Bridge Monitoring Strategies for Sustainable Development with Microwave Radar Interferometry
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1. Introduction
Most constructed structures, including buildings, roadways, and bridges, require regular inspection and assessment so that appropriate action can be taken to maintain their effectiveness. This action could be in the form of Principal Bridge Inspections or general inspections, which are conducted annually and monitored by the Highways Agency. The cost of correcting defects in buildings and civil infrastructure is immense in many countries [1]. A study carried out in the U.S. estimated that the failure of civil infrastructure systems to perform at their expected level can reduce the national gross domestic product, while a study conducted within the UK showed the immense annual cost of correcting defects in buildings and civil engineering structures—a sum greater than the total profits of all UK construction companies [2]. For this reason, in many developed countries, where numerous infrastructure systems have become old and deteriorated, the need for efficient maintenance strategies has become crucial. Thus, on the one hand, the maintenance of highways and associated structures, such as bridges, is a pressing issue, while on the other hand, local authorities who own infrastructure are reluctant to carry out costly maintenance, such as strengthening works [3]. Making good maintenance decisions is, therefore, important, but it relies on the application of years of practical experience.
Traditionally, visualization inspections on-site have been the method for assessing bridge conditions [4]. However, this approach has limitations. It detects visible damage only, overlooks hidden problems before they surface, and relies heavily on the inspector’s experience and judgment. On-site visual assessments are neither economical nor appropriate for ongoing monitoring [5]. Infrastructure health monitoring has been shown to be quite successful in the past several years in the routine evaluation of structural problems, and it is now more accessible—especially for bridges—due to lower data gathering costs [6]. By proactively identifying possible issues and estimating the remaining lifetime, this method highlights how crucial it is to comprehend the existing condition and load-bearing capability for long-term bridge health. Damage to a bridge’s construction that results from explosions or natural catastrophes, such as earthquakes, affects the material’s characteristics, boundary conditions, and structural integrity [7]. Structural problems are also a result of service load factors, including aging, traffic expansion, increasing degradation, and environmental effects [8,9]. The benefit of putting in place a bridge health monitoring system is that it allows for an objective evaluation of the structure’s state over time. This proactive strategy makes it possible to identify damage in the aftermath of a catastrophic occurrence or before it becomes a serious problem [10].
Various methodologies exist for monitoring the health of bridges at different stages, with a prominent approach being vibration-based testing. This method relies on dynamic testing to detect structural changes, assuming that damage affects stiffness rather than mass loss [11,12,13]. Bridge health may be effectively monitored and detected through the use of dynamic testing, which assesses the mechanical reaction of a structure’s deformation. This method has attracted a lot of research interest. Dynamic testing techniques frequently include piezoelectric accelerometers and fiber optic sensors, which enable the precise and dependable recording of dynamic deformation time series [14,15,16]. However, the precise placement of sensors in predetermined places and the required hardwiring to the data gathering system are prerequisites for deploying these techniques on monitored bridges. Although this method works well, it is expensive, time-consuming, and there is a chance that ancient bridges will be harmed. Balancing the benefits of dynamic testing with the challenges of installation is crucial for optimizing bridge health monitoring strategies [17,18,19,20,21].
Ground-based real aperture radar (GB-RAR) systems and space-borne synthetic aperture radar (SAR) systems are the two primary types of microwave interferometric radar systems, which are essential for a variety of applications. Because of its ability to provide wide-area and semi-continuous monitoring, space-borne interferometry SAR (InSAR) is a popular option for a number of applications [22,23,24,25]. Both ground-based and satellite radar interferometers share fundamental principles, differing only in viewing geometry. Interferometric radar relies on the phase information of back-reflected microwave signals to detect target displacement within the target cone. However, measuring the distance directly from the phase information encounters challenges, including ambiguity phases and sensitivity to atmospheric conditions and sensor position changes. The electrical device’s capacity to detect minute phase rotations and carry out further signal processing determines how accurate the interferometry phase will be. In a perfect world, 0.1 mm precision could be attained with electromagnetic waves in the Ku band [26,27,28]. Space-borne InSAR measures provide weekly updates and great spatial resolution, enabling millimeter-level precision in tracking geometrical changes. Estimating these components from multitrack radar data is made possible by infrastructure deformation, particularly in vertical and horizontal directions. On the other hand, the track direction affects the sensitivity. Balancing accuracy and various factors in radar systems is thus critical for optimizing their effectiveness in monitoring and detecting structural changes.
