Real-scale tests on the cyclic response of trees under toppling loads: experimental investigation and geotechnical interpretative framework

The collapse of trees, exacerbated by extreme weather events associated to climate change, has become a critical issue, particularly in urban environments where the assessment of tree stability is now an integral component of comprehensive risk management strategies. Standard methodologies for assessing tree stability commonly rely on the interpretation of on-site pulling tests, which provide insights into the mechanical response of trees subjected to lateral forces. In this context, geotechnical engineering can provide a significant contribution to evaluate the overturning resistance of trees, particularly in cases where failure mechanisms are governed by root system uprooting. Indeed, the whole root plate can be conceptualized as a shallow foundation, providing mechanical support to the tree against external loads. This paper presents an experimental campaign of pulling tests conducted on mature trees at the Botanic Garden of the University of Dundee (Scotland, UK). Mutually orthogonal loading directions, the effects of load relaxation, as well as the response to cyclic loading up to complete uprooting were systematically investigated. The experimental findings are analyzed within a geotechnical framework, drawing parallels with foundation engineering principles related to cyclic loading effects on direct foundations subjected to rocking actions. In particular, global mechanical parameters at the macro scale of the trunk collar were derived (e.g., rotational stiffness and damping ratio), and their dependence on the applied loading path systematically compared and critically commented. The results suggest the reliability of the idea of assimilating the root plate to a shallow foundation and provide novel insights into the response of trees subjected to lateral loads, contributing to a more comprehensive understanding of their toppling behaviour.