Keywords: railway ballast, traffic-induced densification, mechanical properties, monotonic loading, cyclic loading, large-scale triaxial tests.

Traffic-induced cyclic loading significantly affects the ballast column beneath sleepers by inducing particle rearrangement, rotation, and degradation. As ballast particles break and relocate, smaller fragments fill the voids within the granular assembly, leading to densification, and higher peak failure stress. This study investigates the evolution of peak failure stress in three railway ballast materials with different lithologies and mechanical qualities: high-quality basalt, medium-quality diabase, and poor-quality limestone. A large-scale triaxial testing program was performed in which specimens were subjected to one million loading cycles, approximately corresponding to three years of traffic for a track carrying 30 million gross tonnes. Two load cases were applied to represent moderate and intense traffic-induced densification, with different load amplitudes and frequencies. Subsequently, monotonic displacement-controlled shearing was conducted at a loading rate of 0.5 mm/min. The results show that cyclic densification improves particle interlocking and increases resistance against monotonic loading. Under moderate cyclic loading, all three materials reached comparable peak failure stresses, while their post-peak ductility differed. Under intense loading, material dependency became more pronounced. Basalt achieved the highest peak stress but exhibited sudden vertical stress drops, indicating brittle macroscopic failure, whereas diabase showed a smoother and more stable shearing response.

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