Infiltration rate affects slope stability by determining the rate of water transport to potential failure planes. This note considers the influences of vegetation (grass and willow) establishment and root growth dynamics on infiltration rate, as related to establishing vegetation on bioengineered slopes. Soil columns of silty sand with and without vegetation were tested by constant-head infiltration tests at 2, 4, 6 and 8 weeks after planting. Infiltration rate increased linearly with plant age and below-ground traits including root biomass and root length density. Infiltration rate for willow-rooted soil was an order of magnitude higher than for fallow soil. The plant age effect was more prominent for willow, which grew faster and with thicker roots than the grass. Illustrative seepage analysis suggests that ignoring the plant age effects could underestimate wetting front advancement to greater depths during rainfall, and underestimate suction recovery at shallow depths during internal drainage.

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Pull-out resistance has been identified as one of the key reinforcement mechanisms for a plant root system to increase slope stability, but the effects of root geometry coupled with plant transpiration on pull-out behaviour are not well understood. This letter presents and interprets a set of centrifuge pullout tests on some newly developed plant root models that are capable of simulating the effects of transpiration. Three idealised and simplified root geometries were considered, namely tap-, heartand plate-shaped. All tests were carried out under identical rainfall conditions at high-g, where the stress state of the soil and root dimensions can be modelled more closely to field conditions. The test results revealed that, after a rainfall event, pore water pressure retained by the tap-and heart-shaped roots (which have longer root depths) was much lower than that retained by the plate-shaped root. The presence of soil suction enhanced the pull-out resistance significantly due to increased tendency of constraint dilatancy upon soil-root interface shearing. Among the three root geometries, the tapand heart-shaped roots were identified to be more favourable in resisting pull-out because they consisted of a vertical taproot component that effectively mobilised soil-root interface friction against pull-out.

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Plant root reinforcement in soil bioengineering has gained increasing interest as a means of sustainable and environmentally friendly soil reinforcement and stabilisation. While Chrysopogon zizanioides is widely distributed in tropical regions worldwide and has been advocated for use in slope stabilisation and soil erosion control, C. nemoralis is normally distributed in mountainous areas in Southeast Asian countries, and its potential to reinforce soil has rarely been explored in the soil bioengineering literature. With the importance of root properties in soil bioengineering, this study was carried out to provide a comprehensive dataset of root biomechanical properties, morphological traits, and root reinforcement of these two contrasting vetiver species. A series of experiments, including root observation with a rhizobox system, uniaxial tensile test, and direct shear test, was performed. Results showed that Young's modulus and diameter of C. nemoralis roots were almost 1.4‐ and 1.3‐times greater than those of C. zizanioides roots (p < 0.05). By contrast, no significant difference between the two vetiver species was observed in terms of tensile strength, 'side' root area ratio (RARS), and root orientation (p > 0.05). The diameter–strength (R2 = 0.55–0.56, p < 0.05) and diameter–modulus relationships (R2 = 0.51–0.6, p < 0.05) of both species were consistent with negative power‐law models. Conversely, their diameter–orientation relationship followed a linear model (R2 = 0.85–0.89; p > 0.05). The soil shear strength in terms of cohesion greatly increased in the presence of the roots of C. nemoralis (Δc = 4.9 kPa) and C. zizanioides (Δc = 4.4 kPa). Therefore, C. nemoralis could be an alternative to C. zizanioides in soil bioengineering applications.

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Influences of root-induced soil suction and root geometry on slope stability: a centrifuge study Leung, Anthony; Kamchoom, V.; Ng, C. W. W. Published in:Canadian Geotechnical Journal DOI: 10.1139/cgj-2015-0263Publication date: 2017 Document Version Peer reviewed version Link to publication in Discovery Research Portal Citation for published version (APA):Leung, A. K., Kamchoom, V., & Ng, C. W. W. (2017). Influences of root-induced soil suction and root geometry on slope stability: a centrifuge study. Canadian Geotechnical Journal, 54(3), 291-303. DOI: 10.1139291-303. DOI: 10. /cgj-2015 General rights Copyright and moral rights for the publications made accessible in Discovery Research Portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from Discovery Research Portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain.• You may freely distribute the URL identifying the publication in the public portal. Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. University of DundeeInfluences of root-induced soil suction and root geometry on slope stability: a centrifuge study Leung, Anthony; Kamchoom, V.; Ng, C. W. W. Published in:Canadian Geotechnical Journal DOI:10.1139/cgj-2015-0263 Publication date: 2016 Document Version Peer reviewed version Link to publication in Discovery Research Portal Citation for published version (APA):Leung, A. K., Kamchoom, V., & Ng, C. W. W. (2016). Influences of root-induced soil suction and root geometry on slope stability: a centrifuge study. Canadian Geotechnical Journal. DOI: 10.1139DOI: 10. /cgj-2015 General rights Copyright and moral rights for the publications made accessible in Discovery Research Portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from Discovery Research Portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain.• You may freely distribute the URL identifying the publication in the public portal. Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Influences of root-induced Abstract:Soil bioengineering using vegetation has been recognised as an environmentally friendly solution for shallow slope stabilisation. Plant transpiration induces suction in the soil, but its effects to slope stability are often ig...