Mapping the spatial variation of soil moisture at the large scale using GPR for pavement applications

The characterization of shallow soil moisture spatial variability at the large scale is a crucial issue in many research studies and fields of application ranging from agriculture and geology to civil and environmental engineering. In this framework, this work contributes to the research in the area of pavement engineering for preventing damages and planning effective management. High spatial variations of subsurface water content can lead to unexpected damage of the load-bearing layers; accordingly, both safety and operability of roads become lower, thereby affecting an increase in expected accidents. A pulsed ground-penetrating radar system with ground-coupled antennas, i.e., 600-MHz and 1600-MHz center frequencies of investigation, was used to collect data in a 16 m × 16 m study site in the Po Valley area in northern Italy. Two ground-penetrating radar techniques were employed to non-destructively retrieve the subsurface moisture spatial profile. The first technique is based on the evalu¬ation of the dielectric permittivity from the attenuation of signal amplitudes. Therefore, dielectrics were converted into moisture values using soil-specific coefficients from Topp’s relationship. Ground-penetrating-radar-derived values of soil moisture were then compared with measurements from eight capacitance probes. The second technique is based on the Rayleigh scattering of the signal from the Fresnel theory, wherein the shifts of the peaks of frequency spectra are assumed comprehensive indi¬cators for characterizing the spatial variability of moisture. Both ground-penetrating radar methods have shown great promise for mapping the spatial variability of soil moisture at the large scale

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 signal processing methodology for assessing the performance of ASTM standard test methods for GPR systems

Ground penetrating radar (GPR) is one of the most promising and effective non-destructive testing techniques (NDTs), particularly for the interpretation of the soil properties. Within the framework of international Agencies dealing with the standardization of NDTs, the American Society for Testing and Materials (ASTM) has published several standard test methods related to GPR, none of which is focused on a detailed analysis of the system performance, particularly in terms of precision and bias of the testing variable under consideration. This work proposes a GPR signal processing methodology, calibrated and validated on the basis of a consistent amount of data collected by means of laboratory-scale tests, to assess the performance of the above standard test methods for GPR systems. The (theoretical) expressions of the bias and variance of the estimation error are here investigated by a reduced Taylor's expansion up to the second order. Therefore, a closed form expression for theoretically tuning the optimal threshold according to a fixed target value of the GPR signal stability is proposed. Finally, the study is extended to GPR systems with different antenna frequencies to analyze the specific relationship between the frequency of investigation, the optimal thresholds, and the signal stability

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

Prediction of rutting evolution in flexible pavement life cycle at the road network scale using an air-launched ground-penetrating radar system

In this work, the evolution of damages in pavement life cycle relative to rutting has been modeled in relevant pavement sections. Ground-penetrating radar (GPR) surveys were carried out at the rural road network scale using an air-launched pulsed radar system, 1GHz central frequency of investigation, linked to an instrumented van for collecting data at traffic speed. Surveys were performed in two time periods, six months apart from each other. By knowing the geometrical, traffic, climatic and construction information of each surveyed pavement section, and on the basis of comprehensive literature studies dealing with rutting versus time measurements in several flexible pavement sections during their life cycle, it has been possible to determine a reliable domain of existence by means of rutting versus time prediction curves, in which to locate the pavement section-specific prediction curve, case by case. Results have shown reliable relationships, wherein damage prediction is consistent with those suggested by literature

A review of pavement assessment using ground penetrating radar (GPR)

The use of GPR to obtain information on pavement structures has greatly developed over the past 20 to 30 years. The early 1980’s saw the first major developments of GPR for pavement applications and it is now an accepted technique for pavement investigation. GPR has a proven ability to obtain a variety of information on parameters relating to the structure and materials of the pavement. Despite this, several hindrances to wider use of the technique exist, and there is a requirement to address a number of both perceived and real limitations of GPR use for pavement investigation. This paper aims to provide an up to date discussion and summary of the current and developing uses of GPR for pavement investigation, through reference to previous work and ongoing research,including that conducted by the authors. This paper is intended for both GPR specialists and pavement engineers, and reports the ability of GPR to obtain good data for the various uses described, and discusses the applicability,limitations, and scope of GPR for further developments in pavement investigation

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