Permanent Deformation of Fouled Ballast under Coupled Progress Thawing and Cyclic Loading

Ballasted tracks in seasonally cold regions are subjected to repeated freezethaw cycles that can significantly affect track performance and maintenance. Although frozen ballast exhibits high shear strength under monotonic loading, the permanent deformation behavior of fouled ballast under coupled progressive thawing and cyclic loading remains poorly understood. This study investigates this behavior through largescale triaxial tests on ballast specimens with fouling indices (FI) of 23 and 40 and varying initial water contents (IWCs). Tests were conducted under dry, wet, frozencompletely thawed (FCT), and frozen-progressive thawing (FPT) conditions. Resultsshow that FPT produces the greatest long-term deformation, exceeding that observed under wet and post-thaw conditions. Unlike conventional shakedown behavior, deformation rates during progressive thawing may stabilize or increase rather than decrease, particularly at higher IWC and FI. Three deformation stages were identified: rapid initial rearrangement, transitional restructuring, and post-thaw steady state. The transitional stage becomes more pronounced and prolonged with increasing moisture and fouling, while a steady state may not be reached under high-moisture conditions. These responses are governed by the evolution of particle contacts from ice-bonded to degraded and lubricated interfaces, emphasizing the critical influence of coupled thawing and cyclic loading on ballast deformation in cold regions.