Windthrow resistance of trees: geotechnical engineering approach

Failure of trees in high winds is of interest to a broad array of stakeholders: foresters, meteorologists, homeowners, insurance industry, parks and recreation management. Equally broad is the array of disciplines that contribute to understanding windthrow failure of trees: aerodynamics, forest management sciences, biomechanics, tree biology, and geotechnical engineering. This paper proposes a mechanistic model for assessing the windthrow failure of trees from a geotechnical engineering perspective. The model assumes a homogenized tree root-soil structure enclosed within a cylindrical volume characterizing the root spread and depth. The model predicts the anchorage resistance of a soil-root system by estimating the uprooting resistance of an equivalent circular footing using a 3D load failure envelope with a rotated parabolic ellipsoid shape. The proposed model was validated using the UK Forest Research Tree Pulling Database (UTPD) with 1239 conifer trees of six common species. The results show that the model successfully predicts the windthrow resistance of various tree species and sizes for different soil states. The soil type and state significantly affected the uprooting resistance, with the effective soil unit weight and water table depth being key soil parameters controlling tree anchorage. Conversely, soil friction angle and soil cohesion have only a modest influence on tree anchorage. The influence of desaturation due to negative pore water pressures was also investigated and found to have a significant effect on the uprooting resistance. Although the model shows promise, the paper concludes that further improvements could be made in form and calibration, as discussed in the paper.