GPR applications in mapping the subsurface root system of street trees with road safety-critical implications

Street trees are an essential element of urban life. They contribute to the social, economic and environmental development of the community and they form an integral landscaping, cultural and functional element of the infrastructure asset. However, the increasing urbanisation and the lack of resources and methodologies for the sustainable management of road infrastructures are leading to an uncontrolled growth of roots. This occurrence can cause substantial and progressive pavement damage such as cracking and uplifting of pavement surfaces and kerbing, thereby creating potential hazards for drivers, cyclists and pedestrians. In addition, neglecting the decay of the principal roots may cause a tree to fall down with dramatic consequences. Within this context, the use of the ground-penetrating radar (GPR) non-destructive testing (NDT) method ensures a non-intrusive and cost-effective (low acquisition time and use of operators) assessment and monitoring of the subsurface anomalies and decays with minimum disturbance to traffic. This allows to plan strategic maintenance or repairing actions in order to prevent further worsening and, hence, road safety issues. This study reports a demonstration of the GPR potential in mapping the subsurface roots of street trees. To this purpose, the soil around a 70-year-old fir tree was investigated. A ground-coupled GPR system with central frequency antennas of 600 MHz and 1600 MHz was used for testing purposes. A pilot data processing methodology based on the conversion of the collected GPR data (600 MHz central frequency) from Cartesian to polar coordinates and the cross-match of information from several data visualisation modes have proven to identify effectively the three-dimensional path of tree roots.