Railroad Ballast Fouling Detection Using Ground Penetrating Radar - A New Approach Based on Scattering from Voids

Railroad ballast is the uniformly-graded aggregate between and underneath railroad ties. The purpose of ballast is to provide support for the heavy loading applied by trains. New ballast contains significant void space between the aggregate. As ballast ages it is progressively fouled by fine-grained material that fills the void space. The structural integrity of seriously fouled ballast can be compromised leading to track instability and ultimately, train derailments. For this reason it is very important to be able to detect fouled ballast. Ground penetrating radar (GPR) has been used over the past 25 years for ballast evaluation and has yielded mixed results. Traditionally, GPR data have been interpreted with strategies focusing on reflections from layer interfaces. This can be a severe handicap when obtaining data on a ballast structure which contains gradational fouling. GSSI developed a 2 GHz horn antenna in 2003 which was initially tested on ballast at the Transportation Technology Center, Inc. in Pueblo, CO in 2005 and subsequently on Amtrak Rails near Boston, MA in 2006. It was noted that the antenna is much more focused compared to the commonly used 1 GHz horn antenna and provides data that contain significant scattering energy from the void space in clean ballast. The data from fouled ballast contained minimal scattering from the remaining void space. Comparison of the data obtained using the 2 GHz antenna with available ground truth supports this interpretation. This GPR methodology shows potential for routine inspection of working railroads and is especially suited to assess the ballast fouling condition near the bottom of railroad ties.