Gabion Standard Design
Fine Mesh Metals Ltd are not structural engineers. The designs on this page are suggestions based on industry standard designs. Gabion wall designs are very robust, these designs in general are the basis for any designs produced by structural engineer. however there are ground types and situations where the deigns will not be sufficient If your situation involves having a building, road etc on top of it or you are putting them on very soft clay then it would be advised to contact a suitably experienced and qualified Consulting Engineer, thus ensuring the most appropriate advice being provided and supported by professional indemnity insurance. This has implications in respect of Health and Safety being adequately addressed. The client has an obligation to ensure all persons engaged in the undertaking the works are suitably experienced and qualified, thus being familiar with the construction process and associated risks to health and safety.
A Consultant engineer will require.
a. A topographic survey to conclude wall height and any slope surcharge which could similarly influence the gabion arrangement.
b. Proposed wall location.
c. Ground investigation to conclude retained and foundation soils and associated geotechnical parameters.
This is an additional cost and we can direct you to a Consultant if you require.
The designs given on this page should not be used as a substitute for taking detailed and specific advice from a Consultant engineer. The recommendations and advice are given without Liability on the part of Fine Mesh Metals Ltd and should not be relied upon for any action or inaction.
Fine Mesh Metals Ltd shall not be liable to compensate any other party for any loss, including any consequential loss, arising from following the standard designs.
The standard designs for gabion baskets are very robust, "Belt & Braces". In the Welded mesh variety sold by ourselves, we (at Fine Mesh Metals Ltd) have never seen a failure of a wall installed and filled correctly.
Affect of load behind wall:
A load such as a Carpark for normal vehicles adds a surcharge of 2.5kN/m2 which should not cause any issues with the standard designs.
A road behind the wall which allowed access to HGVs would increase the surcharge to 10kN/m2 , we would advise you take advice from a consultant if this is the case or if a building is within 3m of the retained wall, this distance depends on the soil conditions.
Footings/foundation: Please see Note 1 below for a more detailed explanation.
Construction Design Standards, suggest that a 500mm trench be dug to allow for future excavations in front of the wall, this also gets below the frost layer, and provides a toe in to stop the baskets slipping, (This toe in is not normally used within the structural calculations for the standard designs). Another reason for this is that the top soil contains organic matter that may rot or compress. The majority of installations for short walls are started at ground level, it is down to the customer to make a decision on site looking at the ground conditions.
The worst material for gabion walls to sit on is soft clay, if the ground is likely to sink under baskets weight, a gabion retaining wall may not be possible.
100-200mm of hardcore (type 1) should be compacted with a vibrating plate as a footing for the gabion wall. If 200mm was put in the 500mm deep trench the baskets would start 300mm below ground.
Baskets should be lent back at 6 degrees. BS8002:2015(7.3) Cl. 126.96.36.199 states that A gabion wall should be built to a batter to increase its resistance to overturning and sliding.
The Face of a gabion wall can be flushed or stepped. On taller walls baskets are stepped back to equalize the pressure between the heel and toe of the wall.
Gabions can be cut on site to achieve your required dimensions.
The standard thickness for walls up to 3m in height is 3mm. 4mm is the architectural spec, 5mm is what we would refer to as the military spec or for use on higher walls.
For inexperienced installers we recommend adding a 4mm face for walls 3m and below, this reduces the risk of bulging.
The selection of wire diameter is not straight forward. Below photos include examples of gabion retaining walls up to 10.0m high and constructed using various wire diameters.
BS8002:1994(7.1) comments that The life of a gabion wall is not necessarily limited by the effective life of the cage or basket if the shape of the wall is such that the stone filling remains substantially stable after failure of the cage
And Small gabion walls should be designed on the same principle as a gravity mass wall, no allowance being made for the strength or mass of the wire mesh.
Type of infill stone i.e. rounded or flat and quality of construction can greatly impinge upon the requirement for the stone filling to remain substantially stable after failure of the cage and for the wall to remain a coherent mass with no allowance being made for the strength or mass of the wire. A good standard of infill stone selection and placing notably in the manner intended to ensure the satisfactory performance and serviceability of the installation.
The selection of wire size is thus dependent upon:
- Size and type of infill stone.
- Standard of construction.
- Wall height
The use of stone and construction practice not usually associated with gabion wall construction will have an influence on the performance, serviceability and overall life of the installation.
A cost saving option is to put a false partition in the front of the basket. This way the front can be hand laid with a high quality stone like slate. the back can then be back-filled with a low quality material for example reclaimed brick. However it is worth noting that it is unlikely that the use of low quality or smaller stone behind hand laying partitions would provide the stability of the specified gabion infill stone and as such the service life of the wall is likely to be affected.
Geotextile materials including that of Terram are permeable and will not prevent the passage of water through a gabion retaining wall.
