Soil Desiccation from Vegetation and its Influence on the Cyclic Stiffness and Strength of Clay Soils

When engineers notice vegetation next to a structure, typically the first thought that comes to mind is usually a negative one. Anecdotal evidence from railway employees suggested the opposite; a noticeable improvement in rail track performance was observed where there were substantial stands of trees adjacent to the track. This perception led to a preliminary study conducted by the author and a colleague. This limited study showed an improvement in soil strength and stiffness at the sides and shoulders of the trackbed in the vicinity of trees on sites with highly plastic clays. The current study investigates trackbed properties, in particular the cyclic stiffness and undrained – unconsolidated (UU) shear strength directly underneath and adjacent to the track. Soil samples were taken from vegetated and non-vegetated sites containing expansive clay soils in western Victoria and Queensl and. The vegetated and non-vegetated sites were selected within close proximity of each other to minimize variations in soil properties. This paper reports on the findings of the first half of the project, based on sampling in October on the Victorian sites and in November in Queensland. The results presented in this paper confirm the vegetated sites have improved trackbed strength and stiffness, mainly attributed from an increase in total soil suction (and decrease in soil moisture content).