Gabion baskets full of spectacular looking rock filling is a growing trend as a new approach to dealing with a retaining wall. They can be more cost effective and easier to install than traditional sleepers but less cumbersome and difficult to manoeuvre than concrete or sandstone blocks. Simple wire mesh panels and spiral winders make building a small retaining wall very easy. Here's some tips that can help.
All gabions are made from hot dipped galvanised 5mm gauge steel with a 50 x 50mm aperture (mesh gap). These features may not sound important but it is. Here's why:
Galvanised steel is known for its resistance to weather and ability to withstand corrosive atmospheres, in particularly exposure to coastal air. Read here for more information on galvanised gabions.
5mm gauge steel is thick enough to allow the baskets not to bulge, stretch or warp when filled with rock. Lighter gauge steel (3 or 4mm) can bulge once rocks fill the basket, pushing the mesh panels away from each other and incurring a bow in the basket. Read more on this here.
50 x 50mm mesh aperture goes hand in hand with the 5mm gauge steel. The larger mesh, the more stress placed on the intersections of the steel. The larger the mesh, the less intersections meaning more pressure placed on each intersection. This can directly contribute to bulging baskets once full of rock.
The limits of height you can build a gabion rock retaining is really only limited to your needs. Baskets can be used to divide or step gardens by as little as 300mm or can be used to build a side of a house. It all comes down to stability and above a certain height, engineering approval. Find out more detail on considerations of how high you can build a retaining wall here.
Here are 4 common approaches to think about when determining the height of your rock retaining wall:
Add additional baskets to increase the depth of the wall. This increases the surface area of the base of the wall on the ground which reduces the opportunity for the wall to sway or collapse. The recommended ratio is 2 baskets high to every 1 basket deep.
Install support posts. Support posts help add stability to your wall to also reduce the risk of collapse. They need to be strong enough to properly support the gabions and should be secured in a concrete slab underneath the basket wall. The stablisers can be threaded through the openings in the baskets to create rigidity through interlocking the baskets together. The recommended spacing between the support posts is 1m.
Step your wall. Stepping your retaining wall will add stability and have a provide a stepped look rather than a flush look for your wall. Gabion walls with a stepped front face shall have a minimum 150 mm horizontal set back for each 500 mm vertical lift.
Slope your wall. Sloping your wall back into the incline of the hill will help add stability to the wall. The recommended incline is between 6 and 10 degrees. Compacting of soil under the base will be required. This is not recommended for retaining walls greater than 5m in height.
Each state of Australia has different regulations of the acceptable height of retaining walls without engineering approvals. This allows you to build a retaining wall without approval from your council and an engineering study. In Queensland, councils will allow you to build a retaining wall less than 1 metre in height without a permit or building approval.
This height restriction applies to the following councils: Brisbane City, Ipswitch City, Redland City, Gold Coast City, Logan City, Scenic Rim, Southern Downs, Lockyer Valley, Moreton Bay, Somerset, Toowoomba, Sunshine Coast, Noosa, Gympie and South Burnett Regional council.
Gabion baskets can come in a range of sizes since they're built from a combination of steel wire mesh panels and spiral winder joiners. This gives you a wide variety of flexibility when it comes to finalising on the size basket you need.
There are a number of challenges when using large gabions. Here's some considerations:
The most common form of delivery for all gabion baskets is flat packed on a wooden pallet (see below). The gabions are constructed into their finished shape onsite in situ when ready for filling. You would never build the basket and fill with rock and try to move into position - the basket will be very heavy and not possible to move by hand. The installer building the retaining wall will physically take the steel mesh pieces from the pallet and erect into a basket form and place in it’s final resting position (in retaining wall).
To find out what you get with your gabion basket flatpack
When you consider this, moving steel mesh sheets of 1000mm x 500mm in size are far easier to handle by one person than sheets of 1000mm x 1000mm. Gabion baskets made up of wire mesh panels of 1000mm x 1000mm in size is about the maximum size that can be handled by 1 person. Panels of a larger size will require more people (at least 2) to put together. Refer to the demo video to see an example of a 1000mm x 500mm basket being put together.
Smaller gabion walls (or larger gabion walls built in stages of smaller sections) allow you to physically reach the bottom of the basket. This is very helpful when you're trying to place the rock filling. If you're trying to reach down from a 1m high basket or even worse, get a wheelbarrow up high to tip rock in, you're going to struggle.
