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Earth Surface Dynamics An interactive open-access journal of the European Geosciences Union

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© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
24 Feb 2017
Review status
This discussion paper is under review for the journal Earth Surface Dynamics (ESurf).
Tree-roots control of shallow landslides
Denis Cohen1 and Massimiliano Schwarz2 1Department of Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801, USA
2School of Agricultural, Forest, and Food Sciences, Bern University of Applied Science, 3052, Zollikofen, Switzerland
Abstract. Tree roots have long been recognized to increase slope stability by reinforcing the strength of soils. Slope stability models include the effects of roots by adding an apparent cohesion to the soil to simulate root strength. No model includes the combined effects of root distribution heterogeneity, stress-strain behavior of root reinforcement, or root strength in compression. Recent field observations, however, indicate that shallow landslide triggering mechanisms are characterized by differential deformation that indicates localized activation of zones in tension, compression, and shear in the soil. These observations contradict the common assumptions used in present models. Here we describe a new model for slope stability that specifically considers these effects. The model is a strain-step discrete element model that reproduces the self-organized redistribution of forces on a slope during rainfall-triggered shallow landslides. We use a conceptual sigmoidal-shaped hillslope with a clearing in its center to explore the effects of tree size, spacing, weak zones, maximum root-size diameter, and different root strength configurations. The model is driven by root data of Norway spruce obtained from laboratory and field measurements. Simulation results indicate that tree roots can stabilize slopes that would otherwise fail without them and, in general, higher root density with higher root reinforcement results in a more stable slope. Root tension provides more resistance to failure than root compression but roots with both tension and compression offer the best resistance to failure. Lateral (slope-parallel) tension can be important in cases when the magnitude of these forces is comparable to the slope-perpendicular tensile forces. In these cases, lateral forces can bring to failure tree-covered areas with high root reinforcement. Slope failure occurs when downslope soil compression reaches the soil maximum strength. When this occurs depends on the amount of root tension upslope in both the slope-perpendicular and slope-parallel directions. Roots in tension can prevent failure by reducing soil compressive forces downslope. When root reinforcement is limited, hillslopes form a crack parallel to the slope near its top. Simulations with roots that fail across this crack always resulted in a landslide. Slopes that did not form a crack could either fail or remain stable, depending on root reinforcement. Tree spacing is important for the location of weak zones but tree location on the slope (with respect to where a crack opens) is as important. Finally, for the specific cases tested here, large roots, greater than 20 mm, are too few too contribute significantly to root reinforcement. Omitting roots larger than 8 mm predicted a landslide when none should have occurred. Intermediate roots (5 to 20 mm) appear to contribute most to root reinforcement and should be included in calculations.

To fully understand the mechanisms of shallow landslide triggering requires a complete re-evaluation of the traditional apparent-cohesion approach that does not reproduce the incremental loading of roots in tension or in compression. Our model shows that it is important to consider the forces held by roots in a way that is entirely different than done thus far. Our work quantifies the contribution of roots in tension and compression which now finally permits to analyze more realistically the role of root reinforcement during the triggering of shallow landslides.

Citation: Cohen, D. and Schwarz, M.: Tree-roots control of shallow landslides, Earth Surf. Dynam. Discuss., doi:10.5194/esurf-2017-10, in review, 2017.
Denis Cohen and Massimiliano Schwarz
Denis Cohen and Massimiliano Schwarz
Denis Cohen and Massimiliano Schwarz


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Short summary
Tree roots reinforce soils on slopes. A new slope stability model is presented that computes root reinforcement including the effects of root heterogeities and the dependence of root strength on tensile and compressive strain. Our results show that roots stabilize slopes that would otherwise fail under a rainfall event. Tension in roots is more effective than compresion. Redistribution of forces in roots across the hillslope play a key role in the stability of the slope during rainfall events.
Tree roots reinforce soils on slopes. A new slope stability model is presented that computes...