Evaluation of reduced-scale railway ballast particle size effects for different test sample sizes using monotonic triaxial tests

This study rigorously investigated the mechanical characteristics of railway ballast using largescale triaxial (LST) testing, focusing on shear strength, stress-strain relationships, and volumetric alterations by altering particle sizes under controlled conditions. Samples with a size ratio (D/d(max)) of 4.7 are typically used in LST testing. However, the ballast used in Thailand typically features a maximum grain size of 70.00 mm, whereas LST testing requires samples with diameter exceeding 300 mm, which poses logistical and material challenges. To mitigate this issue, our test introduced a parallel grading method to decrease sample size while preserving essential ballast properties and morphological features. This test applied two grain-size distributions: normal grain-size distribution (NGD) and parallel grain-size distribution (PGD). The study aimed to ascertain whether the smaller-sized samples can still provide accurate assessments of mechanical behaviors. The outcomes demonstrated that both NGD and PGD approaches yielded similar stress-strain behaviors and cohesion values. A marginal difference in friction angle values of 4 degrees was noted, suggesting the occurrence of minor variances in shear resistance, which did not significantly affect the overall mechanical properties. The findings validate the effectiveness of the parallel grading technique, thus offering practical implications for testing procedures in railway engineering.