Description
Live smiles consist in willow fences forming a "smile" shape (catenary curve) aimed to hold slumping soils on the slopes (Figure 1). They are considered more effective than modified brush layers or wattle fences, which are more prone to collapse where mud loads the structure (toppling failure). This is due to their bowed configuration, which transfers the load from a toppling load to one where the cuttings are under tension. In this way, since most plant materials are extremely strong under tension, live smiles can hold far more slumping soil or mud than a traditional wattle fences or modified brush layer (Polster, 2003). In addition to their shape, they also have the advantage of growing their roots which reinforce the soil and stabilize the slope and consequently they are considered critically important in stabilizing the slumps and arresting gullies before they become serious and persistent (Goldsmith et al., 2013). Thanks to their capacity of holding a big amount of material they are mostly used to treat sites with saturated surfaces where flowing mud is expected to frequently affect the slope, preventing the growth of vegetation. Despite the fact that only 10 cm or 15 cm of the surface actually flows, the surface can cover several hectares and thus affect a large area. The catenary curve can cover great distances along the slope, once the ends are firmly secured (Polster 1999).
Advantages:
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Willow cuttings are very strong in tension and therefore a structure in which the plant materials are placed in tension would take advantage of this attribute;
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As these materials dry, they can develop significant strength and thus can stand at much steeper slopes;
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They can cover great distances along the slope surface;
Disadvantages:
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High demand of material;
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Not effective if the cuttings are not tightly tied together;
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Less suitable for coarse and drained material.
Design methods
The width of live smiles should vary between 2 and 5 m depending on the nature of the slope and the size of the cuttings available. If the slope is steep and/or the cuttings are thin, it is preferable to have smaller smiles. On flatter slopes or with larger cuttings the smiles can be wider. The height could be up to 40 cm in the middle, tapering at the ends (Figure 2). The live smiles are supported by rebars firmly established in the underlying in-situ material with spacing intervals around 50 to 60 cm to provide a firm foundation for the structure. To avoid a significant amount of mud flowing on the slope, the live smiles should be spaced in such a manner that there are no zones where mud could accumulate.
Period of installation: Dormant season is preferred
Materials: cuttings, rebars.
Functional suitability criteria
Type of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 0 | Most suited to flow or slides. Due to its structure, it is also well suited to reduce toppling hazard, when compared to traditional brush layers or wattle fences. |
| Topple | 0 | |
| Slide | 6 | |
| Spread | 0 | |
| Flow | 5 | |
Material type |
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| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 6 | This type of measure is mainly applicable on slopes with high percentage of mud or near saturated slopes having slumping soils. |
| Debris | 3 | |
| Rock | 0 | |
Depth of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 6 | Typically, applicable to surficial and/or shallow landslides. It becomes ineffective for high depths of movements. |
| Shallow (0.5 to 3 m) | 2 | |
| Medium (3 to 8 m) | 0 | |
| Deep (8 to 15 m) | 0 | |
| Very deep (> 15 m) | 0 | |
Rate of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 1 | The measure is most suitable to prevent from extremely slow to slow soil movements, it is less effective when the soil movement is higher. |
| Slow | 4 | |
| Very slow | 5 | |
| Extremely slow | 6 | |
Ground water conditions |
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| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 4 | Applicable in all groundwater conditions, particularly suitable in slopes with high soil moisture content. |
| High | 5 | |
| Low | 1 | |
| Absent | 1 | |
Surface water |
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| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 6 | Applicable irrespective to surface water conditions. Live smiles can be an effective treatment for flowing silts as they help draining the moisture, causing the silts to flow. |
| Snowmelt | 4 | |
| Localized | 4 | |
| Stream | 3 | |
| Torrent | 0 | |
| River | 3 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 8 | The reliability of the technique depends on the reliability of the evaluation of the demand in terms of mud flows. |
| Feasibility and Manageability | 7 | Well established technique, even if not so common. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 7 | The construction of the live smiles may take some time, depending on availability of materials in the area. |
| Environmental suitability | 10 | Highly suitable with the surrounding environment, it involves live cuttings of plants generally taken in situ |
| Economic suitability (cost) | 8 | Generally, not expensive measure, mostly because of the use of natural materials |
References
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Goldsmith, W., Gray, D., & McCullah, J. (2013). Bioengineering case studies: Sustainable stream bank and slope stabilization. Springer Science & Business Media.
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Polster, D. F. (1999, November). Soil Bioengineering for Steep/Unstable Slopes and Riparian Restoration. In Fourth Annual Roads, Rails and Environment Workshop, November(Vol. 2, No. 3, p. 1999).
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Polster Environmental Services Ltd (2003). Alternatives for Bank Stabilization – Literature Review. Streambank Erosion BMP Steering Committee, Fisheries and Oceans Canada, Aberta.