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Background:There have been several recent reports of collapsed roads at the various locations throughout the Bangkok metropolitan area. Most of the problems are caused by improper construction of utility networks and poor rehabilitation work. Ground penetrating radar (GPR) technique was selected to investigate the potential presence of subsurface voids under the road surface. In geotechnical and structural applications, GPR is an excellent tool for being able to image steel reinforcing bars, voids and tendon ducts in concrete structures and, more relevantly to this study, voids beneath concrete roads. Objectives:The objective of this study was to survey the area for potential voids that might exist under the road surface using ground penetrating radar technique. Methods:The GPR survey campaign was divided into two stages, which were the preliminary and detailed surveys. The objective of the preliminary survey was to quickly survey the area for potential voids that might exist under the road surface and subsequently a more detailed survey of those areas was performed to confirm the existence and determined the lateral and vertical extension of the potential void(s) identified in the preliminary GPR survey. The GPR data were collected with 400 MHz antenna mounted on a survey cart.Results: Several void-like anomalies were detected from the GPR data and these selected anomalies were drilled to confirm the existence of a void. However, some GPR anomalies were found not to be voids, and mostly came from areas of past road maintenance or manholes with a hidden cover (asphaltic concrete overlay). One example of a large void under the road surface was detected in this study, being clearly seen in the GPR data and then confirmed by drilling. Conclusion:The GPR method was successfully used for void detection under a main road in Bangkok city. In this study the 400 MHz ground-coupled antenna was used to image potential subsurface voids and these were then confirmed (or not) by drilling boreholes in that area through the road surface. In the example shown in this report as a case study, the identified void was approximately 4 m long, 2 m wide and 1.5 m deep. As a result of its discovery it was subsequently treated by backfilling and a new road surface was then constructed.