Ballast Settlement Ramp to Mitigate Differential Settlement in a Bridge Transition Zone
Differential settlement in railroad track transitions, often associated with differences in track stiffness, may apply considerable impact load and may cause rapid deterioration of track geometry. Such differential settlement commonly seen in bridge approaches may lead to problems in ride comfort, track safety, and reliability. Ballast and subballast layers have been identified as a major cause of differential settlement related to the particulate nature of the aggregate deformation behavior causing degradation and breakdown associated with increased track usage. This paper describes an innovative field approach that successfully demonstrated the use of engineered ballast materials for reducing or mitigating the differential settlement problem in a bridge transition zone. Discrete element method (DEM) simulations were used to predict full-scale track deformations of four ballast materials having different gradations and aggregate shape properties. An imaging device was used to create accurate particle size distributions and shapes for measuring and quantifying by indexes the ballast aggregate shape properties, that is, particle flatness and elongation and angularity. On the basis of the results of the DEM modeling study, a bridge approach settlement ramp was constructed and tested at the Transportation Technology Center’s Facility for Accelerated Service Testing. The results showed that the settlement ramp approach could be achieved by constructing ballasted track with proper selection of materials in each section of track transition zone. This study also demonstrated that the DEM simulation approach combined with image analysis could be used as a quantitative tool for predicting the ballast performance and providing help in design of track transitions.