Evaluation of cyclic response of local subgrade materials in Thailand conforming to Chinese high-speed rail standard

The permanent deformation of subgrade materials is a critical factor governing the long-term performance of high-speed railway (HSR) track systems under repeated loading. This study investigated two locally available subgrade materials in Thailand, low-quality crushed stone (CS) and clayey gravel (GC), both classified as Group B according to TB 10001-2016. Multistage cyclic triaxial tests were conducted at effective confining stresses of 20, 50, and 100 kPa with stepwise increasing cyclic deviator stresses. The results showed markedly different deformation responses despite the identical empirical classification. CS exhibited lower permanent strain accumulation and greater cyclic stability due to its lower plasticity and stronger particle interlocking, whereas GC showed progressive deformation associated with its higher fines content and plasticity. Shakedown analysis further indicated that CS remained predominantly within the plastic shakedown and creep ranges, while GC progressively transitioned into incremental collapse under higher cyclic stresses. A four-parameter stress-dependent power-law model incorporating effective confining stress, cyclic deviator stress, and loading cycles was developed, achieving R 2 = 0.96 for CS and R 2 = 0.91 for GC. External assessment using the clayey gravel dataset of Chen et al. (2020) yielded R 2 = 0.79, demonstrating the applicability of the model formulation while confirming its material-specific parameters. Under a representative project-specific condition of σ′ 3 = 25 kPa, Δq cyc = 25 kPa, and N = 10,000 cycles, the predicted permanent axial strains were 0.584% for CS and 4.572% for GC. These findings demonstrate that performance-based cyclic evaluation is necessary to distinguish the deformation resistance of locally available HSR subgrade materials beyond conventional index-based classification.