Detection of Fouling in Ballast by Electromagnetic Surveying

During the service life of railway tracks, particle breakage, surface intrusion, and infiltration of fines from subgrade caused by traffic loading Increase the amount of fines in ballast, known as ‘fouling’. Fouling increases plastic deformation of ballast which leads to deteriorate a track surface. To avoid progressive deterioration of the track, continuous and prompt maintenance is required when fouling approaches a critical level. Maintenance operations are scheduled on the basis of measured track deformation and the level of fouling. Recently, electromagnetic surveying and visual observation through boreholes have been proposed to detect and quantify the fouling content. However, systematic studies are required to validate the application of electromagnetic-based techniques in the quantification of fouling levels in the rail substructure. The change of electromagnetic parameters in simulated fouling processes was evaluated in a small-scale laboratory study using time domain reflectometry (TDR). An increase of fouling depth increased the relative dielectric permittivity from 1.2 to 2.0. An increase in water content of the fouling material from 5 to 10% increased the electrical conductivity of fouled ballast from 10 to 24 mS/m. The TDR results were combined to develop a qualitative approach to assess the gradual change of fouling content of ballast. INTRODUCTION Ballast is the large-size aggregate layer in the railway substructure that distributes the train load to the foundation soil, while also providing proper drainage for the track. Unlike base layers in pavement structures, railway ballast breaks and deteriorates under the cyclic and loads from trains. Throughout this deterioration process, ballast properties deviate from the specifications and form ‘fouled ballast’. Fouling starts with internal degradation of the ballast, and continues with subgrade infiltration and surface spillage (1,2,3,4). Ballast deterioration increases with number of loading repetitions and the load intensity. A major concern facing the US freight rail transportation industry is the high maintenance costs due to heavy freight loads and substandard track substructure (5). Proper maintenance of railway track is confounded by inadequate non-destructive techniques to estimate the condition of track (6). In the US, there are over 371,200 km of railroad track that must be periodically assessed (6). The current assessment methods include forensic investigations of the track and use of ground penetrating radar, GPR (7,8,9,10). As the amount of fouling increases in the ballast layer, GPR continuously images the reduced thickness of track bed (6,8). However, as ballast becomes progressively more fouled and retains more moisture, the reflection becomes less defined and horizons become more difficult to track. GPR studies illustrate the difficulties in data interpretation and the sensitivity to the water content of fouled ballast (6,8,11). Studies by Ebrahimi (12) showed that the fouling content of ballast increased the plastic deformation of track, while the water content also had a significant effect on the long-term deformational behavior of the track. The objective of this paper was is to characterize electromagnetic (EM) parameters of fouled ballast using time domain reflectometry (TDR) methodology to assess the depth of fouling and moisture content of fouled ballast. Two main sources of fouling, including internal ballast crushing (i.e., ‘mineral fouling’) and surface spillage of coal dust (i.e., ‘coal fouling’) were tested. The correlation between the EM properties of ballast with a gradual increase in the depth of fouling was derived. EM WAVE PROPAGATION AND TDR MEASUREMENTS In TDR measurements, an electromagnetic (EM) step voltage is propagated along the length of TDR probes. The time and the amplitude required for the EM wave to propagate and reflect back to the source is measured and, if the geometry of the probes is known, the permittivity and conductivity of the material between the probes can be estimated. The propagation velocity (V) of an EM wave depends on the electrical conductivity (σ), complex dielectric permittivity (ε=ε’-j·ε”), and complex magnetic permeability (μ=μ’-j·μ”) of the medium (13). If the material is assumed to be ferromagnetic (i.e., μ=μo where μo=4π·10 H/m is the magnetic permeability of free space) and the imaginary component of the dielectric permittivity is assumed negligible (i.e., ε”<< σ/ω), the EM wave velocity is: