The price of building a retaining wall can be quite expensive and after looking into (and doing) it myself, it’s easy to see why. There are so many variables that you need to account for and if not done right, will only end up causing more work and costing you more money.
How To Plan Your Retaining Wall Build
It’s important for me to mention that a majority of retaining walls will require a permit when over a certain size and can be different for each local council, so if that applies to your wall, it’s best to check first.
The first item was what type of materials I preferred to use and, in my case, I chose treated pine sleepers. It is much lighter and easier to manage by myself, plus the cost was much cheaper. There are many alternatives such as, concrete panels, limestone blocks or hardwood sleepers, each having their own challenges but for now we will talk about treated pine.
I needed to know how I was going to connect and support the horizontal sleepers, with my options being to either butt join them and bolt them to a 200 x 75mm post or to sleeve them into galvanized pre-fabricated posts. I chose the galvanized posts because it was, in my opinion easier to install.
The size of my wall was about 15m, so I decided to use 3m long sleepers and went for 200 x 50mm because my wall is not going to be what I want long term (we are planning to demolish the house and I will get large limestone retaining blocks), so for now this will suffice. If I was planning on keeping the retaining for 20 years, I would have chosen 200 x 75mm sleepers.
So, once you know what types of materials you want to use, you can start making your shopping list. For my 15m x 600mm high wall:
- 15 x 200 x 50mm x 3m sleepers
- 2 x 50mm galvanized corner posts
- 4 x 50mm galvanized posts
- 3 x 200 x 75mm x 2.7m (cut in half, makes 2 midway posts)
- 36 bags of rapid set concrete equals 3 bags per post (call it over kill, but it will retain)
- M10 x 150mm galvanized cup head bolts with washer and nut for midway posts
- M10 x 65mm treated pine or galvanized cobalt screws for galvanized posts
Steps For Building Your Retaining Wall
My wall was next to a Colorbond fence, so I made an allowance of 400mm away to allow for the concrete around the fence posts. I started by digging away as much sand as I could to give me a clear area and leveling as best as I could. I would recommend compacting the ground and possibly adding some sub-base material for good drainage.
Next, I set posts either end with a string line attached and set the height using a bubble level, so I had a height and a distance off the fence to set my posts to. Starting with my corner post and using one of my sleepers as an accurate distance, I set in each post. The recommended depth of posts for 600mm high is 600mm deep, but the posts I had were 1500mm, so each post was 900mm in the ground. Before putting in each post, including my mid posts, I added about 100mm of blue metal gravel to allow water to drain from the bottom of the post and cemented each one with 3 bags of cement. My holes were roughly 400mm square.
After setting each one of the galvanized sleeved posts, I put in the sleepers allowing for a small gap between the end of the sleeper and the post. This method allows for a small amount of expansion and makes it easier to put the sleepers in, as if you post isn’t dead level, they may bind up. I attached 2 x M10 x 65mm cobalt screws per sleeper, per end to each galvanized steel post to secure them.
The mid way posts holes were then dug halfway along each sleeper and I used 2.7m x 200 x 75mm sleepers cut in half, so these were anchored about 750mm in the ground with 100mm of gravel and 3 bags of concrete. I secured these to the sleepers using 2 x M10 x 150mm cup head bolts with washer and nut to be able to pull the sleepers to each post, as each sleeper tends to have slight bows. It makes sense to use the bows to your advantage; I like to face the bow into the area to be retained as this allows the weight of the backfill to push the bow straight.
Retaining Wall Drainage and Backfill
Drainage needs to be considered when building a retaining wall because water doesn’t move through impervious objects and will damage your wall due to build up of hydrostatic pressure. Using biddum (geo-fabric), I cover the entire depth of the wall, stapling it to the back of the sleepers, laid it at the bottom of the trench and up past the first rail of the Colorbond fence.
I laid a 65mm socked agricultural pipe behind the wall giving it a slight fall to the outside of my wall and bedded it with soil. The backfill consisted of 300mm wide by 400mm deep gravel stone fill behind the wall to allow for good drainage and then wrapped the stone with the overlapping bits of biddum, which will stop any sand from entering and filling my drainage area.
Lastly, the top layer of about 200mm was filled with my garden mixture of choice, ready for any plants.
How Easy Is It to DIY a Retaining Wall?
Well let me say that even though I have much experience in many aspects of building and maintenance, this was by no means an easy task. The digging was tiring because of the depths of my posts, not to mention the 8m3 of sand I had to move away from the original fence and the sleepers were both heavy and awkward to move.
Would I do it again? The answer to that will always be a yes from me, but if you don’t have the strength, tools or experience, then I would definitely suggest you leave this type of job to a professional. After all, you don’t want to spend that much time, money and effort into something that may fail because you forgot one simple step, or you didn’t use the right materials for the job!
How Much Does It Cost to Build a Treated Pine Sleeper Retaining Wall?
All up, this wall cost me $1850 Australian to build and took me 4 full days. Now when you get a quote for a retaining wall and think that it is too expensive, consider that.