Deformation and Degradation Mechanisms of Railway Ballast under High Frequency Cyclic Loading

The increased demand for higher speeds and increased freight capacity in railroad transportation exacerbates the rate of ballast degradation that leads to unacceptable track deformation and frequent maintenance. In response, a series of large-scale cyclic triaxial tests was performed to investigate the combined effect of train speed (frequency), axle load, and confinement on the deformation and degradation of ballast. In these tests, the load frequency $f$ was varied from 5 to 60 Hz to simulate train speeds of $40–400\mathrm{km}/\mathrm{h}$. Two sets of deviator stress magnitude $q_{\max ,cyc}$ (230 and 370 kPa) were applied to resemble axle loads of 25 and 40 t, respectively, and the effect of three levels of confining pressure ${\sigma }_3^{\prime }$ (10, 30, and 60 kPa) was examined. The results indicate that three distinct categories of permanent deformation mechanisms exist at various levels of $f$, $q_{\max ,\mathrm{cyc}}$, and ${\sigma }_3^{\prime }$. Ballast degradation was more pronounced at higher $f$, and it had a more profound effect on cyclic densification. The resilient modulus also increased with an increase of $f$, $q_{\max ,\mathrm{cyc}}$, ${\sigma }_3^{\prime }$, and number of load cycles ($N$). An empirical relationship for the dynamic amplification factor (DAF) was proposed to properly reflect the role of train speed on axle load.