Degree Name

Doctor of Philosophy


School of Civil, Mining and Environmental Engineering - Faculty of Engineering


Bioengineering including native vegetation is an ancient method of improving the stability of slopes. In modern railway engineering, this technique is re-captured for increasing the soil stiffness and shear strength of sub-grade beneath rail tracks. Currently this practice has become increasingly popular in Australia for stabilising railway corridors built over expansive clays and compressive soft soils. The tree roots provide three stabilising functions: (a) reinforcement of the soil, (b) dissipation of excess pore pressures and (c) establishing sufficient matric suction to increase the shear strength. The main focus of this research is to investigate the effects of vegetation on soil matric suction, ground settlement and lateral movement (radial consolidation). A mathematical model for the rate of root water uptake has been developed based on the root growth rate and considering ground conditions, type of vegetation and climatic parameters.The three independent features in the root water uptake model considered in detail are soil suction, root distribution, and potential transpiration. In order to establish a rigorous analysis for estimating the actual transpiration or root water uptake, the above mentioned factors have been quantified through relevant equations to develop the proposed root water uptake model. A two dimensional finite element approach based on ABAQUS has been employed to solve the transient coupled flow and deformation equations. The proposed root water uptake model has been implemented in the coupled analysis by introducing a sink term as a subroutine in the finite element analysis. The finite element mesh can be constructed using partially/fully saturated soil elements, representing the salient aspects of unsaturated permeability and the soil water characteristic curve. The model formulation is based on the general effective stress theory of unsaturated soils. Based on this proposed model, the distribution of the matric suction profile adjacent to the tree has been numerically analysed. To validate the model, an array of field measurements conducted at Miram site in Victoria, Australia and the data have been compared with the numerical predictions. The predicted results calculated using the soil, plant, and atmospheric parameters contained in the numerical model, compared favourably with the field and the associated laboratory measurements, justifying the assumptions upon which the model has been developed. The numerical analysis encompassing the developed root water uptake model can reasonably predict the region of maximum matric suction (away from the tree trunk axis), which has been consistent with the field measurements. Moreover, field measurements taken from the previously published literature have been compared with the numerical predictions. It is found that given the approximation of the assumed model parameters, the agreement between the predicted results and field data is still promising. The influence of different parameters on the maximum rate of root water uptake is investigated through parametric and sensitivity analyses. In addition, the rate of selected parameters such as potential transpiration and its distribution, suction at wilting point, the coefficient of permeability and the distribution of root length density have been studied in detail. The findings of this study confirm that four key parameters, including permeability, wilting point suction, density and distribution of the root length, and the rate of potential transpiration should be estimated or measured accurately in order to predict the behaviour of clayey soils near tree roots. The action of a single tree on improving the soil behaviour has been compared to a vertical drain with applied suction (vacuum pressure). It is seen that root water uptake and associated matric suction is analogous to a prefabricated vertical drain with vacuum preloading, and the lateral inward displacements simulate the radial consolidation process of prefabricated vertical drains. If a pattern of trees can be grown systematically along rail corridors, this may offer a cheaper and more environmentally attractive solution to vertical drains in the long-term. The results of this study provide a valuable and a relatively accurate mean to estimate the influences of vegetation on ground. The numerical model developed herein offers practicing geotechnical engineers an effective tool for designing structures on vadose zones containing vegetation. It is desirable to consider the influence zone of tree roots and the improved soil properties in modern geotechnical designs, benefiting from native vegetation.

02chapter1.pdf (202 kB)
03chapter2.pdf (619 kB)
04chapter3.pdf (2635 kB)
05chapter4.pdf (234 kB)
06chapter5.pdf (3388 kB)
07chapter6.pdf (4716 kB)
08chapter7.pdf (5593 kB)
09chapter8.pdf (1992 kB)
10chapter9.pdf (170 kB)
11bibliography.pdf (179 kB)
12appendices.pdf (1224 kB)



Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.