Uprooting Safety Factor of Trees from Static Pulling Tests and Dynamic Monitoring

Tree fall events are acknowledged as important causes of damage in urbanized areas and tree risk assessment is now part of general risk management analyses. Preserving arboreal heritage also implies biomass and biodiversity conservation, reduction of superficial soil erosion, improving the stability of potentially unstable slopes. In this context, tree risk assessment techniques used by professional agronomists may benefit of a deeper mechanical understanding of the soil-root interaction mechanisms controlling tree stability. From a Geotechnical point of view, the tree root plate is conceptually assimilated to the foundation of a slender structure subject to eccentric loads. In the paper results of static pulling tests and wind dynamic monitoring on two liquidambar trees are described; estimations of their uprooting safety factor are then derived according to the currently available techniques used in the agronomic field. The pull tests were performed using an advanced loading-unloading scheme, along two orthogonal directions. The preliminary interpretation shows that the system is characterized by a marked non-linear and irreversible behaviour, even at low loading levels as usually tested in agronomic practice. The results suggest that dynamic tree stability testing could be a valuable tool for stability assessment, albeit the interpretation of the results requires expertise in the dynamic behaviour of trees.