Georadar: le proprietà meccaniche della sovrastruttura stradale

L’ indagine preventiva delle caratteristiche prestazionali di una pavimentazione stradale risulta di prioritaria importanza nella corretta implementazione di un efficace sistema di manutenzione programmata delle strade o Pavement Management System (PMS), in quanto la determinazione della soluzione manutentiva più idonea può consentire un notevole contenimento dei costi di intervento, specialmente ove questi siano legati ad operazioni di ripristino e riabilitazione delle condizioni strutturali di origine

Road foundation detailing using ground penetrating radar systems with different frequencies

This paper reports on the assessment of the underground construction details of a road pavement using different frequency ground penetrating radar (GPR) antenna systems. In addition to this, the possible presence and location of an underground watercourse was investigated in this work. The existence of the latter problem was perceived due to reoccurrence of longitudinal and traversal road surface cracking as well as subsidence at a particular location of the road. Reoccurrence of this damage was interpreted and related to the possible existence of an underground watercourse. The original design and the construction of the road were as such to prevent this movement. Therefore it seemed necessary to perform a GPR survey to investigate and confirm the underground construction details of the road. To this purpose, the identified area was surveyed using high to low frequency antennas with 2000 MHz, 900 MHz, 600 MHz and 200 MHz central frequencies of investigation. The results were conclusive in terms of construction details provided and evidence of subsidence within the road identified. The maximum depth of penetration achieved by the use of the 600 MHz and the 200 MHz antennas (maximum of 3 m) did not allow to identify or confirm the existence of any underground watercourse

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 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

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

Recent advances in mapping tree roots using ground penetrating radar

Environmental issues and preservation of natural heritage, especially ancient trees and rare plants, are becoming priority objectives to achieve. Unknown pathogens carried along by the wind can lead to epidemic phenomena and often to a quick death of entire forests. To this effect, active and passive methods can be used to reduce the risk of premature death of trees. Passive methods rely on the application of policies for the control and the management of the forestall heritage. These are based on the monitoring of living trees and are aimed at identifying the early-stage symptoms of the disease. Within this context, use of destructive testing methods is increasingly discouraged, and non-destructive testing (NDT) methods are emerging as the only viable solution for a non-intrusive assessment of the disease. The ground penetrating radar (GPR) non-destructive testing method has proved to be one of the most powerful NDT methods, due to a high versatility, rapidity in data collection and the provision of reliable results at relatively limited costs. Applications of GPR in forestry science are related – but not limited to - the effective tree trunk assessment and appraisals, tree roots mapping, soil interaction with tree and plants. This study reports a demonstration of the GPR effectiveness in tracking tree roots. The main objective of the research was to provide an effective and high-resolution mapping of the tree roots. To this purpose, the soils around a fir tree and an oak tree were investigated using a ground-coupled multi-frequency GPR system equipped with 600 MHz and 1600 MHz central frequency antennas. A dedicated data processing algorithm was firstly developed to filter out the data from noise-related information and to highlight deep reflections from attenuated targets. At a later stage, a multi-step algorithm pinpointing the identified targets (i.e., the vertex of the reflection hyperbolas) in a three-dimensional environment was created. Results have proven the viability of GPR in mapping tree roots for different species of trees. The proposed algorithm has allowed to successfully identify both shallow (i.e., within the first 25 cm from the soil surface) and deep (i.e., underneath 25 cm of depth) tree root systems

A comparison between different central frequencies of investigation in buried utility detection through GPR: a study case

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 faces 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 parking 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 different placements than 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

Mapping the root system of matured trees using ground penetrating radar

This study reports a demonstration of the ground penetrating radar (GPR) potential in health monitoring of tree roots. The main aim of the research was to provide effective and high-resolution mapping of tree root systems. To this purpose, a dedicated data processing methodology, based on two main chronological stages, was developed. First, an algorithm was proposed to filter out the data from noise-related information and to enhance deep reflections from attenuated targets. At a later stage, a multi-step algorithm connecting the identified targets (i.e. the vertices of the reflection hyperbolas) in a three-dimensional environment was created. To demonstrate the viability of the proposed methodology, the soils around two different tree species (i.e. fir and oak trees) were investigated using a ground-coupled multi-frequency GPR system equipped with 600 MHz and 1600 MHz central frequency antennas. The method has allowed to identify distinctive features in terms of shallow (i.e. within the first 25 cm from the soil surface) and deep (i.e. lower than 25 cm from the soil surface) tree root systems for different species of trees