The use of a gabion wall to support a building foundation would not be acceptable due to the flexible nature of a gabion. Alternative forms of building foundation should be considered in the event that the proximity of the building and associated traditional foundations are within the near vicinity of the rear of the gabion wall ,where soil strains and hence movements are likely to occur.
Considerations in Respect to health and safety.
Its is paramount when undertaking such works to ensure that the construction can be undertaken with due regard for the Health and safety for all concerned. In this regard the requirements of the Construction (Design and Management) Regulations 2015 herein referred to as CDM Regs apply. These requirements also apply to domestic clients. There have been many incidences of both injuries and fatalities associated with the undertaking of construction works in conjunction with excavations.These have occurred where vertical or near vertical excavated faces faces have been provided and of which have either been unsupported or poorly supported, or where temporary unsupported excavated slopes or batters have been provided which are too steep to remain stable. The influence of other factors which give rise to the potential for instability should also be included in considerations. these factors include but are not limited to water ingress, storage of materials, passage of vehicles.
The use of temporary propping to an excavated face in conjunction with earth retaining wall construction is generally not practical due to the restrictions presented working. The use of a temporary unsupported excavated slope or batter is typically employed and at a gradient of 45 degrees, albeit subject to site conditions. This scenario is the basis of calculation for the standard designs ,where the gap behind the wall is filled with angular material. The angular back-fill should be max of 40mm, so as to be compressed with a vibrating plate without needing heavier equipment, (type 1 would be suitable). If you wish to lawn on top of the back-fill 150-200mm of top soil would be advised with a geotextile underlay to stop the soil drying out. If you wish to plant shrubs a deeper soil pocket will be required.
Highways classify angular back-fill as Class 6N which can be up to 125mm however this will require much heavier machinery to compress and will affect the design of the gabion wall.
1m high wall
A 1m high wall is very simple it can be made of 1m x 1m x 1m or 2m x 1m x1m baskets. The smaller is recommended if the length you have to cover is small as it will reduce transport costs. We can also use 1/2m deep baskets this will save on the cost of fill but will reduce the strength of the wall and is recommended only in situations where the bank is fairly stable. You can also use 1m high walls for terracing a garden in steps. We also provide sets of steps for this scenario.
It is standard to use 3mm wire thickness as the baskets have little weight exerted on them.
2m high wall
Gabion baskets are a mass retaining system thus, the higher the area that needs retaining, the heavier the wall must be. The standard design for a gabion wall is a pyramid. In general, for every 1m increase in wall height, the bottom row basket depth should be increased by half a metre.
For a 2m high wall the bottom row should be 1.5m deep and the top row should be 1m deep.
It is still standard to use 3mm wire thickness for both rows.
This can be made more stable by setting the top baskets slightly further back.
3m high wall
In this example the top 2 rows are the same as a 2m wall but with a .5m increase in the depth of the bottom row.
The bottom row is 2 metres deep middle row 1.5m deep and top row 1m deep. Normally we use 2m x 1m x 1m baskets bottom row 1.5m x 1m x 1m baskets in the middle row and 1m x 1m x 1m or preferably 2m x 1m x 1m baskets going sideways on the top row.
At this height there is quite a lot of weight exerted on the bottom row and unless the bracing ties are installed correctly there is a possibility that the face of the bottom row might bulge out. You may prefer to use 4mm thick wire on the bottom row rather than risk this. If the look of the wall is important than we would then put a 4mm thick face on the 3mm thick top row baskets. This will save on costs of having all baskets in 4mm.
Any higher than 3m you can still use the same principal of increasing the depth by .5m but it may be preferable to stagger the baskets back and getting a structural engineer to assess the stability of the site would be a preferable option.
Curved wall: Staggered effect using 1m or better .5m wide baskets
There are 2 types of curved walls inside and outside bends. When the wall curves around you when looking at it (inside bend ) this can be achieved easily using standard baskets angled away from each other. The wedge shaped gaps that are left are fixed using extra panels that are cut to size and wired in on site.
Outside curves are more difficult and require the gabion to be tapered inwards. This is achieved by taking the side panels off the baskets and sitting the baskets within each other. The lids can be folded or overlapped.
Depending on the depth and speed of the river, different designs are appropriate. In general there is a worry that the river will undercut the baskets if using a standard wall. It is normal to use a gabion mattress (a short basket normally 3m x 2m x .3m high) that goes into the stream and underneath a standard wall. The standard wall does the retaining whilst the mattress stops the foundations being eroded.
Gabion Baskets are permeable so that water can come through them, this stops a build up of water behind them and the ground becoming waterlogged. It is possible to stop water coming through by lining the baskets with a geotextile material, such as Terram. You can have a thin cladding wall basket in front of this so it is not visible. The flood protection offered is only as good as the installation and all joints must be sealed. Care must also be taken to ensure the plastic isn't torn or damaged. Water can still get around the sides of the baskets. This is only suitable in specific location.
