Effects of root geometry and transpiration on pull-out resistance

Pull-out resistance has been identified as one of the key reinforcement mechanisms for a plant root system to increase slope stability. Yet, any 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 pull-out tests on some newly-developed plant root models, which are capable of simulating the effects of transpiration. Three idealised and simplified root geometries were considered, namely tap-, heart- and plate-shaped. All tests were carried out under identical rainfall condition at high-g, where the stress state of soil and root dimensions can be modelled more closely to the field condition. The test results revealed that after subjecting to a rainfall event, PWP retained by the tap- and heart-shaped root (which have longer root depths) were much lower than that by the plate-shaped root. The presence of soil suction enhanced the pull-out resistance significantly due to increased tendency of constraint dilatancy upon intense soil-root interface shearing. Among the three root geometries, the tap- and heart-shaped ones are 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