Evaluating suction profile in a vegetated slope considering uncertainty in transpiration

Understanding root-water uptake in soil slopes is essential because any changes in soil moisture would lead to changes in soil suction and in turn changes in soil shear strength and slope stability. Uncertainties associated with plant characteristics are present in the root-water uptaking process. The objective of this paper is to evaluate the suction profile in a vegetated slope with a grass cover and the stability of the slope during rainfall infiltration. Random field theory is applied to model the spatial variation of the maximum transpiration rate of Bermuda Grass, which is a native grass species in Hong Kong. The slope is subjected to 12 h, 24 h, and 48 h of drying processes before rainfall. A modified Richards equation governing water flow in unsaturated–saturated media is employed to incorporate a sink term of Bermuda Grass. The effects of initial conditions induced by the drying processes of different periods prior to rainfall on the suctions retained in the vegetated slope after rainfall are investigated. Uniform and triangular root distributions are considered to simulate likely root architectures and their influence on the suctions retained. Slope stability analysis is further carried out to investigate how plant transpiration contributes to slope stability. The results demonstrate that, given the same maximum transpiration rate, the root distribution has little influence on inducing or retaining suctions. At a low infiltration flux, the higher the coefficient of variation of the maximum transpiration rate and the longer the pre-drying time, the wider the range of the suctions that a vegetated slope can retain. A pre-drying process of 48 h results in a significant increase in the slope factor of safety.