Effects of root geometry and transpiration on pull-out resistance

Abstract

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 pull-out 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-, heart- and 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 tap- and 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.