Influence of particle size scaling on the static and cyclic behaviour of railway slab track-bed materials: laboratory and DEM study

Accurate characterisation of the static and cyclic behaviour of coarse-grained railway base materials is essential for high-speed rail design and maintenance. Laboratory triaxial testing is often constrained by apparatus size, necessitating particle size scaling to satisfy the one-sixth maximum particle size criterion. This study evaluates the influence of two standard scaling techniques—scalping and parallel gradation—on the mechanical response of gravel–sand mixtures used in slab track-bed foundations. Consolidated drained static and cyclic triaxial tests were performed on specimens with prototype and scaled gradations under confining pressures ranging from 50 to 200 kPa. Complementary three-dimensional Discrete Element Method (DEM) simulations, incorporating non-spherical particle shapes and flexible boundaries, were calibrated against laboratory data to investigate macro- and micro-mechanical behaviours across loading frequencies of 0.1–7 Hz. Results show that scaling alters peak and residual strengths, volumetric behaviour, shear modulus, damping ratio, and micromechanical fabric, leading to deviations from the prototype responses. Coarser gradations exhibited higher contact forces, greater deformation, and more substantial frequency-dependent stiffness changes, while scaled specimens retained stiffness and exhibited higher damping at service strains. Predictive relationships for shear modulus and damping ratio as functions of shear strain, confinement, and dimensionless train speed were derived, highlighting the need for gradation-specific calibration. The findings demonstrate that scaled laboratory results should be applied to prototype predictions with caution, and that appropriate correction factors and safety margins are required essential to avoid bias in the design and performance assessments of high-speed railway infrastructure.