An investigation of plant-induced suction and its implications for slope stability

Anil Yildiz,Sarah M Springman,Frank Graf

doi:10.1680/jgeen.18.00218

Abstract

Both above- and below-ground biomass of plants provide additional strength to soil through different mechanisms, which help to increase the stability of a vegetated slope. Nonetheless, shallow landslides on steep slopes covered with vegetation still occur, often being triggered above the groundwater table, due to loss of suction subsequent to rainfall. Therefore, it is essential to know to what extent vegetation enhances slope stability, and to quantify the contribution of vegetation to the shear strength of soil to determine factors of safety. Results of large-scale direct shear experiments on root-permeated soils and slope geometry from a landslide database were synthesised through an infinite slope analysis under partially saturated conditions to find critical combinations of slope angle and suction stress. Monte Carlo simulations yielded a clear separation of stable and unstable zones, which can be used to define the susceptibility of a slope to near surface failure. This method, based on the simulations, has the potential to be used as a regional early warning system.

Highlights

A slope would be marginally stable when the slope angle is equal to the internal friction angle of dry soil
These values were converted to gravimetric water content by substituting them into the soil-water retention curve (SWRC) for Praettigau soil. The outcome of this retrospective analysis – that is, the separation between the stable and unstable zones of the slope angle–suction stress graph with varying depth, can be used as a tool to carry out a quick determination of slope stability. It requires the monitoring of volumetric water content or matric suction at different depths, knowledge of the SWRC and determination of the slope angle from geographical information system (GIS) applications or field measurements
The second part of the analysis focuses on the understanding of the initiation of each landslide in the Praettigau region after the rainfall event in 2005

Summary

Introduction

A slope would be marginally stable when the slope angle is equal to the internal friction angle of dry soil. Any additional shear strength can be mobilised due to suction, vegetation, or any other stabilising agents for a slope with an angle steeper than the internal friction angle of the soil. This additional strength due to suction can be lost either after a heavy rainfall event over a short duration (Brand et al, 1984; D’Amato Avanzi et al, 2004; Guzzetti et al, 2004) or more moderate rainfall over a long duration (Lumb, 1975). The increase in factor of safety (FoS) of a vegetated slope due to plant-induced suction fluctuates throughout a year due to the seasonal variation effects of vegetation on the hydrological regime of a slope, which has been shown to be most significant during dry periods (Kim et al, 2017; Leung and Ng, 2013)

Methods

Results

Discussion

Conclusion