The subsurface structure of a track system consists of ballast bed and soil foundation. Ballast fouling due to degradation and infiltration of other materials from the ballast surface and the foundation has been a common problem with rail track performance. The efficiency of a track system decreases with time due to ballast fouling and/or loss of the shear strength of foundation soil. Monitoring and identifying the changes in mechanical properties of ballast and foundation soil in a track system are essential to schedule and predict maintenance costs and to improve safety. Unlike qualitative measurements with the electromagnetic waves, for example, Ground Penetrating Radar, wave velocity measurements provide an opportunity to estimate the distribution of rail system stiffness with depth that can be directly in design.

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