Integration of GPR and FWD methods for the assessment of airfield aprons

Airport apron relates to an airfield area dedicated to the parking, loading/unloading, refueling and boarding of aircrafts. The standard conventional pavement solution in apron areas is a concrete rigid pavement with jointed concrete slabs, which is due to two main reasons. First, use of concrete technology helps to prevent the potential viscous behaviour of the hot-mixed asphalt solution. This is caused by long-term and permanent loads, especially at high temperatures. Secondly, use of concrete blocks avoids the decay of the wearing course due to the contact with fuel. Although it is relatively easy to design the working features of hardened concrete for apron surfacing purposes (i.e., following the requirements of pavement quality standards), great attention must be paid to the laying stages and construction process. This is to ensure that the laid concrete attains all the designed properties and no premature decays occur. Decays include, inter alia, uncontrolled cracking throughout the concrete slabs. To that effect, role and magnitude of concrete cracking in affecting strength and durability of a rigid pavement subject to external loads is still under debate. Monitoring and assessment of concrete cracking is a complex task, and several theoretical and experimental models have been developed over the past years. To this purpose, ground-truth information were collected using destructive (e.g., concrete sampling) and non-destructive testing (NDT) methods. In this regard, ultrasonic testing (UT) has been widely used for quality control of concrete and damage detection purposes. On the other hand, the falling weight deflectometer (FWD) technology is commonly used for the assessment of stiffness-related parameters of pavement structures. To this effect, mechanical properties of pavements are usually estimated in combination with the geometric information (i.e., thickness of layers/slabs) collected by the ground-penetrating radar (GPR) NDT method. In this study, a demonstration of the potential of integrating ground-penetrating radar (GPR) and falling weight deflectometer (FWD) non-destructive testing (NDT) methods for the assessment of an airfield apron has been given. The main objective was to provide an effective methodology capable to combine multi-source information from FWD, light falling weight deflectometer (LFWD), GPR, pavement construction stages and development of decay over time (available from the airport maintenance company) in order to assess the mechanical properties of an airfield apron affected by early-stage and widespread cracking. The structure of the apron was a rigid pavement with jointed concrete slabs. To this purpose, an airport apron area with dimensions of 190 m × 90 m, paved by a grid of squared concrete slabs with a side length of 7.5 m, was investigated. FWD, LFWD and a ground-coupled multi-frequency GPR system with 600 MHz and 1600 MHz central frequency antennas were used for testing purposes. The results from the integrated application of the above NDTs demonstrated significant potential for the interpretation of distinctive features of the concrete slabs, including cracking, that may affect the mechanical behavior of the pavement

Inferring strength and deformation properties of hot mix asphalt layers from the GPR signal: recent advances

The great flexibility of ground-penetrating radar has led to consider worldwide this instrument as an effective and efficient geophysical tool in several fields of application. As far as pavement engineering is concerned, ground-penetrating radar is employed in a wide range of applications, including physical and geometrical evaluation of road pavements. Conversely, the mechanical characterization of pavements is generally inferred through traditional (e.g., plate bearing test method) or advanced non-destructive techniques (e.g., falling weight deflectometer). Nevertheless, measurements performed using these methods, inevitably turn out to be both much more time-consuming and low-significant whether compared with ground-penetrating radar’s potentials. In such a framework, a mechanical evaluation directly coming from electromagnetic inspections could represent a real breakthrough in the field of road assets management. With this purpose, a ground-penetrating radar system with 600 MHz and 1600 MHz center frequencies of investigation and ground-coupled antennas was employed to survey a 4m×30m flexible pavement test site. The test area was marked by a regular grid mesh of 836 nodes, respectively spaced by a distance of 0.40 m alongside the horizontal and vertical axes. At each node, the elastic modulus was measured using a light falling weight deflectometer. Data processing has provided to reconstruct a 3-D matrix of amplitudes for the surveyed area, considering a depth of around 300 mm, in accord to the influence domain of the light falling weight deflectometer. On the other hand, deflectometric data were employed for both calibration and validation of a semi-empirical model by relating the amplitude of signal reflections through the media along fixed depths within the depth domain considered, and the Young’s modulus of the pavement at the evaluated point. This statistically-based model is aimed at continuously taking into account alongside the depth of investigation, of both the different strength provision of each layer composing the hot mix asphalt pavement structure, and of the attenuation occurring to electromagnetic waves during their in-depth propagation. Promising results are achieved by matching modelled and measured elastic modulus data. This continuous statistically-based model enables to consider the whole set of information related to each single depth, in order to provide a more comprehensive prediction of the strength and deformation behavior of such a complex multi-layered medium. Amongst some further developments to be tackled in the near future, a model improvement could be reached through laboratory activities under controlled conditions and by adopting several frequency bandwidths suited for purposes. In addition, the perspective to compare electromagnetic data with mechanical measurements retrieved continuously, i.e. by means of specifically equipped lorries, could pave the way to considerable enhancements in this field of research. Acknowledgements - This work has benefited from networking activities carried out within the EU funded COST Action TU1208 “Civil Engineering Applications of Ground Penetrating Radar”