Description
Trees have been planted on many slopes worldwide specifically to increase slope stability. For example, 60.000 fast-growing acacia and gmelina seedlings were planted in an effort to stabilize the historic Cucaracha landslide in the Gaillard Cut of the Panama Canal when it was reactivated in 1986, almost blocking the canal (Rivera, 1991; Berman, 1991).
The way vegetation affects slope stability is through root reinforcement of soil. In fact, roots in soil create a new soil–root composite system in which root fibres with strong tensile strength but weak compressive strength are embedded in soil. This reinforcement is considered as additional soil strength via root cohesion (Wu et al. 1979, 2013; Nilaweera and Nutalaya 1999; Cazzuffi et al., 2006; Leung et al. 2015a).
As consequence, Vegetation is widely believed to improve the stability of slopes, especially on steep slopes and with respect to superficial or shallow movements (where roots usually develop) Planting density is another important parameter influencing soil shear strength. Loades et al., 2010 found that a soil with higher planting density in field shows a bigger shear strength compared to a rooted soil in glasshouse.
The net positive contribution of vegetation to slope stability is supported by a number of case studies where slope failures could be attributed to the loss of reinforcement provided by the tree roots (Wu et al., 1979; Riestenberg and Sovonick-Dunford, 1983; Riestenberg, 1987), while Greenwood et al. (2004) reported a 10% increases in the Factor of Safety of vegetated slopes compared to non-vegetated slopes.
From a mechanical point of view, vegetation can improve the stability of slopes through the anchoring or reinforcement effect provided by the roots Wu (1995). In fact, roots in soil create a new soil–root composite system in which root fibers with strong tensile strength but weak compressive strength are deeply anchored in soil. This reinforcement is considered as additional soil strength via root cohesion (Wu et al. 1979, 2013; Nilaweera and Nutalaya 1999; Cazzuffi et al., 2006; Leung et al. 2015a). To consider the effect of roots on the soil shear strength, from easy to more complex models have been developed in literature: W&W model proposed by Wu (1976) and Waldron (1977), the Fiber Bundel Model (FBM) proposed by Pollen and Simon (2005) and more recently the Root Bundle Model (RBM) proposed by Schwartz et al. (2013).
For example, the W&W model assumed that the increase in shear strength provided by roots as additional cohesion into Coulomb’s law is accepted for the schematic soil root-interaction model showed in Figure 2. The failure plane is assumed horizontal and the roots are subjected to tensile stress due to their elongation trough the sliding surface.
The governing factors are the mechanical properties (tensile strength and elastic modulus) of the roots and their density in the shear zone. The tensile strength of a single root depends mostly on the root diameter, the root species and its state (living or decaying) (Schmidt et al., 2001; Preti, 2013). The anchoring effect of roots depends on the type of vegetation, as well as on the season of the year and the environmental conditions. Moreover, the roots have different properties and grow differently from plant to plant (Figure 4). In general, a denser network of roots in the soil will favour stability and for a given species the diameter of the roots will determine the amount of stress a root can take before breaking.
Notable studies on the reinforcement effects of roots on vegetated slope have been conducted by Greenway et al. (1984), Greenway (1987) and Yin et al. (1988). Wu (1995) shows that roots left after logging continue to have a positive effect on slope stability for many years, with their tensile strength reducing gradually, but it takes time for new trees to establish a new stabilizing root system.
Greenwood et al. (2007) highlight that vegetation may also result in increased suction (negative pore pressure) in unsaturated soil, potentially increasing the apparent cohesion of the soil.
Reference shall be made to the fact sheets in section 1 of this Annex for further detailed description of applicable techniques and discussion of the basis of design for the use of vegetation to improve slope stability.
For considerations on the hydrological effects of vegetation on soil stability reference may be made to fact sheet 3.5 of this Annex.
Information on the mechanical and hydrological effects of vegetation is provided by the Hong Kong Geotechnical Manual for Slopes (Geotechnical Control Office, 1984), reflecting one of the most comprehensive research programs in the world on the engineering role of vegetation for slope stabilization (Barker, 1991).
Although vegetation offers several advantages, there are some limited applications for the following conditions: steep slopes, adverse soil texture (excessive amount of fine and course material), poor nutrient status, adverse soil chemistry, low soil temperature, low soil moisture, and the hostile weather condition (Polster 1997; Withers 1999). Therefore, it is critical to understand and evaluate the project site conditions prior to adopting vegetation as mitigation measure.
