Effect of Grading on Degradation of Crushed-Rock Railway Ballast and on Permanent Axial Deformation

A series of large-scale repeated-load triaxial tests on a crushed metasandstone railway ballast is described. The effects of different gradings on degradation and accumulation of permanent strain are investigated. The material tested was a mechanically strong ballast aggregate with grading curves similar to the envelope curves in the Norwegian railway ballast specification (EN13450, Category E) and the finer grading in the American Railroad Engineering and Maintenance of Way Association's (AREMA) ballast (AREMA 4) commonly used in the United States on railway mainline tracks. The scope of the research was to assess the influence of grading on ballast breakage and permanent vertical deformation. Breakage and production of fines influence ballast service life, and permanent strain from tamping affects the duration of track alignment. As an approach to simulate the impact from ballast tamping, each test specimen was loaded totally with 4 million load repetitions but dismantled and rebuilt after each 1 million load applications. The results of the study indicate that an increase in average grain size to a single-graded ballast material gives more permanent strain, more ballast breakage, and slightly more production of fines less than 0.063 mm. The recompaction after each tamping operation is the main cause of ballast breakage.