Soft soil improvement induced by tree root suction

Behzad Fatahi, Buddhima Indraratna and Hadi Khabbaz

Bioengineering aspects of native vegetation are currently being evolved to improve soil stiffness, slope stabilisation, and erosion control. 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 effects of vegetation on soil matric suction, shrinkage and ground settlement are discussed in this paper. A mathematical model for the rate of root water uptake that considers ground conditions, type of vegetation and climatic parameters has been developed. A conical shape is considered to represent the geometry of the tree root zone. Based on this proposed model, the distribution of the moisture and the matric suction profile adjacent to the tree are numerically analysed. Field measurements taken from literature published previously are compared with the authors’ numerical model. The predicted results, calculated based on soil, plant, and atmospheric parameters contained in the numerical model, compared favourably with the measured results, justifying the assumptions upon which the model has been developed. The findings of this study indicate that due to significant changes in soil moisture content induced by tree roots, the shear strength of the soil will be enhanced. It is desirable to consider the influence zone of tree roots and the improved soil properties in modern geotechnical designs, benefiting from native vegetation.