This PhD thesis is composed of a synopsis and five published papers that are focused on both the research results of studies on electrical properties of road materials and subgrade soils and their seasonal changes and the use of Ground Penetrating Radar technique in traffic infrastructure surveys. The data for this survey was collected mainly in Finland, Texas, Scotland and Sweden and thus presents many kinds of road materials, subgrade soils and climate conditions. This research demonstrates that the GPR technique not only gives valuable structural information on the different types of structures and subgrade soils but it provides a wide range of information of the electrical properties of the materials under survey which can be further related to their mechanical performance. The best information will be gained if GPR data is analysed together with other non destructive testing data collected from the roads, railways and airports.

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Electrical properties of road materials and subgrade soils and the use of ground penetrating radar in traffic infrastructure surveys

This paper will evaluate ground-penetrating radar (GPR) as a non-destructive method to rapidly, effectively, and continually assess the conditions of railroad ballast. Compared to uniformly graded, clean ballast, fouled ballast has a finer, well-graded particle size with fewer air voids. Ballast under different conditions generates various GPR electromagnetic scattering patterns. A field GPR survey with multiple sets of 1 and 2 GHz air-horn antennae was conducted in summer 2005 at the Transportation Technology Center, Inc. (TTCI) in Pueblo, Colorado. The 2 GHz antenna was found to be more sensitive to the change in scattering pattern. Appropriate data processing was used to remove the effects of the rails to obtain clear GPR images of the subsurface layers. From the image analysis, ballast thickness, ballast fouling condition, and trapped water can be assessed.

Scattering analysis of ground-penetrating radar data to quantify railroad ballast contamination

Effective quality assurance and quality control inspections of new roads as well as assessment of remaining service-life of existing assets is taking priority nowadays. Within this context, use of ground penetrating radar (GPR) is well-established in the field, although standards for a correct management of datasets collected on roads are still missing. This paper reports a signal processing method for data acquired on flexible pavements using GPR. To demonstrate the viability of the method, a dataset collected on a real-life flexible pavement was used for processing purposes. An overview of the use of non-destructive testing (NDT) methods in the field, including GPR, is first given. A multi-stage method is then presented including: (i) raw signal correction; (ii) removal of lower frequency harmonics; (iii) removal of antenna ringing; (iv) signal gain; and (v) band-pass filtering. Use of special processing steps such as vertical resolution enhancement, migration and time-to-depth conversion are finally discussed. Key considerations about the effects of each step are given by way of comparison between processed and unprocessed radargrams. Results have proven the viability of the proposed method and provided recommendations on use of specific processing stages depending on survey requirements and quality of the raw dataset.

https://www.mdpi.com/2076-3263/9/2/96/pdf

Signal processing of GPR data for road surveys

Ballast fouling in the railroad substructure is detrimental to the effectiveness of the railroad track and its structural capacity. The early detection of ballast fouling is of utmost importance to the safety of the rail system and its life-cycle cost-effectiveness. Ground-penetrating radar (GPR), a nondestructive evaluation tool, has shown its potential as a means of assessing the condition of the railroad substructure rapidly, effectively, and continuously. However, an unknown ballast dielectric constant and an unclear interface between clean and fouled ballast limit the accuracy of GPR assessment. In the present study, controlled laboratory testing was conducted to measure accurately the dielectric constants of two common ballast types, granite and limestone, under various fouling and moisture conditions. In addition, a time-frequency method, short-time Fourier transform (STFT), was used to demonstrate graphically the frequency energy variation with the depth of the ballast under various conditions. Th...

Railroad Ballast Evaluation Using Ground-Penetrating Radar: Laboratory Investigation and Field Validation

Trial pitting and portable ballast samplers currently provide one means of assessing and monitoring the composition of railway track formations. A technique has been sought in which the lateral continuity of properties observed in these pits and cores can be assessed quickly with a view to characterising sections of the formation or ballast. The technique that is being developed by IMC Geophysics Ltd., in association with Scott Wilson Pavement Engineering, uses Ground Probing Radar (GPR) technology to image attributes of the formation and ballast. Trials have demonstrated that significant horizons can be imaged and that character changes along those horizons can be correlated with quality or structural variations. At sites where the ballast is relatively clean the interface between ballast and subgrade has been clearly imaged. In particular, changes in the quality of the image have correlated well with marked changes observed in trial pits, and with significant changes in track geometry records. From a combination of trial pits, cores, track geometry records and GPR it should be possible to improve the economics of track maintenance.

Imaging attributes of railway track formation and ballast using ground probing radar

Laboratory experiments were undertaken to identify and characterise the dielectric properties of railway track ballast using Ground Penetrating Radar (GPR). Better results were obtained with lower frequency antennas. Clear distinctions were obtained between wet and dry and clean and spent ballast. The laboratory experiment is described in detail and sample radar scan plots given — the final analysis of dielectric constants is also given. The implications of the findings for radar velocity are discussed. The application to identifying defects in railway track bed is discussed.

R. Gillespie

Papers

Electromagnetic properties of railway ballast