An investigation into the railway ballast dielectric properties using different GPR antennas and frequency systems

TLDR: In this article, the relative dielectric permittivity of railway ballast using ground-penetrating radar (GPR) was investigated using a container (methacrylate material) of dimensions 1.5-1.5m.

Abstract: This paper presents an investigation into the relative dielectric permittivity of railway ballast using ground-penetrating radar (GPR). To this effect, the experimental tests are carried out using a container (methacrylate material) of dimensions 1.5 × 1.5 × 0.5 m. GPR systems equipped with different ground-coupled and air-coupled antennas and central frequencies of 600 MH, 1000 MHz, 1600 MHz and 2000 MHz (standard and low-powered antenna systems) are used for testing purposes. Several processing methods are applied to assess and compare the dielectric permittivity of the ballast system under investigation. A comparison of the results identifies critical factors as well as antennas and central frequencies most suitable for the purpose.

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Table 1 High-pass and low-pass filters applied across the whole set of central frequencies of investigation 223 used in this study. 224

Table 2 Main properties of the limestone ballast aggregates assessed by using standard test methods. 291

Frequently asked questions

Q1. What are the contributions in this paper?

This paper presents an investigation into the relative dielectric permittivity of railway ballast using ground13 penetrating radar ( GPR ). 

Q2. Why is it recommended to conduct a survey at the network level?

Due to 466 the higher complexity of the investigation domain in terms of boundary conditions (e.g., presence of 467 tracks/sleepers, fouling, moisture within the track bed etc.), it is recommended to carry out first a survey at the 468 network level to divide the railway track into homogeneous stretches. 

Q3. What is the data processing scheme used to obtain the raw GPR signals?

The data processing scheme 190 A data processing scheme is applied to the raw GPR signals as a sequence of four steps, namely, a) time-zero 191 correction; b) signal stacking; c) zero-offset removal and d) band-pass filtering [46, 47]. 

Q4. What are the main structural components that are subject to the major deformations?

The cyclic loading exerted by the moving trains 34 affects both of these main structural components, although the ballast and the sub-ballast layers are the 35 structural components that are subject to the major deformations. 

Q5. What are the important structural and functional tasks covered by ballast aggregates?

Among the most important structural and functional tasks 29 covered by the ballast aggregates, it can be cited i) the resistance to the vertical, lateral and longitudinal loads 30 exerted on the sleepers; ii) the reduction of the maximum stress from the sleepers area to a minor stress level 31 at the foundation and iii) the improvement of the water drainage across the whole track bed structure [1]. 

Q6. What is the main concern of ballast for the construction of railroads?

The use of ballast aggregates for the construction of railroads is massive in railway engineering and the 25 effective assessment and health monitoring of their geometric, physical and mechanical properties is an issue 26 of major concern in terms of safety of the operations and costs of of the rail asset management. 

Q7. What is the simplest method for estimating the dielectrics of the ballast system?

The volumetric mixing formulae 174 The volumetric mixing formula (VMF) theoretical model [39] is used to compute the dielectrics of the “ballast 175 system” by assuming a multi-phase configuration of the investigated medium. 

Q8. Why do low-power GPR systems exhibit worst performance in terms of SNR?

Due to this lower radiative power, 265 this type of GPR systems exhibit worst performances in terms of signal-to-noise (SNR) ratio [55]. 

Q9. What are the permittivity values of the ballast aggregates?

The permittivity of the ballast aggregates (i.e., 186 limestone aggregates), the methacrylate base of the tank and the air are here assumed equal to 6.50, 4.00 and 187 1.00, respectively.