Gabions Wall Design & Build Tips
Gabion Wall Design
Devoran gabions are transported to site flat packed with lacing wire as standard for on site erection.
On site, the gabions are opened out in the required position. After which the sides, diaphragms, face and rear panels of the gabion box are rotated to the vertical position and joined with a continuous lacing operation (wire supplied as standard). The gabions can be joined using helicals or ‘C’ rings (supplied at additional cost).
Joints between gabion courses are normally continuously laced or clipped with ‘C’ rings.
If you are designing a wall, always seek professional advice for design and suitability – Contact Devoran for assistance.
Design and Build Tips:
- Slopes of retained fill cannot exceed the internal angle of friction of the soil.
- In poor ground bearing conditions opt for a larger base and / or stepped wall configuration to even bearing pressures. Embed a wider founding gabion to spread the loading over a greater area.
- Avoid where possible gabion walls built with a vertical face. The section will increase and deformation on the gabion face will be more visible.
- When using our standard gabion system, nominally step the face of the gabion wall at each course.
- Filling – use our guide to filling gabions.
- Use windlass braces to ensure gabions do not bulge.
- Use corner braces if required.
- Ensure that the gabion wall design is based on factual soil investigations.
- Select the gabion system based on stability, economy of design, volume of earth works required and quality of installation required.
- Contact Devoran for further advice with gabion wall design and gabion wall construction
Often Seen Mistakes
When it comes to filling the gabion baskets with stone (see below for recommended stone fill) it is very important to minimise any large voids within the cells. This is because over time settlement can occur. If there are large voids between the stones, as the wall settles, for example during heavy rain fall, the stones will begin to move and fall into the open voids. As a result the structural integrity of the wall is compromised and if this happens, in cases where a high level of stone movement has occurred, the gabion mesh will begin to deform and give way at its weakest point, the middle of the face, which causes bulging and increased pressure on the weld joints. When this happens it usually requires replacement baskets which can be a very costly fix. To avoid any gaps in the gabions, whether you are machine filling or hand filling the baskets, always check for large voids and re-orientate the stone so it interlocks within the cells. It may be more time consuming during construction but can save a lot of time and money in the long run.
Geotextile Separator Fabrics
In some applications we recommend the use of a geotextile fabric separator to the underside and rear of the gabion wall. Geotextiles are a non-woven permeable fabric sold by the m2, commonly used in the construction of gabion retaining walls to prevent fine soils from passing through the gabion baskets and thus protecting the integrity of the retained structure behind / below the wall. Some soil conditions do not require a geotextile so it is best to discuss this with your engineer, or alternatively, contact Devoran Metals for more assistance.
Wrong Stone Choice
Contrary to some opinions, not any stone can be used to fill gabion baskets. The grading and angularity of fill are the 2 most important factors to consider when choosing gabion stone. The grading of stone should be between 100-200mm in diameter; this can be tightened to 80-150mm providing the control of the grading is tight. Stones that are smaller than the mesh will not be contained by it. Non-frost susceptible quarried stone, which is normally angular, is the preferred fill as the interlock is very good and voids are minimal. It is not always best practise to fill gabions with extracted stone or rock from the existing site as it may not possess the correct qualities. For example, if the stone is susceptible to frost during the winter months the fill will begin to disintegrate causing voids to form and, on walls over 1m high, the gabions will buckle under their own weight compromising the structure and safety of the wall. If you have any questions regarding the stone you want to use please don’t hesitate to contact us.
Seen far too often on large retaining walls is the lack of internal bracing ties which leads to deformation of the mesh face and the unsightly ‘bulge’. There are many ways to protect the aesthetics of the gabion mesh, firstly by installing internal windlass bracing ties at every 1/3 and 2/3 of the basket which involves lacing tying wire (supplied as standard) central to each cell, spanning 2 meshes, on the front and rear faces. Timbers or scaffold tubes can be temporarily wired to the external faces during filling to act as shuttering to prevent bulging and then removed once complete. Secondly, by ensuring that the correct grading of stone has been used and voids are minimised; and thirdly, you can opt for a thicker mesh in either 4mm or 5mm.
Gabion walls, whether retaining or freestanding, should be founded on either a concrete base or compacted hard-core of 300mm. Concrete is best if the ground is subject to significant heave and settlement (if for example the water table is very high). Otherwise you should remove all the top soil to a firm founding, excavate a further 300mm then bring the level back up to where you want to be with rolled hard-core, or type 1 (as used in road sub bases) which you can build directly off. Most retaining walls are designed with a 300mm toe for increased stability at a 6° incline.
Free Standing Walls
If you are thinking about building a freestanding gabion wall there are a few important points to consider before you start. It is often thought that you can just stack narrow gabions and because they are filled with heavy stone, they will be structurally sound – this is not the case. The narrowest width you can use without using internal wind posts is 750mm. An unsupported wall is by far the most cost effective solution. If, however, you require the wall to be narrower than 750mm you need to install internal wind posts inside each gabion cell. The size of the wind post, usually a galvanised square box section, will need to be determined by a structural engineer as it is based on wind/ pedestrian load calculations. If you do not have a structural engineer on the project, Devoran Metals can offer a free design service.
The most common mistake made when building a gabion wall is the quality of the product being used. Despite following all of the advice above, if the gabions are of poor quality it is inevitable that the gabion wall will fail over time. All gabions supplied by Devoran Metals have been independently tested by the British Board of Agreement and conform to industry standards.