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...

Field Measurements of Passive Earth Forces in Steep, Shallow, Landslide‐Prone Areas

“…The observations clearly showed the decay of compression force corresponding to the dissipation of excess pore pressure and the variation of the rheological properties of soil (Ghezzehei & Or, , ), notwithstanding the short time between the loading phases. This process implies both the mobilization of the passive earth force and the force redistribution on a slope, and these factors have to be quantified and integrated into numerical models; moreover, better results are obtained if these factors are related to another variable, such as the volumetric strain rate or the displacement (Cohen & Schwarz, ). Indeed, the interplay between the displacement rate and changes in suction and pore water pressure at the toe of the landslide determine the development of the failure.…”

“…For example, the assumption of a rigid sliding block implies that ultimate states of active and passive forces act simultaneously. To overcome such limitations, the role of soil displacement becomes crucial, and including this factor requires consideration of the timing of when earth forces are actively applied to the sliding soil mass, which may improve the results as shown in Askarinejad et al (, ) and later in Cohen and Schwarz (). For more details, see Text S2 in the supporting information.…”

“…Furthermore, these models were combined with complementary algorithms, such as a spectral clustering search algorithm (Milledge et al, ; Bellugi, Milledge, Dietrich, McKean, et al, ; Bellugi, Milledge, Dietrich, Perron, et al, ), a process‐based fully distributed hydrological model (Anagnostopoulos et al, ), a stochastic procedure (Cislaghi et al, , ; Cislaghi & Bischetti, ; Dorren & Schwarz, ), and a seismicity component (Hess et al, ). Recently, Cohen and Schwarz () implemented the software SOSlope, which is based on force‐displacement calculations of rooted soils under tensile and compressive states using the discrete element method.…”

“…In addition, it is unclear when this type of behavior occurs in nature. Furthermore, recent studies have shown the effects of passive earth forces on landslide triggering (e.g., Cohen & Schwarz, ).…”

Large roots dominate the contribution of trees to slope stability

“…Wu et al, 1979;Gray and Leiser, 1982;Stokes et al, 2009Stokes et al, , 2014. In addition to hydrological effects (e.g., rainfall interception, pore water pressure reduction) the main effect of vegetation on slope stability is the mechanical reinforcement of soils by roots (Sidle and Ochiai, 2006;Stokes et al, 2014;Sidle and Bogaard, 2016;Vergani et al, 2016;Sidle and Ziegler, 2017;Cohen and Schwarz, 2017;Giadrossich et al, 2017).…”