Use of Ground Penetrating Radar for Site Investigation of Low-Volume Roadways and Design Recommendations (4906-1)

This report will present several case studies describing the use of ground penetrating radar (GPR) technology for site investigations. Two types of GPR will be described-the air-launched and ground- coupled systems. The use of air-launched radar is well established within the Texas Department of Transportation (TxDOT). The limitation of this technology is its depth of penetration. While providing very useful information on the surface and base layers, it provides little information on the subgrade soils. The use of low-frequency ground-coupled radar systems will provide little useful near-surface information but it can provide data on subgrade properties and how they vary along a project. Combining both radar types can potentially provide a comprehensive subsurface investigative tool for both new pavement construction and for major pavement rehabilitation projects. In this report a brief description will be provided of the different systems together with the software used to process the GPR signals. Air-launched data are processed with the COLORMAP system developed by the Texas Transportation Institute. The ground-coupled data are processed using the Road DoctorTM system developed by Roadscanners, Inc. of Finland. The case studies presented were collected on actual TxDOT evaluation projects mainly in the Bryan District. They range from near-surface applications where the goal was to identify changes in pavement structure which were not available in construction records to identifying the areas beneath the pavement subsidence associated with strip mining activities.