How to build a gabion retaining wall
Follow our story on how we built a gabion retaining wall.
Our house on a hill
Our house is situated at the top of a hill and the driveway slopes down towards the house. With no off-street parking due to the single track lane, we only had a small area of old, sloping crazy paving to park our two cars. Every time you opened a door it would swing into something – usually a leg or another door!
We did, however, have a grassy area to the side of the driveway, which would be the ideal spot to extend the driveway and build a garage. However, the bank would need a serious retaining wall.
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Best type of retaining wall for our situation
So, what type of retaining wall should we build?
The first option, which I’m sure lots of people investigate, is a dense block wall or a hollow block wall reinforced with rebar and concrete. However, a structural engineer would need to get involved due to the height we required, plus we knew that the finish in brick would be costly. Render wasn’t an option as it would need regular maintenance due to the damp conditions.
I also looked at a railway sleeper retaining wall as they look more natural and would fit in with the feel we were after. At some point, though, this would need to be replaced due to natural aging and deterioration.
Then, I came across gabions baskets. These would be ideal as they could be filled with a heavy material, have a semi-urban/natural look about them, stay permeable and would not rot for a very, very, long time.
Why I decided to do it myself!
We spoke to several builders, but quotes came back between £10,000-20,000 (!) for the 2m high by 20m wide retaining wall.
I knew I could do something for less and a gabion retaining wall system was the answer. We had plenty of brick rubble leftover from our internal house renovations which could fill the gabions. We also had the space to accommodate the 1m deep baskets, leaving us with about 8m of useable driveway.
Which gabion baskets should you buy?
I decided to buy the gabions from gabionbaskets.co.uk (or Fine Mesh Metals is their other name). They have lots of choice, great installation information and lots of case studies to look at.
A common question is how long do gabions last? I discovered that the ones from gabionbaskets.co.uk are Galfan coated which means they last 4 times longer than standard galvanised baskets. So make sure you check for this as some of the cheaper ones available are only galvanised with no extra treatment.
I mocked up our gabion wall design in excel and sent it over to gabionbaskets.co.uk who were able to work out a cost for me. This included a strengthened 4mm front face on each gabion (to help reduce any bulge), the lacing wire, partition panels for the decorative stone and helicoils for easy front joins. The helicoils are more noticeable than the lacing wire, but I think add a nice, framed look to each basket.
Site preparation for a gabion retaining wall
As with any DIY project, the planning and site preparation are the most important steps in the whole process. We hired a man and his digger to remove the tree stumps and cut back into the bank along the line of our boundary. All the top soil was removed until we reached firm, solid ground. The gabion foundation can be a layer of compacted hardcore 100-200mm deep. Or in our case we had reached a flint and chalk layer which was solid enough.
Using a spirit level (or app on a mobile phone), I made sure the ground was level left to right, but also angling 5-6 degrees leaning into the bank. Gabion baskets must lean slightly backwards to aid their retaining powers.
Next, I put down a sheet of woven geotextile to stabilise the ground and added a thin layer of gravel to help find the final angle of 5-6 degrees into the bank.
Gabion wall drainage
As with any retaining wall, you need to consider how to alleviate the build up of pressure caused by excess water in the retained soil. Strictly speaking, as the gabion basket method is completely porous, you might want to skip this step, but it doesn’t hurt to add extra drainage behind the baskets if you can.
I achieved this by adding an 80mm perforated drainage pipe along the back of the bottom row, sloping slightly in the direction I wanted it to drain. The pipe was wrapped in a non-woven geotextile and topped with a good 300-400mm of gravel, similarly to what you would do for a French drain or behind another type of retaining wall.
Regardless of whether you add extra drainage behind the baskets or not, as the back of the gabions is open, non-woven geotextile needs to be pulled up behind the baskets to prevent soil working its way into the stone in-fill.
Please click on this post to find out more about the different types of geotextile and their uses.
Gabion wall construction
Our baskets arrived part made up. I just had to fold out the flaps and secure the joins with either the lacing wire or helicoils (front joins only).
Gabion basket layout:
I used a string line to line up the baskets all in a row on top of the geotextile and gravel. I made sure the baskets were leaning back 5-6 degrees into the bank before starting to lace them together. The whole structure must be ‘as one’.
How to fill a gabion basket:
The cheapest way to fill a gabion basket is with gabion stone bulk bags. I found a great selection at Mainland Aggregates. Gabion stone is just another name for angular limestone. The fancier the colour the more expensive they get, so stick with plain old Buff or York gabion stone. These can be hand placed along the face of the basket.
