Effective maintenance of railways requires a comprehensive assessment of the actual condition of the construction materials involved. In this regard, geophysical methods as GPR offer an excellent alternative to other invasive, more costly and time-consuming traditional techniques.
Ballast condition is an extremely important factor to ensure the safety of traffic and users because it forms the trackbed and supports the track. It also helps with drainage, so rainwater can drain away rather than pooling. Over time, ballast aggregates wear down and become rounded. They lock together less easily, reducing the ballast’s effectiveness. As the ballast aggregates grind together over time through normal wear and tear, it creates a layer of fine-grained material. This process is known as fouling. When these fine particles combine with water in the ballast make drainage more difficult and the ballast less flexible, therefore less able to restrain the track as trains move over it.
The bearing capacity of the substructure stands as a major concern for designers and maintainers, as differential settlements are usually reported to occur within the platform. This is mainly due to the change in the material grading.
Fouling can be detected as a water accumulation within the trackbed. And GPR is a very effective tool to detect water concentrations near the surface. Therefore, by analyzing the spectral response of GPR data collected on a railway track it is possible to determine the degree of fouling.
Sometimes the origin of the problem is deeper (eg, settlement in embankments due to foundation failure) and other methods must be used (ERT or seismic methods). These methods produce resistivity or seismic velocity models that can be related with structural anomalies under the track.

Example of results derived from a 3d-gpr survey on a railway platform. An area with an anomalous ballast thickness is clearly identified on an embankment