Design methods
will be updated
Functional suitability criteria
Type of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 0 | Both rotational and translational |
| Topple | 0 | |
| Slide | 4 | |
| Spread | 0 | |
| Flow | 0 | |
Material type |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 8 | Careful selection of species is required for applications on rock, where roots may open fractures favouring water ingress and instability |
| Debris | 4 | |
| Rock | 2 | |
Depth of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 8 | Limited by root penetration |
| Shallow (0.5 to 3 m) | 4 | |
| Medium (3 to 8 m) | 0 | |
| Deep (8 to 15 m) | 0 | |
| Very deep (> 15 m) | 0 | |
Rate of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 2 | Seeding can be applied remotely, by helicopter if necessary. However, it needs time to become established and this may limit application in moderately to fast movements |
| Slow | 6 | |
| Very slow | 8 | |
| Extremely slow | 8 | |
Ground water conditions |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 8 | Species must be selected to suit agronomical conditions. Irrigation may be necessary in arid soils. |
| High | 8 | |
| Low | 4 | |
| Absent | 2 | |
Surface water |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 8 | May be used to stabilize banks of slow watercourses, but it requires special techniques. |
| Snowmelt | 8 | |
| Localized | 6 | |
| Stream | 4 | |
| Torrent | 0 | |
| River | 4 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 8 | Needs significant maintenance, especially in early stages; inappropriate species selection could be ineffective or even detrimental to stability |
| Feasibility and Manageability | 8 | Impact on mechanical aspects of slope processes not yet fully established. Strong reliance on empiricism. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 8 | Application on steep slopes or moderately to fast slides may be done remotely. The roots can require a growth period (1 vegetative year) to become mechanically effective as soil reinforcement |
| Environmental suitability | 10 | Native species are preferred because highly suitable with the surrounding environment |
| Economic suitability (cost) | 8 | Relatively low installation costs, but it may require significant maintenance or even irrigation |
References
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Barker R.F. (1991) “Developments in biotechnical stabilization in Britain and the Commonwealth” Proceedings of Workshop on Biotechnical Stabilization, University of Michigan, Ann Arbor, 83-123.
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Berman G. (1991) “Landslides on the Panama Canal” Landslide News (Japan Landslide Society) n° 5, 10-14.
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Geotechnical Control Office (1984) “Geotechnical Manual for Slopes” 2nd edition, Public Works Department, Hong Kong.
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Greenway D.R., Anderson M.G., Brian-Boys K.C. (1984) “Influence of vegetation on slope stability in Hong Kong” Proceedings of 4th International Symposium on Landslides. Toronto, Balkema, vol. 1, 399-404.
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Greenwood J.R., Norris J.E., Wint J. (2004) “Assessing the contribution of vegetation to slope stability” Proceedings of the Institution of Civil Engineers, Geotechnical Engineering, 157, 199-207.
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Greenwood J.R., Norris J.E., Wint J., Goodwin A.K., Mc Donald M. (2007) “Discussion: Assessing the contribution of vegetation to slope stability” Proceedings of the Institution of Civil Engineers, Geotechnical Engineering, 160, 51-53.
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Pollen, N., Simon, A., & Collison, A. (2004). Advances in assessing the mechanical and hydrologic effects of riparian vegetation on streambank stability. Riparian vegetation and fluvial geomorphology, 125-139.
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Preti, F. (2013). Forest protection and protection forest: tree root degradation over hydrological shallow landslides triggering. Ecological engineering, 61, 633-645.
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Rienstenberg M.M. (1987) “Anchoring of thin colluvium on hillslopes by roots of sugar maple and white ash” Ph.D. dissertation, University of Cincinnati, Ohio.
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Rienstenberg M.M. and Sovonick-Dunford S. (1983) “The role of woody vegetation in stabilizing slopes in Cincinnati area, Ohio” Geological Society of America Bulletin, vol. 94, 506-518.
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Rivera R. (1991) “Reforestation program – Gaillard cut” Panama Canal Commission, Report n° 1.
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Rogers J.D. (1992). “Recent developments in landslide mitigation techniques”. In “Landslides/Landslide mitigation”, Slosson E., Keene A.G., Johnson J.A., eds.,. Reviews of Engineering Geology, Volume IX, Geological Society of America, Boulder, Colorado.
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Schmidt, K. M., Roering, J. J., Stock, J. D., Dietrich, W. E., Montgomery, D. R., & Schaub, T. (2001). The variability of root cohesion as an influence on shallow landslide susceptibility in the Oregon Coast Range. Canadian Geotechnical Journal, 38(5), 995-1024.
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Schwarz, M., Giadrossich, F., & Cohen, D. (2013). Modeling root reinforcement using a root-failure Weibull survival function. Hydrology and Earth System Sciences, 17(11), 4367-4377.
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Waldron, L. J. (1977). The shear resistance of root-permeated homogeneous and stratified soil. Soil Science Society of America Journal, 41(5), 843-849.
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Wu, T. H. (1976). Investigations of landslides on Prince of Wales Island, Alaska, Geotech. Eng. Rep 5, Columbus, OH: Dept. Civ. Eng,, Ohio State Univ. 94 p.
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Wu T.H., Mc Kinnell W.P., Swanston D.N. (1979) “Strength of tree roots and landslides on Prince of Wales Island, Alaska” Canadian Geotechnical Journal, vol. 16, n° 1, 19-33.
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Yin K.P., Heung L.K., Greenway D.R. (1988) “ Effect of root reinforcement on the stability of three fill slopes in Hong Kong” Proceedings of 2nd International Conference on Geomechanics in Tropical Soils, Singapore, 293-302.
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