Luca Bianchini Ciampoli

An overview of ground-penetrating radar signal processing techniques for road inspections

Ground-penetrating radar (GPR) was firstly used in traffic infrastructure surveys during the first half of the Seventies for testing in tunnel applications. From that time onwards, such non-destructive testing (NDT) technique has found exactly in the field of road engineering one of the application areas of major interest for its capability in performing accurate continuous profiles of pavement layers and detecting major causes of structural failure at traffic speed. This work provides an overview on the main signal processing techniques employed in road engineering, and theoretical insights and instructions on the proper use of the processing in relation to the quality of the data acquired and the purposes of the surveys. GPR is an increasingly used NDT technique in pavement applicationsProper choice of the signal processing techniques according to the GPR data sourceOverview of signal processing techniques to use prior to any post-processing stepOverview of signal processing techniques to perform after the GPR data collection

GPR analysis of clayey soil behaviour in unsaturated conditions for pavement engineering and geoscience applications

Clay content is one of the primary causes of pavement damages, such as subgrade failures, cracks, and pavement rutting, thereby playing a crucial role in road safety issues as an indirect cause of accidents. In this paper, several ground-penetrating radar methods and analysis techniques were used to nondestructively investigate the electromagnetic behaviour of sub-asphalt compacted clayey layers and subgrade soils in unsaturated conditions. Typical road materials employed for load-bearing layers construction, classified as A1, A2, and A3 by the American Association of State Highway and Transportation Officials soil classification system, were used for the laboratory tests. Clay-free and clay-rich soil samples were manufactured and adequately compacted in electrically and hydraulically isolated formworks. The samples were tested at different moisture conditions from dry to saturated. Measurements were carried out for each water content using a vector network analyser spanning the 1 GHz–3 GHz frequency range, and a pulsed radar system with ground-coupled antennas, with 500-MHz centre frequency. Different theoretically based methods were used for data processing. Promising insights are shown to single out the influence of clay in load-bearing layers and subgrade soils, and its impact on their electromagnetic response at variable moisture conditions.

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.

Signal processing for optimisation of low-powered GPR data with application in transportation engineering (roads and railways)

High-frequency air-coupled ground-penetrating radar (GPR) systems are used in road engineering for achiev-ing high-resolution and fast imaging of the shallow layers of pavements. Regulatory policies on the permitted radiated power enacted by some international agencies for information and communication technologies, such as, the Federal Communications Commission operating in the United States, have led manufacturers to market low-powered GPR systems to comply with the standards. The signal collected by these systems is more unstable than ordinary-powered GPRs, with the interpretation of the raw data being misleading or, mostly, totally subjective or even impossible. Thereby, the use of relevant signal processing techniques combined purposely within procedural schemes may help to reach reliability and effectiveness levels close to those granted by standard systems. In this study, a post-processing scheme aimed at maximising the correlation between signals collected by low-powered and standard 2 GHz antenna systems in railway and road surveys is presented.

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.

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.

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.