Interpreting the influence of tree root water uptake on the long term hydrology of a clay fill railway embankment

This paper uses a numerical model to investigate the influence of tree root water uptake on pore water pressures and the vertical displacement of a clay fill railway embankment. The simulated results of soil wetting and drying are compared with long term field measurements from an instrumented railway embankment subjected to a programme of tree removal. The simulations and field measurements show that trees cause significant seasonal variations in pore water pressure and water content near the soil surface, but can maintain persistent soil suctions at depth within the tree rooting zone. Demonstration of this result using a numerical model requires a root water uptake function to spatially separate water infiltration, evaporation and transpiration processes. When the depth of roots and water uptake are reduced through tree removal, the persistent soil suctions established by the trees are lost as water infiltrates from the soil surface. The model is used to show that trees left on the bottom third of the slope can maintain persistent suctions at the slope toe while potentially reducing seasonal ground movements at the crest that may influence railway track quality. A parametric study compares the influence of plant root water uptake depth on displacement of the embankment slope.