Share
We have a sloping front garden bathed in afternoon sun which we wanted to level for a future patio and lawn. This required a new retaining wall behind an existing one. The height was 60cm (600mm) for 4.5m straight, then ~45 degree corner from which point the ground sloped up over a distance of 3.0m so the height went from 60cm to 30cm. This is a beginner's guide for someone like me doing this for the first time.
Materials
H4 treated pine sleepers 200mm h x 75mm d x 3.0m length (cut in 1.5m lengths)
- 9 sleepers for the 4.5m part (3 high)
- 5 sleepers for the sloping 3.0m part (the bottom sleeper of each bay was cut to match the slope)
Galvanised steel posts
- 1200 high for the 4.5m part (2x H for middle, 2x C for ends)
- 1200 to 900 high for the sloping 3.5m part (1x 1200 C for end, 1x 1200H for middle, 1x 900 C for end)
Other
- Cement (40 bags of Bastion non quick dry cement)
- Crushed rock (several bags, to go under the bottom sleeper)
- 100mm Ag slotted drainage pipe (8m, divided and joined to a T pipe, connected to more pipe to fashion a soak well)
- Non-woven GeoTextile fabric
- Scoria (at least ½ cubic metre)
Tools
Ryobi circular saw (to cut the sleepers)
Post hole diggers - metal bar, post hole tool
30mm 2 person petrol Auger (Bunnings hire)
Levels
Builder's string
Measuring tape
Bits of wood (to mark out and level top of wall with string)
Mallet / small sledge hammer
Bondall In-ground Timber Protector (to paint the sleepers)
Steps
Step 1
Preparation, Tools, and Digging the post holes
First, check whether you need a building permit and/or engineer's report. For our project we did not require these as
* the height of the wall is <1m
* it is not retaining a carport, sloping ground, or other significant load
* it is not part of the dwelling or other existing structure. Basically this means that the new retaining wall had to be set back at least 1m from the pre-existing retaining wall which is ~90cm high
* it is >9m from the intersection as we live on a corner (note, this is the distance measured from where the 2 roads would meet in a straight line, NOT measured from the curb which curves around the corner)
Please check with your council if you have any concerns.
To prepare the site we first removed all the bark and weed mat. We then dug back 1m to where the retaining wall would be + another 30cm at least for the drainage, making the new soil base level to the base of the retaining wall or slightly below. Don't dig deeper as the post holes will be up to the soil level.
I then hammered in timber boards to mark out the 2 parts of the retaining wall. I made these level vertically so that I could run 2 string lines, one at the top of the posts to mark the height, and another lower down to assist with making the posts vertical.
As the 2 sections of the wall were 4.5m and 3.0m (luckily), and the sleepers come in 3.0m lengths, I was able to just cut the sleepers in half with a circular saw. I then carefully marked the post hole sites by holding a post, sitting a sleeper against it, then holding the next post and marking the site, repeat. (The distance between post holes is a little more than 1.5m as the post takes some space, and you need 1-2cm gap at the end of each sleeper). Obviously the post should be at the centre of the hole but for at least one of the holes I had to sit the post very close to the edge due to nearby obstruction - just make sure the concrete is all around the post to prevent water getting in.
Then to dig the post holes. For most of the wall it was 60cm high. The C / H galvanised posts were therefore selected to be 120cm high (needs to be at least as deep as the height of the wall). You then need an additional 10cm depth below the bottom of the posts, therefore the post holes were dug to a depth of 70cm. The diameter of the holes were 30cm, after reading and taking advice from the Bunnings community. 45cm is described but seems like overkill for my project given it's only at most 60cm of soil being retained and it's just garden not a sloping section and certainly not holding a car park or some other significant weight.
These are the tools I used. The metal bar and post hole digger were great and the 2 person petrol Auger hired from Bunnings helped a lot. I used the smaller drill attachment first, then swapped for the 30cm attachment, then finished the holes of with the bar and digging tool.
Then it rained a fair bit so I waited for the holes to dry out a bit, and made sure to scoop out any remaining water and dig out any mud until I was back to solid soil.
Step 2
Concreting in the retaining wall posts
For our 6 post holes (30cm wide, mostly 70cm deep) we used in total 40 bags of cement. I read not to use the quick set cement as it is not as strong, also this is our first time using cement so we wanted time to not be stressed! We mixed 2 bags at a time in a wheelbarrow which was fine, but you could hire a cement mixer.
