Non destructive detection of woody root systems in urban contexts

National audienceFor environmental, landscaped and ecological reasons, the planting of trees in cities is becoming increasingly important, in parallel with the growing development of urban areas. Controlling the development of roots, parts not visible, becomes essential for their preservation in the face of urban development work such as burial of pipes and networks, transport routes, parking or place, sometimes near foundations. As part of the redevelopment of a bitumed square in the cultural site of the "Subsistances" of Lyon, where eight lime trees had been planted at the same period with a regular spacing, some showed a particular development. Non-destructive detection of the main roots was necessary in order to understand this difference in evolution and to minimize the impact of future work. Two prospecting methods (electrical and acoustic) were implemented on the asphalt square. If the electrical resistivity tomography (geophysical exploration method) makes it possible to obtain information on the structure of the subsoil, we have developed an acoustic method based on the Rinn method, making it possible to detect the propagation of a wave between a woody root subjected to acoustic excitation and the part of the trunk to which it is attached. The signal propagating through the sapwood can thus be detected by means of accelerometers attached to the trunk, differentiating in amplitude and speed from the acoustic signal passing through the ground only. An estimate of the extent of the root system and, the direction of major roots buried under an asphalt area can thus be obtained. The particular development such as heaving of the bitumen, regular or puny growth of the trunk or root extent can thus be controlled, and a better preservation of trees or buildings can be envisaged