Root strength, growth, and rates of decay: root reinforcement changes of two tree species and their contribution to slope stability

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.

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Root strength comparison between early and late successional trees in a subtropical forest

10.5194/egusphere-egu2020-13073 ◽

2020 ◽

Author(s):

Cang-Wei Chen ◽

Guo-Zhang M. Song ◽

Li-Wan Chang ◽

Chien-Jui Ko ◽

Hsin-Tien Lee ◽

...

Keyword(s):

Slope Stability ◽

Tree Species ◽

Survival Function ◽

Root Diameter ◽

Single Root ◽

Pull Out ◽

Daunting Task ◽

Root Strength ◽

Successional Species ◽

Central Taiwan

ABSTRACT&#160;&#160;&#160;&#160;Slope stability of forested areas is often determined by tree root strength. After landslides, the early successional species emerged first, followed by the late successional species. This study aimed to examine whether tree root strength varies as tree species change along with the succession sequence. The study site is in the Lienhuachi Experimental Forest in central Taiwan, where multiple landslides happened in 2008. Three dominant early (Mallotus paniculatus, Sapium discolor, and Schefflera octophylla) and three late successional species (Cryptocarya chinensis, Engelhardtia roxburghiana, and Randia cochinchinensis) were sampled to conduct the single-root-pull-out tests in the field. Root strength which varies with root diameters was estimated with the Root Bundle Model with the root-failure Weibull survival function (RBMw). Results showed that the overall root strength of the early successional tree species were higher than that of late successional species only when root diameter was lower than 5.44 mm. However, among the six species, the root strength of Sapium discolor, an early successional species, was highest and the species with the lowest root strength was a late successional species (Engelhardtia roxburghiana). To precisely estimate tree effects on slope stability, our results highlighted the need to collect root strength data specifically for each species, even though it will be a daunting task for areas rich in tree diversity.Keyword: landslide, Root Bundle Model, vegetation succession

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A new tool to accurately calculate root reinforcement: the Root Bundle Model software RBM++

10.5194/egusphere-egu2020-20297 ◽

2020 ◽

Author(s):

Ilenia Murgia ◽

Denis Cohen ◽

Filippo Giadrossich ◽

Gian Franco Capra ◽

Massimiliano Schwarz

Keyword(s):

Slope Stability ◽

Tree Species ◽

Root Distribution ◽

Mechanical Characteristics ◽

Model Complexity ◽

Small Scale ◽

Soil System ◽

Root Reinforcement ◽

Essential Information ◽

Pullout Tests

The influence of vegetation on the hydro-geomorphological response is widely recognized, and root reinforcement mechanisms are an important component of slope stability models. The calculation of this essential information is very complex because of the multiple interactions in the root-soil system, but also because of several mechanical characteristics that influence the tension and compression behaviour of the root itself.This contribution has two aims. The first one is to show parameters of root reinforcement effects of Robinia pseudoacacia (L.), a tree commonly used for the mitigation of rainfall-induced landslides at small scale. This species is very widespread because it is able to grow on marginal areas, such as abandoned hillside sites, or on infrastructures, such as road and railway scarps, but its characterization represents a gap in knowledge in the literature. Field pullout tests were performed to collect input data for the quantification of root reinforcement using the Root Bundle Model with Weibull survival function (RBMw, Schwarz et al, 2013). Recent studies have shown how the RBMw is a very efficient model for the evaluation of root reinforcement by considering the heterogeneity of both root mechanical characteristics and their distribution in the soil. However, due to the model complexity and the need for information difficult to obtain, other simpler but less accurate approaches, such as the Wu model, have been preferred.&#160;For this reason, the second aim of the work is to present a new tool written in C++, and called RBM++, easy to use that enables anyone, from Universities to private companies, to quantify the effect of roots on slope stability. RBM++ allows the calculation of root reinforcement using two different methods: the first one by entering own data of the mechanical parameters of the roots, estimated beforehand with pullout tests in the field, and the root distribution in the soil; the second one by selecting the tree species and the data related to the spatial root distribution. For the first method, it is necessary to use a pullout machine to obtain the data. Because this instrument is not commonly available the model has the option to use default parameters for nine tree species based on values found in the literature.&#160;Output from RBM++ comes in tabular format and with a plot that shows, via the graphical user interface, the spatial distribution of forces as a function of the distance from the tree trunk and size of the tree.&#160;&#160;&#160;RBM++ makes it easier to share and exchange knowledge related to root reinforcement. Therefore, it will allow the realization of a database containing standard data on root mechanical behavior of tree species commonly used for shallow landslide mitigation.

