On the effective stress in unsaturated porous continua with double porosity

Borja, Ronaldo I.; Koliji, Azad

doi:10.1016/j.jmps.2009.04.014

Cited by 115 publications

(131 citation statements)

References 37 publications

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“…The increase in the number of redistributions with loading is similar to the process of avalanching in load-controlled selforganized systems like fiber bundle models (Cohen et al, 2009;Lehmann and Or, 2012). The increase in force redistribution across the slope corresponds to the progressive slope failure stage of coalescence of local failure surfaces that eventually leads to global failure (Petley et al, 2005;Cohen et al, 2009). This is equivalent to our phase 3.…”

Section: Displacement and Force Redistributionmentioning

confidence: 69%

“…Bonds between adjacent blocks represent mechanical forces acting across the blocks due to roots and soil (Cohen et al, 2009). These forces can either be tensile or compressive depending on the relative displacements of the blocks.…”

Section: General Frameworkmentioning

confidence: 99%

“…Because of the relative motion between blocks that have moved and blocks that remain stationary, mechanical bond forces between blocks are no longer zero and the force balance changes. This relative motion triggers instantaneous force redistributions across the entire hillslope similar to a self-organized critical (SOC) system of which the spring-block model (Bak et al, 1988;Hergarten and Neugebauer, 1998;Cohen et al, 2009) is a subset. Looping over blocks and moving those that are unstable is repeated until all blocks are either stable (factor of safety greater than or equal to 1) and the system reaches a new equilibrium or some blocks have failed (their displacements are greater than some set value, usually a few meters), triggering a landslide.…”

Section: General Frameworkmentioning

confidence: 99%

“…Our formulation of this concept is empirical and is a simplification of the more common dual-porosity models that employ two flow equations (e.g., Richards' equation) that exchange moisture between the two domains, and mixture equations for water content, hydraulic conductivity, rainfall partitioning based on the volumetric ratio of the fast and slow flow domains (e.g., Gerke and van Genuchten, 1993;Shao et al, 2015). In accord with continuum mixture theory for effective stress (e.g., Borja and Koliji, 2009), we write the mean pore-water pressure of the soil (matrix + macropores), p, as…”

Section: Hydrological Triggeringmentioning

confidence: 99%

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Tree-root control of shallow landslides

Cohen

Schwarz

2017

Earth Surf. Dynam.

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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...

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Section: Displacement and Force Redistributionmentioning

confidence: 69%

Section: General Frameworkmentioning

confidence: 99%

Section: General Frameworkmentioning

confidence: 99%

Section: Hydrological Triggeringmentioning

confidence: 99%

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Tree-root control of shallow landslides

Cohen

Schwarz

2017

Earth Surf. Dynam.

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“…saturated soils (e.g. Khalili 2000&2002;Pereira et al 2005;Gray and Schrefler 2001;Nikooee et al 2013;Borja & Koliji 2009), we wish to explore an alternative path in this paper. The key idea is to (i) observe the micromechanics, (ii) propose an abstract representation of the observed micromechanisms, and then (iii) embed them in a constitutive model.…”

Section: School Of Civil Engineering University Of Sydneymentioning

confidence: 99%