Soil stability : Soil stability and natural hazards: from knowledge to action


In order to quantify the effects of plants and fungi on slope stability, researchers of the "Soil stability” project conducted soil mechanical experiments as well as vegetation and forest structure analysis. New criteria will help to assess susceptibility to landslides.

Background (completed research project)

Healthy and intact vegetation in general and forests in particular have long been known to positively influence slope stability. However, an appropriate quantification of their effectiveness is still lacking, even as the demand for it has continued to increase.


The overall objective of the project was to develop practicable recommendations for sustainable soil use and effective ecological restoration by balancing slope stability, plant and fungal diversity, and land use.


  • Well root-permeated slopes of investigated soil types are stable when more than 5 degrees steeper than the angle of internal friction Φ'.
  • Selected mycorrhizal fungi increase root growth and/or soil aggregate stability.
  • Forest structure, particularly gap length, significantly influences slope stability and is related to root reinforcement.
  • More than 90 per cent of 218 shallow landslides in forests are explained with a three-step filter:
    1. 50 per cent by exceeding the 5 degree threshold between slope inclination and angle of internal friction Φ'
    2. 40 per cent by vegetation parameters, e.g. forest cover > 40 per cent, gap length ≤ 20 m, and species mixture
    3. 7 per cent by terrain morphology types particularly susceptible to slope instabilities

Implications for research

In the project, volume changes (dilatancy) were for the first time properly considered in direct shear tests with root-permeated soil. They should be given more attention in further research addressing the biological effects on soil stability. Furthermore, different root architectures and their specific functions in respect of soil mechanical and hydrological processes have to be taken into account. Finally, more information is needed on mycorrhiza of plant (tree) species, mostly in respect of soil stabilising effects and succession processes.

Implications for practice

Maintenance of vegetation is required to optimise slope stabilising effects and reduce susceptibility to shallow landslides to the greatest extent possible. Important factors in slope stabilisation are suitable diversity of organisms (plants, mycorrhizal fungi), of the structure above (canopy, layering) and below ground (root architecture, soil texture), and of successional stages (eco-engineering).

Above and within areas susceptible to shallow landslides, fertilisation and pasturing should be restricted.

The findings support recommendations from “Sustainability and Monitoring of protections forest (NaiS)” of the Federal Office for the Environment and help to improve the guideliness

Original title

Soil stability and natural hazards: from knowledge to action

Project leader

  • Dr Frank Graf, WSL Institute for Snow and Avalanche Research SLF
  • Dr Peter Bebi, WSL Institute for Snow and Avalanche Research SLF
  • Dr Martin Frei, Office of Forests and Natural Hazards, Canton of Grisons
  • Christian Rickli, Swiss Federal Institute for Forest, Snow and Landscape Research WSL
  • Dr Christian Rixen, WSL Institute for Snow and Avalanche Research SLF
  • Professor Sarah Springman, ETH Zurich