Junjun Ni

Bioengineering for Slope Stabilisation Using Plants: Hydrological and Mechanical Effects

The negative impact of climate change calls for more sustainable and environmentally friendly techniques to be developed to improve the engineering performance of our civil infrastructures such as slopes in urban built environments. Soil bioengineering using plants and microorganisms is considered a low-cost and aesthetically pleasant solution for shallow slope stabilisation. Although extensive research has been conducted on the mechanical effects of root reinforcement in past decades, the hydrological effects of plants on shear strength and water permeability of vegetated soil slopes are not clear. This extended abstract presents an all-round, cross-disciplinary research programme consisting of indoor and field experiments, centrifuge testing and theoretical analysis to examine the plant hydrological effects on slope stability. It was revealed that some plant species native to southern China and Europe could preserve a credible amount of suction after heavy rainfall, which is positively correlated with the leaf area index (LAI), the root area index (RAI) and the ratio of root to shoot biomass. The total amount of water infiltration were found to be considerable higher in bare than those in soil covered by grass and tree. Plant-fungus interaction caused a significant increase in root tensile strength, hence potentially the mechanical reinforcement to soil. By developing novel artificial root systems in the centrifuge and deriving new theoretical closed-form stability equations, it was discovered that heart-shaped roots produced stronger stabilisation effects than either tap- or plate-shaped roots. This root architecture preserved higher suction (hence higher soil shear strength) and provided greater mechanical reinforcement effects due to multiple branching. These findings advance the fundamental understanding of plant-soil interaction and its influence on slope bioengineering applications.

Comparison of single- and dual-permeability models in simulating the unsaturated hydro-mechanical behavior in a rainfall-triggered landslide

Landslide-prone slopes in earthquake-affected areas commonly feature heterogeneity and high permeability due to the presence of cracks and fissures that were caused by ground shaking. Landslide reactivation in heterogeneous slope may be affected by preferential flow that was commonly occurred under heavy rainfall. Current hydro-mechanical models that are based on a single-permeability model consider soil as a homogeneous continuum, which, however, cannot explicitly represent the hydraulic properties of heterogeneous soil. The present study adopted a dual-permeability model, using two Darcy-Richards equations to simulate the infiltration processes in both matrix and preferential flow domains. The hydrological results were integrated with an infinite slope stability approach, attempting to investigate the hydro-mechanical behavior. A coarse-textured unstable slope in an earthquake-affected area was chosen for conducting artificial rainfall experiment, and in the experiment slope, failure was triggered several times under heavy rainfall. The simulated hydro-mechanical results of both single- and dual-permeability model were compared with the measurements, including soil moisture content, pore water pressure, and slope stability conditions. Under high-intensity rainfall, the measured soil moisture and pore water pressure at 1-m depth showed faster hydrological response than its simulations, which can be regarded as a typical evidence of preferential flow. We found the dual-permeability model substantially improved the quantification of hydro-mechanical processes. Such improvement could assist in obtaining more reliable landslide-triggering predication. In the light of the implementation of a dual-permeability model for slope stability analysis, a more flexible and robust early warning system for shallow landslides hazard in coarse-textured slopes could be provided.