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High temporal frequency monitoring of transport infrastructures is crucial for implementing effective maintenance prioritisation strategies and prevent major failures. To this extent, ground-based non-destructive testing (NDT) methods have been successfully applied for decades, reaching very high standards of data quality and accuracy. However, routine and systematic campaigns are required over relatively long inspection times for data collection and implementation into reliable infrastructure management systems (IMSs). On the other hand, satellite remote sensing techniques, such as the Multi-Temporal Interferometric Synthetic Aperture Radar (MT-InSAR) method, have proven effective in monitoring ground displacements of transport infrastructures (roads, railways and airfields), with a much higher temporal frequency of investigation and wider inspection catchment areas. Nevertheless, the integration of information from i) satellite remote sensing and ii) ground-based NDT methods is still an area to be explored in civil engineering. Within this framework, this paper aims to review significant stand-alone applications in these two areas of technology for transport infrastructure monitoring. Furthermore, recent advances, main challenges and future perspectives arising from their integration are discussed. Contents of this paper are organised within the context of an invited keynote talk given at the 2021 IEEE Asia-Pacific Conference on Geoscience, Electronics and Remote Sensing (AGERS – 2021).

Linear transport infrastructures, as bridges and viaducts, are exposed to natural hazards or endogenous events, which can affect their operation and structural integrity. Recent unpredicted bridge failures and collapses highlight the need for effective structural monitoring operations, especially for aged concrete structures. Non-Destructive Testing (NDT) methods, such as Ground Penetrating Radar (GPR) and Mobile Laser Scanner (MLS), have been used for the assessing and monitoring of such structures in recent years. Our paper reports on the outcomes of the integrated monitoring method based on the use of GPR and MLS technologies for the structural assessment of bridges and the prevention of damages induced by structural subsidence. The analyses we made aim to assess the structural integrity of the Olivieri Viaduct, located in Salerno, Italy. Designed for this, a GPR inspection was made using multifrequency GPR systems equipped with both ground-coupled and air-launched antennas. In addition, MLS surveys were carried out to analyze and quantify pavement surface irregularities. The surface and structural condition of the pavement as measured by MLS has been integrated with GPR outcomes to identify and classify potential damage sources likely responsible for layer deterioration and surface decay. This study confirms that an integrated nondestructive monitoring approach based on GPR and MLS technologies can be successfully implemented to assess the health-condition of critical assets, clearing the way for integrated approaches in continuous monitoring of transport infrastructure.

In recent years, the Building Information Modeling (BIM) methodology has emerged as a pivotal approach for the design and management optimization of infrastructures. The need for fully integrated models has led to develop various tools to support the designers throughout the decision-making process of the entire life cycle of an infrastructure project. In particular, the design of transport infrastructures is a complex process that involves issues concerning all the different fields of civil engineering. In this framework, the BIM method can be efficiently applied to support the project of drainage systems capable of protecting railways and roadways from potential hydraulic risks. According to the above, this study presents an optimized BIM-based methodology for the integrated analysis of the flood risk and the design of the drainage system components. Specifically, the proposed method allows to implement hydraulics and hydrological numerical models as part of the different design phases. Such an approach holds two main advantages for the designer. On one hand, the computational modelling provides a decisive support in both defining water surface profiles over the designed area and identifying the spots exposed to major hydrological risks. On the other hand, the hydraulically guided modelling of each component of the drainage system allows for an increase of the overall productivity and efficiency of the project. A case study is presented to highlight the great potential of the proposed approach in ensuring both the integrity of datasets and the rapid clash detection of critical interferences between each system component. In conclusion, new perspectives about a fully integrated hydraulics analysis in the infrastructures design process are stressed out.

