Improvement of soil stability along rail corridors through native vegetation

Field studies conducted along the railway lines in Australia have revealed the ability to implement the green corridor concept as a soil stabilization method. Native trees grown along rail corridors are capable of increasing the matric suction of the subgrade soil underneath the track substructure via root water uptake, in conjunction with the tree canopy evapo-transpiration. Moreover, these trees are capable of providing significant mechanical reinforcement through the anchoring effect provided by the root network plus the additional cohesive increment due to hair roots generating osmotic suction. Much of the previous research carried out to quantify the mechanical strength generated by tree roots has been mainly based on empiricism. In many cases, empirical relations have been developed for a given tree species grown under known soil conditions. The extrapolation of such empirical relations from one tree-soil system to another can be misleading. Furthermore, the effect of transpiration by tree canopy and its influence on the sustained suction equilibrium generated at the root zone for stabilising soft subgrade has not been considered rationally. To accommodate the above natural phenomena, a novel computational model has been developed to quantify the overall suction effect provided by the tree roots and its continual link with the rate and magnitude of canopy evapo-transpiration. Root based suction of a tree improves the shear strength; accelerates the pore water pressure dissipation and it may alter the potential failure conditions of the soil-root system from a saturated to an unsaturated domain. Therefore, it is necessary for the root based suction and the mechanical properties of the root network to be analysed within a coupled multiphase framework. Accordingly, this paper will present the requirement of an advanced shear strength model that captures and combines the root reinforcement effect with both osmotic and matric suction components generated in the soil through naturally coupled osmotic evapo-transpiration phenomenon.