Effect of load–unload–reload cycles on the shear resistance of railroad ballast in triaxial monotonic tests

Abstract Railroad ballast, composed of unbound coarse aggregates, is subjected to repeated vertical load–unload–reload (LUR) actions due to passing trains, which leads to significant particle rearrangement within the track bed. Shear resistance is a critical property governing the stability of ballast, yet conventional investigations predominantly rely on monotonic triaxial tests that do not adequately capture the effects of LUR-induced particle rearrangement. This study examines ballast shear behavior through novel large-scale triaxial tests incorporating axial LUR loading schemes, distinct from traditional cyclic loading, to replicate the mechanical effects of repeated train loads. Experimental results reveal that shear strength is strongly influenced by LUR loading paths. Increased LUR cycles enhance shear resistance due to the development of fabric anisotropy driven by particle rearrangement. Under identical confining pressures and initial densities, monotonic triaxial tests without LUR loading may underestimate ballast shear strength by up to 72.7%. This is attributed to the suppression of particle rearrangement under higher confinement. Furthermore, the shear failure envelope evolves with increasing LUR cycles, rendering failure criteria derived from monotonic tests inadequate for LUR conditions. These findings highlight the limitations of conventional triaxial approaches and underscore the necessity of accounting for particle rearrangement when evaluating the shear strength and failure behavior of railroad ballast and similar granular materials.