Considerations in respect of the noted 500mm trench is in respect of ‘Unplanned excavation’. BS8002(7.1) states:-
In checking the stable equilibrium and soil deformation, retaining walls should be designed assuming a depth of unplanned excavation in front of the wall. The depth of the excavation should be not less than 10% of the total height retained for cantilever walls or of the height retained below the lowest support level for propped or anchored walls, but the depth of the excavation may be limited to 0.5m. This recommendation for an addition excavation as a design criterion is to provide for unforeseen and accidental events. The recommended values should be reviewed for each design; more adverse values should be adopted in particular critical or uncertain conditions but smaller values may be adopted where adverse conditions are beyond reasonable doubt. Foreseeable excavations such as service or drainage trenches in front of a retaining wall, which may be required at some stage in the life of the structure, should be treated as a planned excavation. Actual excavation beyond the planned depth is outside the design considerations of this code.
The following is of note:-
- In checking the stable equilibrium and soil deformation, retaining walls should be designed assuming a depth of unplanned excavation in front of the wall
- This recommendation for an addition excavation as a design criterion is to provide for unforeseen and accidental events.
- The recommended values should be reviewed for each design;
- Foreseeable excavations such as service or drainage trenches in front of a retaining wall, which may be required at some stage in the life of the structure, should be treated as a planned excavation
BS 8002: 1994(7.1) has as noted been superseded and withdrawn and replaced by Eurocode 7 (7.2) and BS8002:2015(7.3).
The following is noted with reference to Eurocode 7(7.2) :-
Cl. 188.8.131.52 Ground Surfaces
(1)P Design values for the geometry of the retained material shall take account of the variation in the actual field values. The design values shall also take account of anticipated excavation or possible scour in front of the retaining wall.
Cl. 184.108.40.206 (2) discusses considerations in respect of cantilever and supported walls where stability is dependant upon ground resistance in front of the structure and lowering by an amount defined as Δa.
BS8002:2015(7.3) Cl. 4.6.4 – Unplanned Excavation comments on the requirements of Eurocode 7 Cl. 220.127.116.11 (2) i.e. notably in respect of cantilever and supported walls.
The design of gravity retaining walls e.g. gabion earth retaining walls are required to comply Eurocode 7 Clause 9 and Clause 4 and Clause 5 of BS8002:2015 notably Eurocode 7 Cl. 18.104.22.168(1)P in respect of unplanned excavation.
In the design of a gravity earth retaining wall e.g. gabion wall we are required to consider all aspects which would affect the normal serviceability and performance of the wall. Such considerations would include but not limited to:-
- Ensuring that the wall formation level is not located in soil containing organic matter as such materials are subject to volumetric change due to degradation which could give rise to movement and potential instability in the wall. Similarly non engineered fill i.e. made ground which could include either non compacted or poorly compacted material and material subject to volumetric change due to degradation.
- The wall formation is at a level where foundations soils are not subject to volumetric change due to frost heave and thaw, i.e. typically 450mm below ground level.
It is of note that a gabion earth retaining wall is flexible and can accommodate differential settlement and movement without structural distress. Criterion normally associated with rigid structures e.g. brick in particular the requirement to provide the foundation formation at a depth below a level where volumetric change due to seasonal variation in moisture can occur notably in clay soils where a minimum depth of 900mm is required, is not applicable to a flexible gabion earth retaining wall.
Any resistance to sliding provided by ground to the face of the wall is typically ignored in design due to the potential for such soils to be non-engineered, potentially partly organic and not always present when construction of the wall is completed.
The incorporation of the foundation formation at 0.5m below finished ground level to the face of the wall would satisfy the requirement to:-
- Maintain stable equilibrium and soil deformation
- Avoid potential disruption due to frost penetration
- Typically be below the depth of any organic soil.
Commencement of construction of the gabion wall directly upon the excavated soil surface is likely to prove unacceptable due to excessive irregularity. The use of a well graded granular material such as SHW Type 1 compacted to ensure a dense stable mass or concrete would provide a suitable blinding/levelling medium and thus maintain acceptable wall alignment. Both materials can be considered non-frost susceptible and as such complying with the requirement to avoid potential disruption due to frost penetration.
Soils at and below formation level are significant in concluding the suitability of a mass gravity retaining wall including a gabion earth retaining wall. Some soils e.g. soft and very soft silts and clays and soils containing organic matter are unlikely to demonstrate satisfactory geotechnical characteristics necessary to provide and maintain the satisfactory design, performance and serviceability of the wall. Some loose cohesionless materials e.g. sands and gravels can similarly prove unacceptable unless treated.
Example Images of good stone placing in stepped and flush walls.
A 6m high flush faced gabion wall
A 10m high flush faced gabion wall
A stepped gabion wall on a curve
A good example of stone placing when filling a gabion.
A close up of a good gabion stone placing.
The below example is of a narrow gabion facing to sheet piling.