For gabions placed on the ground, you can simply wheelbarrow your rock filling directly into the top of the basket then attach the lid mesh. You do not require any loaders or bobcat’s to scoop the rock and fill the basket from an unmanageable height.
Here's a good example: Say you wanted to build a achieve a 2m long gabion rock wall that is 300mm deep and 500mm high. Rather than trying to achieve this with one large basket, we can use a combination of different standard panel sizes as follows:
4 off 1000x500mm panels (acting as the front and back facing panels)
4 off 1000x300mm panels (acting as the bottom and top facing panels)
3 off 500x300mm panels (acting as the 2 end panels and 1 centre panel)
Rather than try and handle one 2m monster basket, you can build this in situ one panel at a time. Notice how the 1 off 500x300mm panel is used as a central panel? This means that this setup is more cost effective than buying 2 individual baskets butted up together because one end panel is now redundant. It also ties all the other panels in together nicely with the spiral winder (not shown).
Smaller gabion baskets installed on top of one another allow for a centre support that runs horizontally through the middle of your retaining wall. This ensures your rock filling stays consistent throughout the basket, preventing bellowing. Over time, rock filling cannot slowly sink deeper into the basket causing bellowing due to the weight and pressure of the rock. The larger the basket, the more likely bellowing is to happen because the less wire mesh is available to support it. Such an effect will eventually destroy the shape of the wall and its functionality.
Step 1 - Laying Out The Site
Start by laying out the wall using a string line and line level or a carpenters level mounted on a long straight edge (usually 2"x4" - 12' long) and check that the pad is level in both directions.
Step 2 - Provide A Firm And Level Base
Using a hoe or flat shovel, remove any grass, weeds or top soil. If the soil is hard, you may be able to put your gabion wall directly on the existing soil. If you have sandy or clay soil, you will need to replace about 6" depth with compacted gravel or a concrete footer, otherwise the soil might slump under the weight. If you are not planning to vegetate the wall, use brush killer (Round Up) to destroy any weeds or roots then cover over with geotextile fabric.
Step 3 - Gabion Reinforcement - Install Support Columns
There is virtually no structural integrity in the wire of a narrow width gabion wall. These walls will require internal support posts imbedded in concrete and spaced on appx. 6' centers.
Depending on the installation and local codes, typical columns may be sections of galvanized pipe, pressure treated wood posts or structural steel members.
Be sure to bury the column 3-4' deep, add dry mix concrete, then wet it up and return in a day or two and start your installation.
Step 4 - Assembling The Gabion Baskets
Start by unfolding and laying out the wire mesh panels with its bottom panel laying flat on the prepared base by sliding the gabion over the internal support posts. Our baskets are held together with a spiral of galvanized steel that you wind through the adjacent edges of the mesh panels. It’s easy to do since the spiral matches the gabion's mesh grid spacing. When the spiral reaches the end, use pliers to pinch each end to secure and prevent it from slipping out.
6 ft long baskets are the most commonly used size. They have a middle dividing panel to reduce bulging, which is wired in along its bottom edge when the cage is open on the flat area, and then down each vertical side once the sides are brought up and the basket is in place. The baskets generally need bracing ties across each corner 1/3 and 2/3 up depending on the height of the basket, which are positioned and pinched closed.
Step 5 - Filling The Gabion Baskets
If you have indigenous rocks available, filling with these on-site materials will produce a gabion wall that fits well into its surrounding landscape.
To prevent spillage, the exterior rocks placed against the exposed faces need to be slightly larger than the mesh openings. Typical 3" x 3" mesh requires a minimum of 4" rocks and limit the largest rock at 8". Use angular rocks to fit into corners and smaller rocks or used bricks and concrete materials may be inserted in the center of the structure as they will not to be seen when finished.
Careful packing of the rock takes time but is important!
Work in layers, using smaller pieces to fill any voids and to wedge the best looking rocks against the face of the basket. Orient them as closely as possible, then filling in behind to hold them in place.
Step 6 - Adding Layers
To obtain fence heights taller than 3' you generally stack multiple baskets on top of each other. Once you have packed the lower basket, wire close its lid and then assemble the second layer on top. Fill the second layer like the first using the best looking rocks on the face and the less attractive fill rocks in the center.
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