Degree Name

Doctor of Philosophy


School of Civil, Mining and Environmental Engineering


In prehistoric times humans improved ground using native vegetation, and in modern engineering, this methodology is also used to improve the stiffness and shear strength of soil. This technique has slowly become accepted and is now widely practiced to stabilise slopes and railway corridors constructed on compressive soft soils or expansive clays. Tree roots stabilise soil in three ways: (a) they reinforce the soil, (b) they help disperse surplus pore pressure, and (c) they provide an adequate matric suction that increases the shear strength. The main focus of this study is to investigate how tree roots reinforce vegetated ground, and since this interaction between soil and roots influences the shear behaviour of reinforced soil, it needs to be examined. Previous studies of soil reinforced with tree roots focused mainly on saturated soil because it is simple to reproduce and test in a laboratory, but vegetated ground is generally unsaturated due to the soil moisture variations caused by tree transpiration, so how the soil matric suction affects the strength of a root-soil composite must be examined if vegetation is to be used as an effective technique for ground improvement.

A series of field experiments were carried out to investigate the temporal and spatial variations of the soil matric suction close to a mature gum tree in Wollongong, Australia. A comprehensive analysis to identify how climatic data affected the variations in suction was carried out and a method to quantify the monthly evapotranspiration was proposed because it is considered to be the key factor controlling suction in the vadose zones.

A mathematical model was developed to simulate the pullout behaviour of a simplified root system that incorporates the soil matric suction and other important soil and root parameters. The model algorithms developed were coded using the FORTRAN computer programing language to determine how the root and soil parameters affect the pullout capacity of a root-soil block. A sequence of laboratory pullout tests were performed with a specially designed set up and the model results were validated using the laboratory results. The expected results computed using the soil and tree root parameters contained in the analytical model compared favourably with the laboratory measurements, thus validating the assumptions upon which were used for developing the model.

A computer model was developed to predict the stress-strain behavior of vegetated ground using ABAQUS finite element code. According to the author’s knowledge, this is the foremost study to observe the deformation characteristics of tree root reinforced soil incorporating both root induced suction and mechanical reinforcements. For the analysis, a two-dimensional model, developed using the general effective stress theory of unsaturated soils, was used.

The outcomes of this research study deliver important and comparatively precise methods to estimate the effects of vegetation on grounds, and the numerical model proposed herein provides practicing engineers with a useful tool for designing structures on vegetated grounds.



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.