Tree-root control of shallow landslides
Tree roots have long been recognized to increase slope stability by reinforcing the strength of soils. Slope stability models usually 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 het...
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doaj-1e221db11a7842f3a0806de6bdc2d7962020-11-24T22:07:29ZengCopernicus PublicationsEarth Surface Dynamics2196-63112196-632X2017-08-01545147710.5194/esurf-5-451-2017Tree-root control of shallow landslidesD. Cohen0M. Schwarz1M. Schwarz2Department of Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801, USASchool of Agricultural, Forest, and Food Sciences, Bern University of Applied Science, 3052 Zollikofen, SwitzerlandEcorisQ, 1205 Geneva, SwitzerlandTree roots have long been recognized to increase slope stability by reinforcing the strength of soils. Slope stability models usually 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. 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. 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. The variation in root stiffness with diameter can, in some cases, invert this relationship. 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 this force is comparable to the slope-perpendicular tensile force. In this case, 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, a crack parallel to the slope forms near the top of the hillslope. 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, intermediate-sized roots (5 to 20 mm in diameter) appear to contribute most to root reinforcement. Our results show more complex behaviors than can be obtained with the traditional slope-uniform, apparent-cohesion approach. A full understanding of the mechanisms of shallow landslide triggering requires a complete re-evaluation of this traditional approach that cannot predict where and how forces are mobilized and distributed in roots and soils, and how these control shallow landslides shape, size, location, and timing.https://www.earth-surf-dynam.net/5/451/2017/esurf-5-451-2017.pdf |
collection |
DOAJ |
language |
English |
format |
Article |
sources |
DOAJ |
author |
D. Cohen M. Schwarz M. Schwarz |
spellingShingle |
D. Cohen M. Schwarz M. Schwarz Tree-root control of shallow landslides Earth Surface Dynamics |
author_facet |
D. Cohen M. Schwarz M. Schwarz |
author_sort |
D. Cohen |
title |
Tree-root control of shallow landslides |
title_short |
Tree-root control of shallow landslides |
title_full |
Tree-root control of shallow landslides |
title_fullStr |
Tree-root control of shallow landslides |
title_full_unstemmed |
Tree-root control of shallow landslides |
title_sort |
tree-root control of shallow landslides |
publisher |
Copernicus Publications |
series |
Earth Surface Dynamics |
issn |
2196-6311 2196-632X |
publishDate |
2017-08-01 |
description |
Tree roots have long been recognized to increase slope stability by
reinforcing the strength of soils. Slope stability models usually 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. 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. 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. The variation in root stiffness with diameter can, in some
cases, invert this relationship. 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 this force is comparable to the
slope-perpendicular tensile force. In this case, 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, a crack parallel to the slope forms near the top of
the hillslope. 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, intermediate-sized roots (5 to 20 mm in diameter) appear to
contribute most to root reinforcement. Our results show more complex
behaviors than can be obtained with the traditional slope-uniform,
apparent-cohesion approach. A full understanding of the mechanisms of shallow
landslide triggering requires a complete re-evaluation of this traditional
approach that cannot predict where and how forces are mobilized and
distributed in roots and soils, and how these control shallow landslides
shape, size, location, and timing. |
url |
https://www.earth-surf-dynam.net/5/451/2017/esurf-5-451-2017.pdf |
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