Railways are important assets requiring continuous and effective monitoring. Within this context, non-destructive testing (NDT) methods are gaining momentum including, amongst others, the ground-penetrating radar (GPR) technique. GPR has proven its viability at providing effective condition-based assessment of railway ballast and identifying several different sources of decay. In this paper, the main challenges related to the data collection and processing stages for railway ballast investigations are reported. In addition, a review of main survey protocols and data processing strategies, including state-of-the-art research in this area of endeavor is presented, in terms of the issues related to the configuration of the track-bed structure (i.e., the effects of rails and sleepers on the signal) and the main inspection targets (i.e., ballast fouling, water content and segregation of the aggregates).

Use of non-destructive testing (NDT) methods has tremendously increased over the last decades in a wide range of civil and environmental engineering applications. Research on stand-alone use of electric, electromagnetic, optical and acoustic NDT techniques has rapidly progressed and potentials of these methods have been comprehensively explored and evaluated, making the majority of techniques established in many areas of endeavour. Research focus areas have been mostly on developing new theories, the advancement of hardware and software components, and on introducing novel surveying protocols, data processing and interpretation methods. Accordingly, the standard of quality and accuracy of data has reached very high levels with available technology.\ud Parallel to this, the integration between sensing technologies, in terms of collecting and modelling multi-source, multi-scale and multi-temporal data, has become a very challenging task in research. This concept is in fact being acknowledged as a fundamental area of research development that could highly contribute towards the enhancement of the capabilities of existing NDT methods in new and non-conventional scenarios. This trend is motivated by the need to provide more effective solutions for the investigation of complex scenarios. At the same time, a balance among project costs and time requirements on one hand, and resource management-related constraints due to the involvement of multiple methods, equipment and interdisciplinary expertise in the other, must be maintained.

This work proposes an experimental-based model for the assessment of the stiffness of a road flexible pavement using ground-penetrating radar (GPR – 2 GHz horn antenna) and light falling weight deflectometer (LFWD) non-destructive testing (NDT) methods. It is known that the identification of early decay and loss of bearing capacity is a major challenge for effective maintenance of roads and the implementation of pavement management systems (PMSs). To this effect, a time-efficient methodology based on quantitative and qualitative modelling of road stiffness is developed. The viability of using a GPR system in combination with LFWD equipment is also proven.

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

Ground Penetrating Radar (GPR) has proved to provide a high reliability in detecting several subsurface features such as water and gas pipes, energy and telecommunication supplies, water reservoirs or air voids. The present work uses a comparison between different central frequencies of investigation to reconstruct the network of utilities located underneath a paved surface and to understand the best strategy of analysis to undertake. To this purpose, a 757 m2 paved carpark situated in London was used as test site and divided into three smaller areas. Central frequencies of investigation of 250 MHz, 400 MHz, 500 MHz, 1000 MHz, 2000 MHz, and 4000 MHz were selectively employed over these areas, and the outcomes from the 250 MHz, 500 MHz, and 1000 MHz are here analyzed. The analysis of the data has detected the presence of several utility lines with placements different from those represented within the design charts. Useful insights about the performances of different central frequencies of investigation are here discussed, as well as the usefulness of GPR in validating information collected by visual inspections and available from cartographic maps. Ground Penetrating Radar (GPR) has proved to provide a high reliability in detecting several subsurface features such as water and gas pipes, energy and telecommunication supplies, water reservoirs or air voids. The present work uses a comparison between different central frequencies of investigation to reconstruct the network of utilities located underneath a paved surface and to understand the best strategy of analysis to undertake. To this purpose, a 757 m2 paved carpark situated in London was used as test site and divided into three smaller areas. Central frequencies of investigation of 250 MHz, 400 MHz, 500 MHz, 1000 MHz, 2000 MHz, and 4000 MHz were selectively employed over these areas, and the outcomes from the 250 MHz, 500 MHz, and 1000 MHz are here analyzed. The analysis of the data has detected the presence of several utility lines with placements different from those represented within the design charts. Useful insights about the performances of different central frequencies of investigation are here discussed, as well as the usefulness of GPR in validating information collected by visual inspections and available from cartographic maps.

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.