Optimising the use of ground penetrating radar (GPR) for urban road investigations

Although ground penetrating radar (GPR) technology has existed for many decades, it has only been in the last 15 - 20 years that it has undergone great development for use in near surface ground investigations. GPR is now a commonly used geophysical technique for assessing layer thicknesses and material condition of road structures and foundations. Assessing the condition of road structures, foundations and the supporting ground, to plan subsequent maintenance, is essential to allow the efficient long-term functioning of an urban transport network. Intrusive investigations provide vital information, but are often costly and time consuming, and also have the limitation that only data at discrete points is obtained. The nature of urban sites means that the ground conditions are often highly variable. Existing road structures have often been subject to much maintenance and re-construction, and many urban roads are constructed over ground that has had a previous use. This can result in roads and their foundations containing several layers and sections of material of different ages and condition, often overlying discrete buried objects or structures. Also, it is common for urban sites to contain buried service pipes of various materials and purpose. Various other un-controllable site specific parameters can affect the quality of GPR data obtainable, including road and ground material type and moisture condition, but it is possible to tailor a GPR survey to obtain the optimum data from a site by adjusting factors relating to the in-situ investigation methodology. Using examples of recent urban road investigations, this paper outlines how the whole process of GPR investigation has to be carefully managed from the planning stage, through the site investigation methodology, to the data processing and presentation, to ensure optimum benefit to the end user of the information obtained.