Effects of soil saturation and suction on root reinforcement performance: pull-out experiments on six native Australian plants

Abstract

Improving shallow slope stability with vegetation requires an understanding of root reinforcement performance, in addition to consideration of local ecological impacts. Existing root reinforcement models have not accounted for the influence of soil water content, due to insufficient experimental evidence and theoretical understanding. In this study, the root reinforcement behaviour of six Australian native plants (A. costata, B. integrifolia, E. reticulatus, P. incisa, C. citrinus and M. thymifolia) are examined through vertical pull-out tests under various levels of volumetric water content (VWC) and suction. Additionally, this study employed two root reinforcement models to illustrate the impact of VWC on comparing the performance of these models with experimental results. The study also employs an innovative approach by making an analogy to soil nails or piles and normalising pull-out force against the peripheral surface area of root-soil bundles, defining this as pull-out stress. The results show that VWC and suction have a strong influence on reinforcement, with a roughly linear inverse relationship observed between VWC and pull-out force of root bundles recorded for all species. The pull-out stress followed a nonlinear inverse relationship with VWC and suction as the pull-out force. Furthermore, discrepancies between established-model predictions and experimental data widen with increasing VWCs. It is also found that inadequate sampling can also lead to substantial errors in estimating the actual water content of the soil. The study demonstrates that VWC and suction significantly impact root reinforcement performance, with pull-out strength decreasing as VWC increases. The study also highlights the importance of accurately recording soil water content in root reinforcement experiments and modelling.