Characterization of ballast particle’s movement associated with loading cycle, magnitude and frequency using SmartRock sensors

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SmartRock has recently been adopted in obtaining the particle-scale characteristics of ballast track and gives microscopic explanation of track performance under cyclic loads, such as degradation and reinforcement. We notice that the contact state of individual ballast particles changes with loading cycles, which in reverse gives rise to different particle movements. The changes of contact state and particle movements may accumulate as loading cycle increases and could be considerable. This makes the response of individual particles uncertain and thus brings practical difficulties in evaluating the performance of ballast track via particle-scale characteristics, especially for long-term loading. In order to learn more about the response uncertainty of individual particles, this study conducts a full size model test to investigate the variation of particle movement and inter-particle normal contact force with loading cycle, amplitude as well as frequency by using SmartRocks. The equivalent axle load ranges from 14 to 36 t, the frequency of the cyclic loading varies from 1 to 18 Hz. Two smart rocks were placed below the sleeper end and the shoulder region respectively. It is found that the long-term variation of particle’s movements and contact force with loading cycle show obvious fluctuation due to the continuous accumulated changes of particle’s contact state. While particle’s movements are generally stable during short-term loading. The movements of SmartRock beneath sleeper end are positively correlated with loading magnitude and frequency, nevertheless also shows a characteristic independent on loading cycles, magnitude as well as frequency: particles’ rotation in horizontal plane is mainly permanent rotation (accumulated rotation), while particle’s rotation in planes perpendicular to the lateral and longitudinal directions is mainly recoverable rotation. The influences of loading magnitude and frequency on ballast particle movements are found different. The results provide effective particle-scale indicators for investigations related to ballast track.