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Tree-root control of shallow landslides

Earth Surface Dynamics ◽

10.5194/esurf-5-451-2017 ◽

2017 ◽

Vol 5 (3) ◽

pp. 451-477 ◽

Cited By ~ 24

Author(s):

Denis Cohen ◽

Massimiliano Schwarz

Keyword(s):

Slope Stability ◽

Traditional Approach ◽

Shallow Landslide ◽

Shallow Landslides ◽

Tree Roots ◽

Root Reinforcement ◽

Apparent Cohesion ◽

Root Strength ◽

Weak Zones ◽

Landslide Triggering

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 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.

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Root Reinforcement in Slope Stability Models: A Review

Geosciences ◽

10.3390/geosciences11050212 ◽

2021 ◽

Vol 11 (5) ◽

pp. 212

Author(s):

Elena Benedetta Masi ◽

Samuele Segoni ◽

Veronica Tofani

Keyword(s):

Soil Moisture ◽

Slope Stability ◽

Regional Scale ◽

Moisture Gradient ◽

Future Research ◽

Spatial And Temporal Variability ◽

Soil Behavior ◽

Root Reinforcement ◽

Soil Moisture Gradient ◽

Root Strength

The influence of vegetation on mechanical and hydrological soil behavior represents a significant factor to be considered in shallow landslides modelling. Among the multiple effects exerted by vegetation, root reinforcement is widely recognized as one of the most relevant for slope stability. Lately, the literature has been greatly enriched by novel research on this phenomenon. To investigate which aspects have been most treated, which results have been obtained and which aspects require further attention, we reviewed papers published during the period of 2015–2020 dealing with root reinforcement. This paper—after introducing main effects of vegetation on slope stability, recalling studies of reference—provides a synthesis of the main contributions to the subtopics: (i) approaches for estimating root reinforcement distribution at a regional scale; (ii) new slope stability models, including root reinforcement and (iii) the influence of particular plant species, forest management, forest structure, wildfires and soil moisture gradient on root reinforcement. Including root reinforcement in slope stability analysis has resulted a topic receiving growing attention, particularly in Europe; in addition, research interests are also emerging in Asia. Despite recent advances, including root reinforcement into regional models still represents a research challenge, because of its high spatial and temporal variability: only a few applications are reported about areas of hundreds of square kilometers. The most promising and necessary future research directions include the study of soil moisture gradient and wildfire controls on the root strength, as these aspects have not been fully integrated into slope stability modelling.

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Modeling root reinforcement using a root-failure Weibull survival function

Hydrology and Earth System Sciences ◽

10.5194/hess-17-4367-2013 ◽

2013 ◽

Vol 17 (11) ◽

pp. 4367-4377 ◽

Cited By ~ 45

Author(s):

M. Schwarz ◽

F. Giadrossich ◽

D. Cohen

Keyword(s):

Mechanical Properties ◽

Slope Stability ◽

Root Distribution ◽

Survival Function ◽

Root Diameter ◽

Root Turnover ◽

Diameter Class ◽

Root Reinforcement ◽

Root Strength ◽

Steep Slopes

Abstract. Root networks contribute to slope stability through complex interactions with soil that include mechanical compression and tension. Due to the spatial heterogeneity of root distribution and the dynamics of root turnover, the quantification of root reinforcement on steep slopes is challenging and consequently the calculation of slope stability also. Although considerable progress has been made, some important aspects of root mechanics remain neglected. In this study we address specifically the role of root-strength variability on the mechanical behavior of a root bundle. Many factors contribute to the variability of root mechanical properties even within a single class of diameter. This work presents a new approach for quantifying root reinforcement that considers the variability of mechanical properties of each root diameter class. Using the data of laboratory tensile tests and field pullout tests, we calibrate the parameters of the Weibull survival function to implement the variability of root strength in a numerical model for the calculation of root reinforcement (RBMw). The results show that, for both laboratory and field data sets, the parameters of the Weibull distribution may be considered constant with the exponent equal to 2 and the normalized failure displacement equal to 1. Moreover, the results show that the variability of root strength in each root diameter class has a major influence on the behavior of a root bundle with important implications when considering different approaches in slope stability calculation. Sensitivity analysis shows that the calibration of the equations of the tensile force, the elasticity of the roots, and the root distribution are the most important steps. The new model allows the characterization of root reinforcement in terms of maximum pullout force, stiffness, and energy. Moreover, it simplifies the implementation of root reinforcement in slope stability models. The realistic quantification of root reinforcement for tensile, shear and compression behavior allows for the consideration of the stabilization effects of root networks on steep slopes and the influence that this has on the triggering of shallow landslides.

