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.
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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