Characterization of Degraded Ballast Strength: A Field Application of Ground Penetrating Radar and Dynamic Cone Penetration

Ballast performance and lifecycle are influenced by aggregate breakdown-related degradation or fouling and moisture accumulating in track substructure, necessitating effective monitoring and characterization methods. Recent research supported by the U.S. Army Engineer Research and Development Center has focused on investigating condition assessment technologies and developing innovative solutions to ensure railway infrastructure performance. This paper presents a combined application of ground penetrating radar (GPR), the Smart Dynamic Cone Penetrometer (DCP) with an automatic depth measurement system, and the PANDA® penetrometer for comprehensive ballast condition evaluations. The GPR scans covered 43.5 km (27 mi) of railway track, and two types of DCP tests were executed at 15 distinct locations within the Fort Campbell Army installation in Kentucky. The analyses focused on understanding the depth profiles of the track substructure with particular attention given to ballast depth, fouling index (FI), moisture content, and ballast strength. The GPR surveys yielded relative moisture content (RMC) and FI depth profile heatmaps, revealing a correlation between these two variables. A comparative analysis of the Smart DCP and PANDA penetrometer results was conducted, identifying interfaces between the ballast and subballast layers. The study explores the impact of RMC and FI on ballast strength, particularly beyond a critical penetration depth from the ballast surface. Finally, a Gaussian function model is proposed to represent the California Bearing Ratio as a function of the FI and RMC. These findings offer potential for predictive assessments of ballast strength based on FI and moisture content.