Description
Vegetated gabions are specific gabion walls with incorporated vegetation for a better integration with the surrounding environment. Usually they are made of rectangular containers fabricated from a triple twisted, hexagonal mesh of steel wire. In some cases, the containers can have trapezoidal or cylindrical shapes (Brunet et al., 2005). Once the empty gabions are placed in position, they are filled with stones and then folded shut and wired at the ends and sides (Rossow, 1992). The natural porosity of the gabion varies between 30-40% voids that can be filled with topsoil to retain the moisture in the structure for vegetation as well as to give a substrate available for roots growth between the stones (Brunet et al., 2005). The structure and the function of the retaining measure is the same described in 7.2. Advice on planting vegetation in gabion walls can be found in Coppin and Richards (1990).
Advantages
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If a dense plant system is installed, the presence of gabion walls structures can be totally covered by the vegetation
Disadvantages
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Trees are unlikely to contribute much to the strength of the structure until the wire has corroded seriously;
Design methods
Design methods for the gabions structure are described in 7.2.
In addition, construction guidelines for the installation of vegetation are reported by Coppin and Richards (1990) and Rossow (1992). Live branch cuttings are placed perpendicular to the slope on the wire baskets with the growing tips oriented slightly existing from the gabion (Figure 2). The back of the stakes should extend beyond the baks of the wire baskets into the fill material. In some cases, when hardwood is used, first a hole must be made right through the gabion into the original ground below. When seeds are used, they are directly sown into the gaps between the stones using approximately a rate of 25 seeds per cubic meter (Howell, 1999). Finally, a topsoil is placed over the cuttings and compact it.
When the tree spacing is highly dense, there is need of periodical maintenance with replacement of trees after a few years.
Functional suitability criteria
Type of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 3 | Most suited to rotational or pseudo-rotational slides. May be useful to reduce toppling hazard in certain conditions. Live stakes must be included between the stones in the gabion basket. Inserted them between the baskets it is not suitable. Living stakes must be in contact with the ground behind the gabions. Assume walls are constructed in contact with slope, not outboard as containment structure. |
| Topple | 3 | |
| Slide | 8 | |
| Spread | 2 | |
| Flow | 2 | |
Material type |
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| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 6 | Mainly applicable to landslides involving earth and debris. Applicability in rock limited by typical slope geometry and failure mode. Use of adequate vegetal species is recommended. |
| Debris | 5 | |
| Rock | 3 | |
Depth of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 4 | Typically, applicable to shallow to intermediate depth landslides, the presence of vegetation on the shallowest layers increase the stability |
| Shallow (0.5 to 3 m) | 8 | |
| Medium (3 to 8 m) | 6 | |
| Deep (8 to 15 m) | 4 | |
| Very deep (> 15 m) | 2 | |
Rate of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 0 | Should be carried out preferably on slow landslides; with due care it can be carried out in slow landslides |
| Slow | 2 | |
| Very slow | 5 | |
| Extremely slow | 7 | |
Ground water conditions |
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| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 6 | Applicable in all groundwater conditions. Stone filled gabion baskets are intrinsically free draining. Adequate drainage must be provided at the interface between low permeability backfills, if any, and natural soil. The presence of root network can increase the soil suction at the interface between low permeability soil and natural soil. Adequate drainage must be provided at the interface between low permeability backfills. Use of adequate vegetal species is recommended. |
| High | 6 | |
| Low | 6 | |
| Absent | 2 | |
Surface water |
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| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 4 | Mechanical damage of facing from solid transport typically precludes use near torrents. |
| Snowmelt | 4 | |
| Localized | 4 | |
| Stream | 4 | |
| Torrent | 1 | |
| River | 4 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 8 | The reliability of the technique depends on the reliability of the evaluation of the stability of the treated slope and of the foundations. |
| Feasibility and Manageability | 8 | Relatively simple technique. Potential benefits and limits of applicability are well established. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 8 | Downgrade to 6 where pre-filled gabion baskets need to be lifted using cranes in confined workplaces or on steep slopes |
| Environmental suitability | 8 | The use of a dense system of indigenous trees or shrubs installed between the spaced of the gabions improve the suitability of the measure with the surrounding environment. Sometimes the gabions are totally covered by the established vegetation. |
| Economic suitability (cost) | 8 | Low to moderate, provided local stone is used and the work does not involve diversion of major water courses or interference with existing infrastructure. |
References
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Brunet, G., & Shuey, R. (2005). Stream bank stabilization with vegetated gabions. Land and Water, 49(1), 18-22.
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Coppin N.J., Richards I.G. (1990) ”Use of vegetation in civil engineering” CIRIA Book 10, CIRIA/Butterworths, London .
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Howell, J. (1999). Roadside bio-engineering. Site handbook. Department of Roads, His Majesty's Government of Nepal.
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Rossow, M. (1992). Soil Bioengineering for Slope Stabilization - cedengineering.com