Maria Giulia Brancadoro

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.

GPR applications in mapping the subsurface root system of street trees with road safety-critical implications

Street trees are an essential element of urban life. They contribute to the social, economic and environmental development of the community and they form an integral landscaping, cultural and functional element of the infrastructure asset. However, the increasing urbanisation and the lack of resources and methodologies for the sustainable management of road infrastructures are leading to an uncontrolled growth of roots. This occurrence can cause substantial and progressive pavement damage such as cracking and uplifting of pavement surfaces and kerbing, thereby creating potential hazards for drivers, cyclists and pedestrians. In addition, neglecting the decay of the principal roots may cause a tree to fall down with dramatic consequences. Within this context, the use of the ground-penetrating radar (GPR) non-destructive testing (NDT) method ensures a non-intrusive and cost-effective (low acquisition time and use of operators) assessment and monitoring of the subsurface anomalies and decays with minimum disturbance to traffic. This allows to plan strategic maintenance or repairing actions in order to prevent further worsening and, hence, road safety issues. This study reports a demonstration of the GPR potential in mapping the subsurface roots of street trees. To this purpose, the soil around a 70-year-old fir tree was investigated. A ground-coupled GPR system with central frequency antennas of 600 MHz and 1600 MHz was used for testing purposes. A pilot data processing methodology based on the conversion of the collected GPR data (600 MHz central frequency) from Cartesian to polar coordinates and the cross-match of information from several data visualisation modes have proven to identify effectively the three-dimensional path of tree roots.

An Investigation into the railway ballast grading using GPR and image analysis

This study reports on an investigation into the grain size distribution of the railway ballast using ground-penetrating radar (GPR) and image analysis. The proposed approach relies on the hypothesis that the dimension (grading) of the ballast aggregates can influence the back-reflected spectrum received by the use of GPR. This assumption was confirmed by the finite difference time-domain (FDTD) simulations of the GPR signal, which were run by using the numerical simulator package gprMax 2D. A regression model was developed which related the “equivalent” diameter of the ballast aggregates and the frequency of the peak within the received spectrum. The model was validated in the laboratory environment by means of a 155 cm × 155 cm × 50 cm methacrylate tank, filled up with railway ballast. An air-coupled GPR system equipped with a 2000 MHz central frequency antenna was used for testing purposes. A total of three spatial distributions of the ballast aggregates within the tank were investigated, by emptying out and filling up thrice the tank with the same material. The geometric information on the ballast grading obtained from the simulation-based regression model was compared to the actual grading curve of the ballast. To this effect, an algorithm based on the automatic image analysis was developed. The comparison showed that the modelled aggregate diameter corresponded to the 70 % of the grading of the material sieved out in the laboratory. This contribution paves the way to new methodologies for the non-destructive assessment and the monitoring of segregation phenomena within the ballast layers in railway track-beds.

A comparative investigation of the pavement layer dielectrics by FDTD modelling and reflection amplitude GPR data

The present work focuses on the application of the ground-penetrating radar (GPR) technique on a flexible pavement structure for the assessment of the layer dielectrics. Two air-coupled GPR systems, with antennas operating at 1 GHz and 2 GHz central frequencies have been used for testing and simulation purposes. The ef-fectiveness of the combination of i) the Finite-Difference Time-Domain (FDTD) technique for the simulation of the GPR signal, and ii) the GPR reflection amplitude technique, for the estimation of the dielectrics of the pavement layers, has been analyzed. Three steps of processing are proposed and the results are compared each to one another. In the first stage, the signal has been simulated using design project data for the cross-section investigated and dielectric permittivity values for the (design) construction materials, derived from the litera-ture. In the second stage, the dielectrics have been computed by the signal collected within a real-life flexible pavement. Both the two-way travel time and the reflection amplitude techniques were performed. The third step was focused on analyzing the accuracy of the reflection amplitude method combined with the optimized simulation of the GPR signal. The results demonstrate potential on the use of the proposed approach with re-spect to the application of the reflection amplitude technique to the real-life GPR signal.

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 TimeDomain (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 simulation 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. enables to analyse, in a controlled environment, differing conditions that can be hardly reproduced in the laboratory, as well as to generate a large amount of ballast samples with differing physical conditions. On the other hand, it is relatively complex to simulate a GPR signal for a railway track bed. Electromagnetically speaking, it is necessary to calibrate the physical properties of the investigated materials and to create samples with representative threedimensional (3D) characteristics in order to ensure consistency between the real and the simulated sample. The reproduction of such an irregular volumetric ensemble of coarse aggregates represents a nonnegligible numerical problem, which necessarily requires simplifications to limit the computational efforts. By literature, the most acknowledged method for the simulation of railway ballast is based on the representation of the polyhedral-shaped aggregates using a cluster of smaller simple shapes (Thakur et al. 2009, Indraratna et al. 2016, Sharif et al. 2016). In practical terms, this “clump logic” method allows to simulate the bi-dimensional (2D) or 3D irregular geometry of the ballast grains through the connection and overlapping of a number of smaller spheres (i.e., 3D domain), or circles (i.e., 2D domain), all of which are characterized by differing sizes and positions. In this study, a novel methodology for the simulation of railway ballast is proposed. The method is based on the Random Sequential Adsorption (RSA) paradigm (Feder 1980), which ensures the random location of randomly-sized ballast particles within a simulation domain consistent with the actual dimensions of ballast layers in the real-life environment. The size of the ballast grains was generated according to the grading of the aggregates used in the realcase test. The GPR signal of the produced scenario was subsequently obtained using the FiniteDifference Time-Domain (FDTD) paradigm for the generation of synthetic GPR signals.