Unfortunately, after trying out the gabion stone, it didn’t feel modern enough for our residential setting. It looked more like that seen at industrial sites or on the side of a motorway!
Scottish cobbles, on the otherhand, would provide a more modern look to the retaining wall. However, they are very expensive! One bulk bag doesn’t even fill a 1m3 basket. So, to reduce costs, I used a partition panel set back 100mm from the front of the basket along with the 80-120mm Scottish cobbles.
Using the cobbles and panel also meant I could use recycled stone as the in-fill, which was even cheaper than the gabion stone. We had lots of rubble leftover from our internal house renovations, which was perfect for filling up the baskets. I also ordered more recycled brick from Mainland Aggregates to finish the job.
Adding a partition panel:
Before adding the cobbles, I cut a piece of black woven geotextile just larger than 1m2 for each gabion and placed it inside the basket against the partition- inside the main section of the basket, not the thin partition itself. This blocked the view of the recycled bricks and the baskets looked like they were filled completely with cobbles.
Bracing the gabion basket:
Bracing the gabion basket is a step that cannot be skipped as it reduces any gabion bulge.
The lacing wire was cut into approx. 1.5m lengths, then threaded through the same place in each gabion, eg 4 spaces up, 4 spaces from the left, 4 spaces up, 4 spaces from the right etc. I then looped the wire back through the way it came, and twisted the wire around itself in the middle of the basket to prevent it coming loose. A set of pliers was used to grip the middle of the wire and twist it to make it taught.
It’s important to do this both ways, though I have to say I did fewer side to side, it was the front and back ones that were the most important to stop the bulge on the front face.
Adding a second layer to the retaining wall:
As our retaining wall was 2m high it required a second layer. This layer can step back a little into the bank, or, as in our case, the fronts of the baskets were kept in line.
Remember to make sure the baskets on the bottom row are filled right to the top before adding the second layer. Otherwise, the weight of the material in the top baskets will sink down into the bottom layer and misshape the lid.
I also laced the lids shut before adding the next row. When attaching the front helicoil, this must be woven into the lid of the bottom basket and the base of the top basket so that they are completely joined together. Top and bottom layers must be laced together on all sides with the lacing wire, so again the whole section is ‘as one’.
Back filling behind the gabion wall:
As the second layer got fuller and more stable, I started to backfill behind the gabions with soil. The non-woven geotextile was pulled up taught behind the baskets so that no soil entered the gabions. The soil can be washed through the basket to the front and would look unsightly if this wasn’t in place.
Finishing off the gabions:
With regards to finishing off the gabions, make sure that all sides are laced together and the lids are secured with the helicoils. I didn’t in the end fill the tops with cobbles. They were a leaf trap in the Autumn, and it was difficult to fish debris out. At some point, I plan to lay more white non-woven geotextile over the top with a thin layer of soil and grow rockery type plants near the edge so that they drape over in time.
If you have a basket side that will remain visible after the build, don’t forget to purchase a cost saving panel for it, use wire cutters to reduce its size, and use it to leave a decorative cobble finish.
Alternative gabion fill - gravel bags
Unfortunately, I ran out of brick fill for the second 1.5m tall section of the retaining wall. So, I decided to try an alternative to the recycled brick and ordered a couple tonnes of 20mm shingle gravel and lots of woven rubble bags, large and small.
I filled a gravel bag ¾ full and then tied it off with a cable tie. The large bags were too heavy to lift into place, so I started them off with a few scoops and then placed them inside the gabion before filling them up. I used the small bags to fill in the gaps in between the large bags.
This did make inserting the bracing wires more difficult. Plus, I did find that some of the top baskets moved when the soil was back filled as the weight of the gravel just wasn’t enough to hold them in place. I would avoid this method unless it is a decorative gabion basket and not a retaining wall.
Gabion wall cost per metre
In total we spent £5207 on the 20m wide 2m tall gabion retaining wall, which was approx. 26.75 cubic metres in size – giving a total cost of just under £200 per cubic metre. These figures include the man and his digger and the tree surgeon (£1250), so you can exclude those costs from your project which brings the cost down even further, below £150 per cubic metre.
As you can see, we saved between £5000-£15000 off the builders quotes by doing it ourselves!
This is definitely a project a keen diy-er can do.
The finished gabion retaining wall!
Here is our finished gabion retaining wall!
I hope you’ve found this useful. Please feel free to leave any questions in the comments and I’ll do my best to answer them. Remember we’re just DIY’ers doing it our way in our set of circumstances. Please seek professional advice for your own retaining wall project.