ABSTRACT
The source of most track geometry deviations measured at the rails is due to settlement within or movement of the geotechnical track components, of which only the uppermost surface is available for visual examination. Therefore, reliable measurements are needed that can provide information on the substructure condition, well below this surface, to diagnose the cause of an instability and prescribe an appropriate remedy. This chapter first discusses soils investigation and methods to assess existing conditions, and then it describes the use of a remote sensing technique known as ground penetrating radar (GPR) that can probe the upper substructure layers nondestructively. Next, track geometry measurements are described and related to substructure behavior and vehicle-track interaction. The last section discusses track deflection-based measurements and what they indicate about the track substructure.
