Track Substructure Characterization Using 500 MHz and 2 GHz Ground Penetrating Radar: Results from over 250 Miles of Track in

In July and August, 2007, a configuration of ground penetrating radar (GPR) antennas mounted on a hi-rail vehicle was used to assess the ballast condition and characterize the subballast and subgrade along over 250 miles of track in Wyoming and Alaska. The GPR setup utilized 2 GHz horn antennas mounted over the shoulders and a 500 MHz horn antenna mounted between the rails. The 2 GHz data were processed to assess the ballast fouling condition on the shoulders. The 500 MHz data were obtained with the intent of mapping the subballast/subgrade interface and anomalous features typically associated with perched water or the water table. Twenty ground truth holes were dug at each location and samples were extracted typically at 6-in. depth increments through the ballast layer and in most instances into the subballast and subgrade. The maintenance concerns associated with the Alaska Railroad track were focused on frost susceptible sections. Whereas ballast fouling associated with coal dust was the major maintenance priority of the track in Wyoming. These two structurally different railroads, exhibiting different substructure maintenance requirements, were effectively characterized using one GPR setup. Good correlation was also observed between the processed GPR data and available ground truth