Characterization of Railroad Ballast Behavior under Repeated Loading

Characterizing railroad ballast behavior under repeated train loading is of significant importance for evaluating field settlement or permanent deformation potentials of unbound aggregate ballast layers. For the proper characterization of ballast behavior under dynamic loading, a new triaxial test setup was recently developed at the University of Illinois at Urbana–Champaign. Capable of accommodating cylindrical specimens with a diameter of 305 mm (12 in.) and a height of 610 mm (24 in.), this closed-loop servohydraulic test setup used a load cell and four displacement transducers mounted on the specimen to quantify deformation behavior under loading. Preliminary test results evaluating effects of different applied stress states as well as geogrid reinforcement on ballast behavior established the consistency and repeatability of this new test equipment. Laboratory findings are presented from an ongoing research study aimed at investigating the effects of different ballast types and field degradation trends on permanent deformation accumulation. The ballast type with the highest mill abrasion value was found to accumulate the highest permanent deformation under repeated load triaxial testing. Permanent deformation trends observed for four other ballast types showed direct correlations to the degrees of particle degradation observed in track sections constructed with these ballast materials and trafficked for approximately 18 months with a total track usage of 320 million gross tons.