Root systems of native shrubs and trees in Hong Kong and their effects on enhancing slope stability

• Root distribution of four Hong Kong native shrub and tree species was studied.
• Root tensile strength of the species was investigated.
• The contribution of roots to slope stability was quantified.

ObjectivesMechanical reinforcement by plant roots is believed to have an important role in stabilizing highly saturated slopes against shallow failure. In this study, the root system of four Hong Kong native shrubs (Rhodomyrtus tomentosa and Melastoma sanguineum) and trees (Schefflera heptaphylla and Reevesia thyrsoidea) with height that ranged between 1 and 1.5 m was sampled and their characteristics were studied.MethodsThe distribution of roots and root area ratio (RAR) with depth, relationship between root tensile strength (Tr) and root diameter (d), and also the variation of root cohesion (cr) with depth of the four species were investigated and statistically compared.ResultsRoots of the studied trees were found to extend deeper into the ground (up to 0.8 m) as compared to the shrubs (up to 0.4 m). RAR lies between 0.03 and 0.14% for the top 0.1 m soil and decreased with depth. The obtained Tr–d relationship of all the studied species fell into the same order as compared to some commonly reported European species. Besides, conventionally adopted power relationship between Tr and d was confirmed to be applicable for the studied species. The variation of root cohesion with depth was investigated for each species. Root cohesion of less than 1.5 kPa was evaluated for even the top 0.2 m soil when roots with a diameter that ranged only between 1 and 10 mm were considered. The contribution of roots to slope stability was studied on infinite slopes with and without vegetation under two hydrological scenarios (dry and wet slopes).Conclusions and implicationsIt was found that the studied young vegetation can bring an unsafe slope to marginal safety (factor of safety slightly larger than unity). Moreover, the studied tree species did not outperform the shrubs.