Effects of particle size distribution and lithology on the resistance to breakage of ballast materials

Abstract

The particle-size distribution (PSD) of the ballast material directly influences its behavior when subjected to the loads generated by trains and is considered a key factor in the life span of the railway track. However, most PSD specifications provide limits with unclear criteria, without considering the lithology and morphology of the aggregates, covering materials with different behaviors. This paper analyzes the particle breakage for gradation curves within and without the translated limits of AREMA 24, after performing cyclic triaxial tests with three materials from different sources in ballast small-scale triaxial tests. Morphological analyses of samples before and after test are performed to identify the main breakage mechanisms that occurred for the testing conditions adopted. Breakage indexes are compared to the PSDs and a good correlation between percentage of fines generated, lithology, and coefficient of uniformity (Cu) is observed. Additionally, considering the morphological characteristics of one of the materials evaluated, virtual samples are simulated using a Discrete Element Method (DEM) model to determine contact parameters, such as the coordination number and the total number of contacts, which are key information to characterize interparticle load transfer and stress distribution in granular materials. The results obtained in the paper indicated considerable differences in the adoption of different breakage indexes. A morphological analysis before and after test helped with the selection of the optimized procedure to quantify the sample breakage and highlighted the importance of parameters such as the average number of contacts per particle for the prediction of the ballast deterioration due to breakage.