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Modeling root reinforcement using root-failure Weibull survival function

Hydrology and Earth System Sciences Discussions ◽

10.5194/hessd-10-3843-2013 ◽

2013 ◽

Vol 10 (3) ◽

pp. 3843-3868 ◽

Cited By ~ 3

Author(s):

M. Schwarz ◽

F. Giadrossich ◽

D. Cohen

Keyword(s):

Mechanical Properties ◽

Slope Stability ◽

Root Distribution ◽

Survival Function ◽

Root Diameter ◽

Root Turnover ◽

Diameter Class ◽

Stability Calculation ◽

Root Reinforcement ◽

Root Strength

Abstract. Root networks contribute to slope stability through complicated interactions that include mechanical compression and tension. Due to the spatial heterogeneity of root distribution and the dynamic of root turnover, the quantification of root reinforcement on steep slope is challenging and consequently the calculation of slope stability as well. Although the considerable advances in root reinforcement modeling, some important aspect remain neglected. In this study we address in particular to the role of root strength variability on the mechanical behaviors of a root bundle. Many factors may contribute to the variability of root mechanical properties even considering a single class of diameter. This work presents a new approach for quantifying root reinforcement that considers the variability of mechanical properties of each root diameter class. Using the data of laboratory tensile tests and field pullout tests, we calibrate the parameters of the Weibull survival function to implement the variability of root strength in a numerical model for the calculation of root reinforcement (RBMw). The results show that, for both laboratory and field datasets, the parameters of the Weibull distribution may be considered constant with the exponent equal to 2 and the normalized failure displacement equal to 1. Moreover, the results show that the variability of root strength in each root diameter class has a major influence on the behavior of a root bundle with important implications when considering different approaches in slope stability calculation. Sensitivity analysis shows that the calibration of the tensile force and the elasticity of the roots are the most important equations, as well as the root distribution. The new model allows the characterization of root reinforcement in terms of maximum pullout force, stiffness, and energy. Moreover, it simplifies the implementation of root reinforcement in slope stability models. The realistic quantification of root reinforcement for tensile, shear and compression behavior allows the consideration of the stabilization effects of root networks on steep slopes and the influence that this has on the triggering of shallow landslides.

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Shallow landsliding, root reinforcement, and the spatial distribution of trees in the Oregon Coast Range

Canadian Geotechnical Journal ◽

10.1139/t02-113 ◽

2003 ◽

Vol 40 (2) ◽

pp. 237-253 ◽

Cited By ~ 170

Author(s):

Joshua J Roering ◽

Kevin M Schmidt ◽

Jonathan D Stock ◽

William E Dietrich ◽

David R Montgomery

Keyword(s):

Spatial Distribution ◽

Slope Stability ◽

Empirical Studies ◽

Coast Range ◽

Root Reinforcement ◽

Oregon Coast Range ◽

Oregon Coast ◽

Landslide Occurrence ◽

Hardwood Trees ◽

Root Strength

The influence of root reinforcement on shallow landsliding has been well established through mechanistic and empirical studies, yet few studies have examined how local vegetative patterns influence slope stability. Because root networks spread outward from trees, the species, size, and spacing of trees should influence the spatial distribution of root strength. We documented the distribution and characteristics of trees adjacent to 32 shallow landslides that occurred during 1996 in the Oregon Coast Range. Although broadly classified as a conifer-dominated forest, we observed sparse coniferous and abundant hardwood trees near landslide scars in an industrial forest (Mapleton) that experienced widespread burning in the 19th century. In industrial forests that were burned, selectively harvested, and not replanted (Elliott State Forest), swordfern was ubiquitous near landslides, and we observed similar numbers of live conifer and hardwood trees proximal to landslide scarps. We demonstrate that root strength quantified in landslide scarps and soil pits correlates with a geometry-based index of root network contribution derived from mapping the size, species, condition, and spacing of local trees, indicating that root strength can be predicted by mapping the distribution and characteristics of trees on potentially unstable slopes. In our study sites, landslides tend to occur in areas of reduced root strength, suggesting that to make site-specific predictions of landslide occurrence slope stability analyses must account for the diversity and distribution of vegetation in potentially unstable terrain.Key words: slope stability, vegetation, root strength, shallow landslide, debris flow, Oregon Coast Range.

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