Understanding the capabilities of GPR for the measurement of ballast fouling conditions

Ground penetrating radar (GPR) has become a widely used trackbed inspection tool for railroad maintenance engineers. The method is attractive, due to the ability to autonomously collect continuous trackbed quality related information at traffic speeds (20-200 km/h). Information on ballast quality, formation settlement and the presence of saturated materials is used to plan maintenance and budget material replacement. Ballast fouling levels and moisture content are key properties of ballast quality that affect the stiffness of the trackbed. Due to the often highly heterogeneous composition of trackbed materials, extracting related information from GPR data alone is challenging. This paper considers in detail how electromagnetic properties of trackbed materials, relating to varying degrees of ballast fouling and moisture content, affect attributes derived from reflected and scattered GPR signals. It is shown that the inherent ambiguities in the response of GPR to fouling and moisture within the trackbed mean that neither can be uniquely determined by the method. We present the less ambiguous measure of the thickness of clean ballast, a useful proxy for the overall fouling condition of the trackbed. (C) 2019 Elsevier B.V. All rights reserved.