Railway track condition assessment at network level by frequency domain analysis of GPR data

The railway track system is a crucial infrastructure for the transportation of people and goods in modern societies. With the increase in railway traffic, the availability of the track for monitoring and maintenance purposes is becoming significantly reduced. Therefore, continuous non-destructive monitoring tools for track diagnoses take on even greater importance. In this context, Ground Penetrating Radar (GPR) technique results yield valuable information on track condition, mainly in the identification of the degradation of its physical and mechanical characteristics caused by subsurface malfunctions. Nevertheless, the application of GPR to assess the ballast condition is a challenging task because the material electromagnetic properties are sensitive to both the ballast grading and water content. This work presents a novel approach, fast and practical for surveying and analysing long sections of transport infrastructure, based mainly on expedite frequency domain analysis of the GPR signal. Examples are presented with the identification of track events, ballast interventions and potential locations of malfunctions. The approach, developed to identify changes in the track infrastructure, allows for a user-friendly visualisation of the track condition, even for GPR non-professionals such as railways engineers, and may further be used to correlate with track geometric parameters. It aims to automatically detect sudden variations in the GPR signals, obtained with successive surveys over long stretches of railway lines, thus providing valuable information in asset management activities of infrastructure managers

A simulation-based approach for railway applications using GPR

In this work a numerical model capable to predict the electromagnetic response of railway ballast aggregates under different physical conditions has been calibrated and validated by a simulation-based approach. The ballast model is based on the main physical and geometrical properties of its constituent material and it is generated by means of a random-sequential absorption (RSA) approach. A finite-difference time-domain (FDTD) simulator is then employed to calculate the ground-penetrating radar (GPR) signal response to the scenario. The calibration of the model has been performed by taking into account the main physical properties and the grain size characteristics of both the reference ballast material and a fine-grained pollutant material, namely, an A4 soil type material, according to the AASHTO soil classification. The synthetic GPR response has been generated by using the gprMax freeware simulator. Several scenarios have been considered, which in turn were reproduced in laboratory environment and used for the validation of the model. Promising results have demonstrated the high potential of such approach in characterizing the simulated response of complex coarse-grained heterogeneous materials

Efficient practices in railway ballast maintenance and quality assessment using GPR

The need for effective and efficient railway maintenance is always more demanded all over the world as the main consequence of aging and degradation of infrastructures. Primarily, the filling of air voids within a railway ballast track-bed by fine-grained materials, coming up from the subballast layers by vibrations and capillarity effects, can heavily affect both the bearing and the draining capacity of the infrastructure with major impacts on safety. This occurrence is typically referred to as “fouling”. When ballast is fouled, especially by clay, its internal friction angle is undermined, with serious lowering of the strength properties and increase of deformation rates of the whole rail track-bed. Thereby, a detailed and up-to-date knowledge of the quality of the railway substructure is mandatory for scheduling proper maintenance, with the final goal of optimizing the productivity while keeping the safety at the highest standard. This paper aims at reviewing a set of maintenance methodologies, spanning from the traditional and most employed ones, up to the most innovative approaches available in the market, with a special focus on the Ground Penetrating Radar (GPR) non-destructive testing (NDT) technique. The breakthrough brought by the application of new processing approaches is also analyzed and a methodological framework is given on some of the most recent and effective maintenance practices

An entropy-based analysis of GPR data for the assessment of railway ballast conditions

The effective monitoring of ballasted railway track beds is fundamental for maintaining safe operational conditions of railways and lowering maintenance costs. Railway ballast can be damaged over time by the breakdown of aggregates or by the upward migration of fine clay particles from the foundation, along with capillary water. This may cause critical track settlements. To that effect, early stage detection of fouling is of paramount importance. Within this context, ground penetrating radar (GPR) is a rapid nondestructive testing technique, which is being increasingly used for the assessment and health monitoring of railway track substructures. In this paper, we propose a novel and efficient signal processing approach based on entropy analysis, which was applied to GPR data for the assessment of the railway ballast conditions and the detection of fouling. In order to recreate a real-life scenario within the context of railway structures, four different ballast/pollutant mixes were introduced, ranging from clean to highly fouled ballast. GPR systems equipped with two different antennas, ground-coupled (600 and 1600 MHz) and air-coupled (1000 and 2000 MHz), were used for testing purposes. The proposed methodology aims at rapidly identifying distinctive areas of interest related to fouling, thereby lowering significantly the amount of data to be processed and the time required for specialist data processing. Prominent information on the use of suitable frequencies of investigation from the investigated set, as well as the relevant probability values of detection and false alarm, is provided

How to create a full-wave GPR model of a 3D domain of railway track bed?

