Tree-root control of shallow landslides

Abstract

Abstract. 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 selforganized 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, loc...

Mechanical Characteristics of the Fine Roots of Two Broadleaved Tree Species from the Temperate Caspian Hyrcanian Ecoregion

“…In general, we found that the roots of the trees with larger DBH were more resistant in tension compared with the roots of the medium or small DBH classes, which concurs with [11]. This may be explained by the report of [65], who showed that root resistance was lower in the early growth stage and increased in older plants.…”

Mechanical Characteristics of the Fine Roots of Two Broadleaved Tree Species from the Temperate Caspian Hyrcanian Ecoregion

“…In general, we found that the roots of the trees with larger DBH were more resistant in tension compared with the roots of the medium or small DBH classes, which concurs with [11]. This may be explained by the report of [65], who showed that root resistance was lower in the early growth stage and increased in older plants.…”

Implications of hornbeam and beech root systems on slope stability: from field and laboratory measurements to modelling methods

“…Root reinforcement of large trees is the highest among all the tree DBH classes, concurs with Cohen and Schwarz (2017). The roots of the oldest trees were the most resistant in tension compared to the middle age and young trees (Genet et al 2006).…”

Controls of Root-System Overlap on Hillslope Stability

“…strength that supports the stability of the slope 10 . Numerous studies have quantified root contributions to slope stabilization, ranging from individual roots to the mass of roots at hillslope scales.…”

Mechanical Characteristics of the Fine Roots of Two Broadleaved Tree Species from the Temperate Caspian Hyrcanian Ecoregion

“…In general, we found that the roots of the trees with larger DBH were more resistant in tension compared with the roots of the medium or small DBH classes, which concurs with [11]. This may be explained by the report of [65], who showed that root resistance was lower in the early growth stage and increased in older plants.…”

Implications of hornbeam and beech root systems on slope stability: from field and laboratory measurements to modelling methods

“…Root reinforcement of large trees is the highest among all the tree DBH classes, concurs with Cohen and Schwarz (2017). The roots of the oldest trees were the most resistant in tension compared to the middle age and young trees (Genet et al 2006).…”

Controls of Root-System Overlap on Hillslope Stability

“…strength that supports the stability of the slope 10 . Numerous studies have quantified root contributions to slope stabilization, ranging from individual roots to the mass of roots at hillslope scales.…”

Mechanical Characteristics of the Fine Roots of Two Broadleaved Tree Species from the Temperate Caspian Hyrcanian Ecoregion

“…In general, we found that the roots of the trees with larger DBH were more resistant in tension compared with the roots of the medium or small DBH classes, which concurs with [11]. This may be explained by the report of [65], who showed that root resistance was lower in the early growth stage and increased in older plants.…”

Implications of hornbeam and beech root systems on slope stability: from field and laboratory measurements to modelling methods

“…Root reinforcement of large trees is the highest among all the tree DBH classes, concurs with Cohen and Schwarz (2017). The roots of the oldest trees were the most resistant in tension compared to the middle age and young trees (Genet et al 2006).…”

Controls of Root-System Overlap on Hillslope Stability

“…strength that supports the stability of the slope 10 . Numerous studies have quantified root contributions to slope stabilization, ranging from individual roots to the mass of roots at hillslope scales.…”