The GB-RAR interferometry method is a widely embraced technology for measuring the dynamic deformation of bridges [29,30,31,32,33,34,35,36]. This methodology delivers accurate sub-millimeter deformation monitoring in a non-contact manner. Notably, it offers a straightforward and rapid setup for investigations while enabling a detailed assessment of the condition of the entire bridge structure. In this paper, we focus on the use of ground-based SAR (GB-SAR) interferometric sensors, since they offer flexible application and greater degrees of freedom in terms of geometric view and temporal baselines. The GB-SAR technique has reached maturity in the deformation monitoring of landslide-prone areas and dams, both for periodic measurement and for implementation with continuously operating systems.
Research within the field of GB-SAR data processing remains ongoing, especially for the robust estimation of displacements and vibration frequencies and the development of sensor networks and methodologies for data integration, potentially offering the opportunity to analyze different observations in a spatial and temporal context. In this research work, we describe the application of GB-SAR interferometry to the monitoring of bridges. Three case studies applying different types of microwave radar systems to cases relating to a metallic bridge across a motorway, a stone bridge, and a concrete bridge crossing a river are discussed. The structure of the paper is as follows. Section 2 introduces the basic principle of GB-SAR interferometry for the measurement of aerial displacements and vibration frequencies. Section 3 summarizes the GB-SAR results for the three case studies. Section 4 draws some conclusions and suggests new research directions for the InSAR monitoring of bridges.
4. Conclusions
Ground-based microwave radar interferometry stands at the forefront of recent innovations in infrastructure health monitoring. This technique offers the unique advantage of providing comprehensive insights into the tested structure’s global behavior. However, a notable drawback is the challenge of precisely localizing measured displacements on the structure due to the contribution of all points within the same resolution cell.
This paper introduced innovative strategies to overcome the challenges associated with ground-based microwave radar interferometry in bridge monitoring. The study showcased monitoring results from various bridges using different radar systems. A long-span metallic railway bridge was monitored under a polarimetric real aperture radar system, while a stone bridge and a concrete road bridge underwent measurement via a synthetic aperture radar system, employing a linear rail and MIMO array.
Novel signal processing methodologies were proposed, emphasizing their capability to provide precise location and vertical deformation in structure measurements. The advantages of the microwave radar system—specifically the MIMO array design methodology—and its application to bridge monitoring were elucidated. The MIMO radar system offers not only full-range and cross-range resolution but also an exceptionally high acquisition rate, catering to both dynamic and static monitoring purposes.
The research delved into the dynamic responses of three bridge types—a passing metallic bridge, a stone bridge, and a concrete bridge—under loading conditions. The findings underscore the capability of the microwave radar interferometer, coupled with advanced algorithms, to effectively monitor any bridge type with unprecedented spatial and temporal resolution. This breakthrough marks a significant advancement in bridge health monitoring, promising enhanced accuracy and efficiency through radar technology.
In summary, ground-based microwave radar interferometry emerges as a powerful tool for assessing infrastructure health, offering a holistic view of a structure’s behavior. Despite challenges in precise localization, this paper presented inventive strategies and showcased successful monitoring results on various bridges. The proposed signal processing methodologies and the application of the MIMO radar system further demonstrated the potential to revolutionize bridge monitoring, setting a new standard for spatial and temporal resolution in the field.
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