Breakage characteristics of ballast particle in uniaxial compression tests incorporating shape and size effects

Ballast breakage under the long-term train load is a typical track disease source that would cause ballast bed hardening, ballast fouling, mud pumping and track settlement. However, the effect of ballast morphology on its breakage characteristics including breakage patterns and probabilities remains unclear to date. In this study, a series of uniaxial compression tests on 186 randomly selected ballast particles of different sizes and shapes was conducted. During each test, video and digital images were taken to capture the breakage processes and the morphological variation before and after breakage. Based on morphological analyses, it was found that the ballast could be mainly categorized into three types, i.e., cubic, flat, and elongated-flat types. According to the breakage appearance and force-displacement curves, four breakage modes were observed for the 186 sets of breakage tests and were defined with their breakage characteristics, i.e., splitting, explosive, edge breakage and edge abrasion. The explosive breakage exhibited higher strength than the splitting and edge breakage. As a special form of breakage, the edge abrasion presented the lowest strength. Regardless of ballast shape, the breakage strength of ballast decreased with the increase of particle size. Moreover, the strength of cubic ballast was higher than flat and elongated-flat ballast, especially for those with diameters ranging from 31 to 50 mm, indicating a notable shape impact. However, such effect on breakage decreased for other cases. It was also found that the strength survival probabilities of ballast with different shapes and breakage modes were all conformed to the Weibull distribution. With the deduced parameters, the individual probabilities causing the four breakages modes with known particle size and shape under arbitrary loads could be effectively predicted. These testing results and Weibull distribution parameters would be the benchmark for further clarification of breakage mechanism of ballast within trackbed and supply data base for Discrete Element Method-based researches.