Effects of plant roots on soil-water retention and induced suction in vegetated soil

Plant evapotranspiration (ET) is considered to be a hydrological effect that would induce soil suction and hence influence the stability of geotechnical infrastructure. However, other hydrological effect, such as the change of soil water retention curve (SWRC) induced by roots, is generally ignored. This study aims to investigate and compare the effects of root-induced changes in SWRC with the effects of ET on suction responses in clayey sand. Two series of laboratory tests together with 21 numerical transient seepage analyses were conducted. A tree species, Schefflera heptaphylla, which is commonly used for ecological restoration in many subtropical regions, was selected for investigation. In order to consider any effects of tree variability on induced suction, six tree individuals with similar age were tested with and without the supply of light. It is revealed that under dark condition when ET was minimal, vegetated soil could induce higher suction than bare soil by 100% after subjecting to a wetting event with a return period of 100 years. This may be explained by the increases in the air-entry value and the size of hysteresis loop induced by roots. Water balance calculation from the numerical analyses shows that even under the supply of light, the amount of ET was only 1.7% of the total volume of water infiltrated. This means that during the wetting event, the contribution of root-water uptake to induced suction in vegetated soil was relatively little, as compared with the effects of root-induced change in SWRC.