Ground-penetrating radar (GPR) investigations of railway track beds are becoming more important nowadays in civil engineering. The manufacturing of representative full-scale scenarios in the laboratory environment for the creation of databases can be a critical issue. It is difficult to reproduce and monitor the effect of differing physical and performance parameters in the ballast layer as well as to evaluate the combination of these factors in more complex scenarios. In addition, reproducing full-scale tests of railway ballast implies to handle huge amounts of aggregates. To this effect, the use of the Finite-Difference Time-Domain (FDTD) simulation of the ground-penetrating radar signal can represent a powerful tool for creating, extending or validating databases difficult to build up and to monitor at the real scale of investigation. Nevertheless, a realistic three-dimensional simulation of a railway structure requires huge computational efforts. This work focuses on performing simula-tion of the ground-penetrating radar signal within a railway track bed by using a two-dimensional cross-section model of the ballast layer, generated by a Random Sequential Adsorption (RSA) paradigm. Attention was paid on the geometric reconstruction of the ballast system as well as on the content of voids between the aggregate particles, which complied with the real-world conditions of compaction for this material. The resulting synthetic GPR signal was subsequently compared with the real signal collected within a realistic track bed scenario of ballast aggregates recreated in the laboratory environment

GPR applications across Engineering and Geosciences disciplines in Italy: a review

In this paper, a review of the main ground-penetrating radar (GPR) applications, technologies, and methodologies used in Italy is given. The discussion has been organized in accordance with the field of application, and the use of this technology has been contextualized with cultural and territorial peculiarities, as well as with social, economic, and infrastructure requirements, which make the Italian territory a comprehensive large-scale study case to analyze. First, an overview on the use of GPR worldwide compared to its usage in Italy over the history is provided. Subsequently, the state of the art about the main GPR activities in Italy is deepened and divided according to the field of application. Notwithstanding a slight delay in delivering recognized literature studies with respect to other forefront countries, it has been shown how the Italian contribution is now aligned with the highest world standards of research and innovation in the field of GPR. Finally, possible research perspectives on the usage of GPR in Italy are briefly discussed

Assessment of layer thickness and uniformity in railway embankments with Ground Penetrating Radar

In the aim of a national research project entitled “Interaction soil-rail track for high speed trains”, a protocol was established between the National Railway Network and four national research institutions to develop the knowledge concerning the methodology for the construction and quality control of the railway embankments and railtrack layers for high speed trains. One of the objectives of this protocol is to establish a methodology for quality control of construction layers by different available test methods. Nondestructive testing (NDT) methods are currently very attractive due to their ability to provide information about layer thickness and state condition without causing damage or requiring the removal of material samples. Within the NDT available, ground penetrating radar (GPR) is a very fast and reliable technique, whose advantage is the repeatability and the capability of acquiring continuous data. To reach the proposed goal, a trial embankment was constructed with different materials, layer’s thicknesses, water contents and compaction energy levels. GPR was used in two embankments, in order to detect the thickness of the sub-ballast layer located over the compacted sand layer and its uniformity along the track, but also along the cross-section of the track. In order to control some parameters of the sub-ballast layer, like thickness and uniformity, several metallic plates had been used in the base of the sub-ballast layer, along an alignment. It shows clearly the ability of GPR to detect the sub-ballast layer and its thickness variations along the profile

The application of Ground-Penetrating Radar to transportation engineering: recent advances and new perspectives (GI Division Outstanding ECS Award Lecture)

Ground-penetrating radar (GPR) is one of the most acknowledged and established non-destructive testing (NDT) techniques within the context of the health monitoring and assessment of transportation infrastructures. GPR is being increasingly used for the effective management of infrastructural assets as it weakens the case for using other destructive monitoring methods, such as digging holes, and allows for rapid and reliable detection of many causes of the subsurface damage. Thereby, its usage favours the optimisation of the economical expenditure for the effective maintenance of great infrastructures as well as it improves the public safety by preventing or not raising the risk of accidents. GPR has been used in highway, railway and airfield engineering as well as for the monitoring of critical infrastructures, such as bridges and tunnels. It has found established use in the assessment of the geometric properties of the subsurface, such as in the case of the evaluation of the pavement layer thicknesses, or the size of the rebars in concrete-made structural components. Major physical-based investigations have been focused on the evaluation of the moisture ingress in flexible road pavements and in concrete structures, as well as on the detection of the rebars corrosion caused by the ingress of chloride. The majority of these parameters are evaluated using methods of signal analysis and data processing based on the signal in the time domain. The sophistication of the hardware and software of the GPR systems over the last few years as well as the recent advances achieved in the research have contributed to raise the high potential of this non-destructive technique and paved the way towards new application areas in transportation engineering. In particular, GPR is nowadays finding major application when used with complementary non-destructive testing techniques, although it has still proved to provide reliable results in various self-standing applications. This work aims at presenting the recent advances and the new perspectives in the application of GPR to transportation engineering. This study reports on new experimental-based and theoretical models for the assessment of the physical (i.e. clay and water content in subgrade soils, railway ballast fouling) and the mechanical (i.e. the Young’s modulus of elasticity) properties that are critical in maintaining the structural stability and the bearing capacity of the major transport infrastructures, such as highways, railways and airfields. With regard to the physical parameters, the electromagnetic behaviour related to the clay content in the loadbearing layers of flexible pavements as well as in subgrade soils has been analysed and modelled in both dry and wet conditions. Furthermore, it is discussed a new simulation-based methodology for the detection of the fouling content in railway ballast. Concerning the mechanical parameters, experimental based methods are presented for the assessment of the strength and deformation properties of the soils and the top-bounded layers of flexible pavements. Furthermore, unique case studies in terms of the methodology proposed, the survey planning and the site procedures in rather complex operations, are discussed in the case of bridges and tunnels inspections

"Test-site operations for the health monitoring of railway ballast using Ground-Penetrating Radar"

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