Use of Ground Penetrating Radar for Transportation Infrastructure Maintenance

Abstract

Routine inspection of transportation infrastructure is an essential component of maintenance and rehabilitation programs. However, it often necessitates a significant amount of work and resource allocation. In this study, the feasibility of the Ground Penetrating Radar (GPR) technology to assess and monitor the condition of infrastructure is investigated. The thesis first introduces the basic principles of GPR, its advantages and limitations. The technology, based on the propagation of electromagnetic waves, has the ability to penetrate surfaces and see what is behind. The detection of hidden and underground objects depends on the contrast in the dielectric properties between the surveyed object and the traversed medium. In addition to being non-destructive, GPR has unique advantages such as the rapidity and continuity of measurements. However, traditional obstacles are conductive environments, the cost of data collection, and a relative complexity of data interpretation. After a comprehensive background research, laboratory and field testing, the main conclusions of this thesis are: The step-frequency GeoScope GPR, currently the only system available in Norway for infrastructure surveys, is a very accurate instrument. The error in GPR readings for pavement thicknesses is evaluated to 3,3 %, in line with other published studies. However, as for ground-coupled antennas, it requires more calibration effort with coring than air-coupled models that can calculate the dielectric permittivity continuously. As a general rule, the accuracy of GPR systems is affected by the interpretation process of the collected data. Best results are obtained from manual interpretation. Semi-automatic layer tracking tools are time-saving but may increase inaccuracies in data analysis. GPR is well-suited for geological inspections in tunnels, especially in remotely mapping the cavity behind concrete linings. The distance to the bedrock is very accurately determined since the thickness of the concrete walls is generally known and the dielectric permittivity of air is constant (ε = 1). Ground-coupled antennas are also found to be effective in detecting loose rocks that have fallen from the tunnel roof onto the concrete lining. Such surveys may help detect early signs of rock instability. Given the number of tunnels in Norway and type of construction, the use of GPR can ultimately result in improved tunnel evaluation and considerable cost savings. In the light of the recent focus on environmental damages caused by salt, it is of interest to examine the ability of GPR to measure remaining salt on winter roads. Preliminary results are encouraging and indicate that the technology could have the potential to address this need. A research approach consists in establishing a relationship between GPR amplitudes and salt quantities. In that respect, the accuracy of the salt measurement device SOBO-20 and the water retention characteristics of pavements (not included in this thesis) are studied. However, further investigations of these issues are beyond the scope of this PhD thesis and are not researched. Other applications of GPR in Norway are forensics studies related to frost heave of soils and pavements, and condition assessment of concrete airfields. The conclusion that emerges from this PhD thesis is that Ground Penetrating Radar can be used in various ways across many fields of engineering. Some applications have undergone great development worldwide, while others are innovative and relevant to Norwegian distinctive specificities. It is hoped that this PhD work will contribute to both research and implementation of the GPR technology in Norway.