Description
Gabions are wire mesh boxes filled with stones, placed side by side and laced together in order to form a gravity structure; gabion walls can be built with either the front face or rear face stepped; where possible, it is desirable to incline the wall 6 to 8° from the vertical towards the backfill materials; typical schemes are shown in Figure 1; typical applications are shown in Figures 2 and 3.
Gabions are manufactured either from woven continuous wire or welded mesh; the steel mesh is protected against corrosion by a zinc or zinc-aluminium coating; in highly aggressive environments, additional bounded plastic, thermoplastic or epoxy resin polymer coating can be provided; further details on steel protection are provided by Chapman et al (2000).
The materials used to fill gabions must be durable, e.g. resistant to erosion and frost.
Gabion walls are constructed in 1 to 0.5 m high courses; gabions are typically supplied flat and assembled on site (Figures 4 and 5). Filling can be carried out mechanically (Figure 6) or by hand, depending on the finish required. In order to facilitate construction the backfill is placed and compacted keeping it to the same level reached by the wall. Gabions are suitable also for underwater uses; in this case, prefilled gabions are lowered and put in place by a crane, using a lifting frame; this construction process may be adopted also for other areas with poor access.
Gabion walls are permeable and will allow retained fill to drain freely; where appropriate or necessary, surface and/or deep drainage systems will be provided to keep the backfill materials free from groundwater pressures.
Gabion walls can be designed to support vegetation using growing pockets; root growth within or near the gabion structure is not normally detrimental. Advice on planting vegetation in gabion walls can be found in Coppin and Richards (1990). Care should be taken both in the choice of plants suitable for locations within, above or below the wall and for the suitability of the growing medium (usually loose topsoil or growbags) which may require special water retention measures.
Unprotected gabions are susceptible to vandalism, accidental damage and fire due to the small section size of the wire mesh.
Design methods
The wall specification should stipulate the materials to be considered for both gabion walls and for backfilling.
Stones for filling gabions should conform to BS 5390 for hardness, crushing strength and resistance to weathering (frost susceptibility in particular); they should conform also to specification provided in paragraph 8.1.2. of Chapman et al. (2000) regarding grain composition in relation also to the sizes of the gabion compartment and of the wire mesh.
The properties of backfill will depend on whether or not locally-won backfill is to be used, and if material is required to be free-draining. Optimum backfill is: easy to compact, giving high strength and stiffness; and free-draining, to minimize the build-up of groundwater pressure.
Backfill should not include: natural or contaminated soil which will be chemically aggressive; frozen materials; degradable materials such as topsoil, peat, wood, vegetation, etc.; materials which could be toxic, dangerous or prone to spontaneous combustion; soluble material or collapsible soils. The use of clays prone to swelling should be carefully considered as they can exert very high pressures on the back of retaining walls; the same applies for materials derived from argillaceous rocks such as shales and mudstones.
Walls design shall put special consideration on aspects related to water pressure and drainage. Rationale for drainage systems and related details can be found for example in Geotechnical Engineering Office (1993) and Chapman et al. (2000).
The following ultimate limit states (ULS) need to be verified:
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Bearing resistance failure at the base of the wall;
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Sliding failure at the base of the wall;
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Failure by toppling of the wall;
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Loss of overall stability around the wall;
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Overall stability of the slope, including the wall;
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Unacceptable leakage through or beneath the wall;
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Unacceptable transport of soil grains through or beneath the wall;
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Internal stability. Gabion walls shall be also proportioned so that the resultant forces at any horizontal section lies within the middle third of that section. No allowance should be made in the design for the strength of the wire. Analyses should be made on horizontal sections above the base of the wall to check that there is adequate resistance to sliding using a design friction angle for the gabion fill sliding against itself, ignoring the effect of the wire mesh.
Functional suitability criteria
Type of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 1 | Most suited to rotational or pseudo-rotational slides. May be useful to reduce toppling hazard in certain conditions |
| Topple | 2 | |
| Slide | 8 | |
| Spread | 2 | |
| Flow | 1 | |
Material type |
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| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 8 | Mainly applicable to landslides involving earth and debris. Applicability in rock limited by typical slope geometry and failure mode |
| Debris | 6 | |
| Rock | 4 | |
Depth of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 6 | Typically applicable to shallow to intermedite depth landslides. |
| Shallow (0.5 to 3 m) | 8 | |
| Medium (3 to 8 m) | 7 | |
| Deep (8 to 15 m) | 2 | |
| Very deep (> 15 m) | 0 | |
Rate of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 2 | Should be carried out preferably on very or extremely slow landslides; with due care it can be carried out in slow landslides |
| Slow | 5 | |
| Very slow | 8 | |
| Extremely slow | 8 | |
Ground water conditions |
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| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 7 | 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 |
| High | 8 | |
| Low | 8 | |
| Absent | 8 | |
Surface water |
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| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 7 | Mechanical damege of facing from solid transport typically precludes use near torrents. |
| Snowmelt | 6 | |
| Localized | 6 | |
| Stream | 6 | |
| Torrent | 1 | |
| River | 5 | |
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 | 6 | will be updated |
| 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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Chapman T., Taylor H., Nicholson D. (2000). “Modular Gravity Retaining Walls – Design Guidance”. Publication C516, CIRIA, London.
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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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Geotechnical Engineering Office (1993) ”Geoguide 1 – Guide to Retaining Wall Design” Civil Engineering Department, The Government of the Hong Kong, Special Administrative Region.