Effects of root architecture on pore-water pressure distributions and slope stability : analytical and experimental study

Abstract

Vegetation reduces pore-water pressure in soil by root water uptake. The reduction of pore-water pressure results in higher shear strength but lower water permeability of soil, affecting slope stability in vegetated slope and water percolation in landfill cover. Effects of different root architectures on root water uptake and hence pore-water pressure responses are not well understood. The principle objective of this research is to investigate the effects of root architectures on pore-water pressure distributions and slope stability. Analytical and experimental studies were carried out to investigate the effects of root architecture on pore-water pressure response. Analytical solutions of pore-water pressure distributions in single and multi-layer vegetated slopes were derived considering different root architectures, namely the uniform, triangular, exponential and parabolic roots. Then soil column tests were conducted to investigate effects of root characteristics of two grass types (Bermuda grass and vetiver) on pore-water pressure distributions in the three-layer landfill cover. The measurements were used to verify the analytical solutions. Moreover, dimensional analysis was conducted to investigate the governing parameters of pore-water pressure distributions in vegetated slope. Finally, a series of analytical studies were performed to investigate the effects of root architecture on pore-water pressure distributions and slope stability. Measurements from soil column tests show that vetiver has larger ability to reduce pore-water pressure than Bermuda grass due to larger root area and longer root depth. Hence vetiver is more effective than Bermuda grass in preventing water infiltration. Parametric studies show that exponential root architecture induces the highest negative pore-water pressure in the soil, followed by the triangular, uniform and parabolic roots. Exponential and triangular root architectures show similar behaviour on pore-water pressure distributions, while uniform and parabolic give almost the same results. Exponential root architecture has higher ability to maintain slope stability than parabolic one. Under light rainfall (i.e, 20 mm/day) for 24 hours, the hydrological effects of vegetation are more important inside root zone than outside root zone, while it is opposite for rainfall intensities of 181 mm/day and 394 mm/day over the same duration. The hydrological effects of vegetation are more pronounced outside root zone, when subjected to prolonged rainfall. For a specific root architecture, three dimensionless numbers controlling pore-water pressure distributions in vegetated slope were proposed, including capillary effect number (CN; describing the relative importance of water flow driven by pore-water pressure gradient), root water uptake number (RN; representing effects of root water uptake) and water transfer-storage ratio (WR; representing the importance of water flow compared with water storage rate). During drying period, effects of root water uptake and root architecture on pore-water pressure distributions and the influence zone become more significant as CN and RN increases. During wetting period with the same initial conditions, the larger the WR, the deeper the wetting front and more reduction of negative pore-water pressure.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology