Tree stability has been a major concern in development of urban forestry especially in the recent times which has evidenced an increase in the high intensity typhoon events due to climate change. Despite more than 250 years of research conducted on tree stability, there are limited understanding of large deformation subsurface soil-root system biomechanics and its impact on overall stability of tree. The thesis aims is to investigate the subsurface soil-root system biomechanical properties variation of a tree under lateral mechanical loading and its impact on the overall stability of the tree under different environmental and loading conditions. Two types of trees pulling tests have been conducted to obtain variation of rotational stiffness with deformation: 1) unidirectional destructiv...[ Read more ]

Tree stability has been a major concern in development of urban forestry especially in the recent times which has evidenced an increase in the high intensity typhoon events due to climate change. Despite more than 250 years of research conducted on tree stability, there are limited understanding of large deformation subsurface soil-root system biomechanics and its impact on overall stability of tree. The thesis aims is to investigate the subsurface soil-root system biomechanical properties variation of a tree under lateral mechanical loading and its impact on the overall stability of the tree under different environmental and loading conditions. Two types of trees pulling tests have been conducted to obtain variation of rotational stiffness with deformation: 1) unidirectional destructive loading-unloading, and 2) cyclic non-destructive loading-unloading tests. The first type of tests were conducted on 23 trees consisting of four different species grown in natural forest, roadside forest and urban environments. The second type of tests were conducted on 3 urban roadside trees. A novel approach was adapted to utilise the field-measured rotational stiffness and rotation path parameters along with energy-based approach to define stability around an equilibrium through an archetype Augusti model to investigate the tree stability. Biomechanical properties of coarse woody tree roots were also tested by developing a new technique to form standard elementary test samples (ETS). Extrinsic and intrinsic parameters of each ETS were measured and their effect on its biomechanical properties obtained from two different types of loading actions, namely axial pulling, and axial torsion followed by axial pulling, were studied.

The investigation revealed that the biomechanical properties of the subsurface soil-root system continuously vary as the deformation increases, as reflected from the decline of the rotational stiffness with increase in base rotation deformation. Due to this continuously varying behaviour, an initially stable tree could become unstable at later stage of deformation even before the peak base bending moment resistance is achieved. If the lower branch of an initially stable tree is pruned in a way such that the overall centre of mass of above ground tree structure is shifted upwards in height, it may render the tree to be potentially more unstable. The use of initial biomechanical behaviour to investigate the overall stability of the tree structure is shown to be deceptive as the tree which are more stable at the initial undisturbed mechanical state may end up becoming less stable than its comparison counterpart due to continuously varying biomechanical properties. This finding has been observed more explicitly for the urban trees that have shallow plate like root architecture where the rotational stiffness of the subsurface soil-root system showed a drastic decline within initial 0.5º of base rotational deformation.

The continuously varying behaviour of rotational stiffness during the large deformation reloading cycle of a tree is different from initial first cycle of large deformation loading. The rotational stiffness variation during reloading cycle could be divided into three parts. An initial quick decline in the rotational stiffness at very small deformation close to 0º in the first part followed by plateau like behaviour in the second part and a relatively lower decline gradient in the third part. Due to such biomechanical behaviour variation, a tree during the reloading cycle could become more stable than it would have been in its first loading cycle at the later stages of rotational deformation. The rotational stiffness at small deformation cyclic loading within initial 0.25º of base rotational deformation have been found to increase with every cycle indicating possible hardening effect due to permanent deformations taking place in the subsurface soil-root system at small deformation.

The investigation of biomechanical properties of the woody tree roots revealed multiple orientations of fracture planes of ETS, probably due to cell wall ultrastructure of woody roots. The initial slope of force-displacement curve decreases with an increase in initial moisture content, implying that the initial rigidity of the ETS originates from the internal cellular structure deformation with pore water acting as an incompressible cushion. This phenomenon suggests that waterlogging in compacted urban soil condition could make the woody roots less rigid thereby making it less stable. Roots subjected to torsion prior to tensioning does not show major impact on the small-deformation biomechanical behaviour but leads to an increase in moisture content loss, indicating that the root torsion enhances the cell wall fracture mechanism at large deformation biomechanics.