Physical modelling of root-soil interaction of trees under lateral loads

  • Xingyu Zhang

Student thesis: Doctoral ThesisDoctor of Philosophy


Understanding the stability of trees under lateral loads arising from natural hazards (e.g. extreme weather, rainfall-induced landslides, rock-fall and debris flows) is important as fallen trees can become a potential threat to life and infrastructure and affect forest productivity. Because of the hidden root system below-ground and the difficulty involved in conducting full-scale testing on trees with closely controlled variables, previous research has focused mainly on tree-wind interaction rather than root-soil interaction. In this thesis, centrifuge testing was used as the primary method of investigating the behaviour of root systems when trees were subjected to lateral loads.

Two 1:20 scale 3D printed analogue root system models with different architectures (namely, deep and narrow, and shallow and wide) were reconstructed and 3D printed based on field-surveyed root architecture data. Preliminary push-over tests were performed both at elevated-gravity (centrifuge 20g) and normal-gravity (1g) conditions. It was found that the shallow and wide model showed higher anchorage strength than the deep and narrow model. Regardless of the root architecture, the root anchorage strength measured from dry soil was higher than that from saturated soil. However, once the effective stress was the same, regardless of water conditions, the root anchorage strength was the same. The root-soil interaction in 1g model tests was overestimated, however this could be corrected for excessive dilation at low confining stress by using modified scaling laws.

An extensive series of controlled push-over tests at 20g exploring a range of variables associated with the response of trees under lateral loading was conducted on 1:20 scale 3D printed tree root models. The peak overturning moment obtained from the centrifuge tests was verified against data from field winching tests. It was revealed that the horizontal roots orientated in the loading direction and the central taproot complex contributed most to the overturning resistance. Increasing soil matric suction due to a lowering of the water table, increasing the loading rate and considering the presence of the fine root fraction all resulted in higher moment capacity and rotational stiffness of the root systems. The overturning behaviour in fully saturated soil was ductile, whereas that in partially saturated cases was more brittle, with more root breakages in the windward horizontal roots and the taproot complex. These results suggest that it is important to measure the groundwater conditions when conducting winching tests and demonstrated a connection between soil effective stress, total root breakage area and peak moment resistance.

In physical modelling of root-soil interaction using small-scale models, it is desirable to choose a large scale factor that can avoid unwanted boundary effects from a model container, however, this in turn may result in scale effects when the root is over-scaled and surrounding coarse granular soil could no longer be considered as a continuum. The Discrete Element Method was used to investigate scale effects potentially arising in physical modelling of root-soil interaction. Both lateral and sinker roots of the model root system were analysed, and the horizontal and vertical behaviour was compared with results from the Finite Element Method, where soil is modelled as a continuum. According to the scaling law, with the same prototype scale and particle size distribution, different scale factors/g-levels were applied, in which case the ratio of root diameter (dr) to mean particle size (D50) was changed. It was found that even at the lower dr/D50 investigated in this study (i.e., from 6 to 21), scale effects on the bearing capacity were negligible for both horizontal and sinker roots when displaced downwards. However, grain-size effects were observed during uplift of the horizontal root as the dimension of the affected soil volume above the root was found to depend on both root diameter and mean particle size. Additionally, scale effects on the shaft resistance of the sinker root when displaced upward was found to be well interpreted using a combination of cavity expansion and root-grain size ratio and was also a function of both root diameter and mean particle size.
Date of Award2021
Original languageEnglish
SponsorsChina Scholarship Council
SupervisorJonathan Knappett (Supervisor) & Matteo Ciantia (Supervisor)


  • Root-soil interaction
  • Centrifuge
  • 3D printing
  • Push-over
  • Moment capacity
  • Grain size effects

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