analysis in a frequency domain to the measurement results confirmed that dynamic load components not only of the 10-30 Hz low-frequency band of the passage of an axle load but also those over 100 Hz of higher frequency for the natural-frequency modes of a sleeper acting on a ballast layer. Moreover, for impact-load components over 100 Hz, the ballast layer provides resistsance because of its high rigidity. It can reduce the impact load amplitude to 1/3-1/5 per 10 cm of thickness. However, the ballast layer is almost non-resistant against the low-frequency wave components of the dynamic loads. In addition low-frequency load components are not reduced either.

Moreover, although the results were roughly divisible into two features related to the stiffness characteristics of the ballast layer. The ballast layer has high rigidity and resisted loads in the frequency region higher than about 100 Hz. Against impact-load components with a frequency of 10 Hz or less, the ballast behaviour is determined mostly only by an inertial term. Therefore the ballast layer develops resistance forces as a consequence of its mass.

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