A study of track settlement behavior at critical transitions such as road crossings and bridge approaches has yielded a model to predict the amount of differential track settlement at these transitions caused by ballast and subgrade deformation. This AAR ballast and subgrade deformation model has been developed from field and laboratory test data, and has been validated with field settlement measurements at bridge approaches where the ballast and subgrade properties were well documented. As an analytical tool the model can determine the rate of track differential settlement at transitions with traffic loading and to predict when tamping is required to correct track geometry. Also the model can determine the relative amount of deformation from the individual ballast and subgrade layers. These capabilities provide a means to assess whether the main source of deformation is the subgrade or the ballast and the best means to reduce differential settlement and decrease the frequency of tamping at these track transitions.

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