Put at least 10cm of concrete into the hole to fill the base. The post will sink into the concrete a little at first so you want the post to sit high initially. We found the post sunk quite a bit so we put a large stone on top of the 10cm concrete and placed the post onto this, which meant that the post didn't sink.
You need to agitate the concrete to remove air pockets and make sure it sets strong and solid. As you do this the post will sink a little, ~1cm we found. So we agitated first, then if needed lowered the post by hitting the top with a mallet / large hammer until it was at the correct height. Make sure throughout these steps that it remains vertical in both axes ('plumb').
If you find that it isn't right at this point, maybe the post needs to move sideways a bit, you don't need to pull it out and start again. Just grasp the post firmly and then jiggle it up and down in the concrete. This will turn the surrounding concrete liquid and you will find that you can move the post as you need, then re-level and plumb.
You don't need to brace the posts, even though the concrete takes 3 days to set completely. They didn't move. We did however put some crushed rock between the posts enough to sit a sleeper on, to make absolutely sure that the spacing was correct.
Step 3
Engineering and Batter
These videos further describe my process of determining whether a building permit / engineering report was required. Again, check if you have any questions at all. I read the requirements for my council, then gave them a call to make sure.
Further comments on permission. Also my learning as to whether the posts can be vertical or whether they need to be tilted back (called a 'batter'). If the retaining wall is higher, or supports a greater load, then your wall then you may need to include a batter and you certainly will if the engineer says to. If you do need it, then I'm told it's 5-10mm tilt back per 10cm of post height above the ground. So for my 60cm high wall that would have been 3 to 6cm of backwards tilt (batter).
Step 4
Placing the sleepers
I had marked on the side of the posts where the bottom of the first sleeper should sit. I then placed crushed rock (available from landscape suppliers or you could get bags of similar from Bunnings) at the base and compacted / levelled this for the bottom sleeper to sit on.
To compact it I used a wooden board first as it is much lighter and easier to move around than a timber sleeper. Once I was pretty happy with the height and level, I placed the bottom sleeper on and used the hammer to make sure this was well seated at the correct height and level.
You want the top sleeper to be 1-2cm above the post. Mine were too high so I had to take them out, scrape out a bit of the crushed rock, and start again. This didn't take too long thankfully.
Step 5
Soakwell
Ensuring good drainage of water from behind the retaining wall is very important as the weight of water adds significant load onto the wall. Best practice is to connect the drainage to a drain / stormwater pipe but for me this was at the bottom of the previous retaining wall which I did not want to have to expose. I believe the alternative is to use a soakwell, a space to which the water can drain into and then from there into the surrounding ground. I'll be interested to see comments regarding my approach to this, please be kind! You can buy soakwells from Bunnings but this is a big rectangular box which would get in the way of the hedge that I plan to plant in front of the retaining wall so I made a soakwell by laying slotted Ag pipe within a bed of scoria surrounded by the GeoTextile fabric. I placed this behind the previous retaining wall, deep enough that it should drain into the drainage backfill of the previous retaining wall and from there into the existing drain.
Step 6
Drainage
To further protect the H4 treated pine, you can paint it with a waterproofing paint available from Bunnings. This is a water-based paint which goes on easily with brush or roller and does not need an undercoat. It finishes with a lovely deep black colour. I painted this at the bottom of the bottom sleeper, both ends of all sleepers (especially as one of these was where I had cut the 3m sleeper in half), and the back of all sleepers except going only ½ way up the back of the top sleeper. You're supposed to paint to 10cm above the soil line but I didn't really want the black paint too visible from the house.
Before laying the drainage, you want to ensure that the ground behind the retaining wall slopes evenly downwards to the drainage point (which for me was where the drainage pipe connected to the soakwell pipe).
Then place the non woven GeoTextile fabric (available from Bunnings of course) down the back of the retaining wall (cover the gaps between sleepers), along the bottom of the 30cm sloping space behind the retaining wall, then up the other side behind the backfill with enough to end on top of the backfill. Inside this layer (which keeps the soil out of the scoria and drainage pipe) put 1-2cm of scoria at the bottom, then the slotted AgPipe on top of this (100mm diameter).
Then you fill with scoria up to the point that you want the topsoil. Once you are at this point, wrap the fabric over the top of the scoria sealing out the soil, then place your topsoil on top of the fabric. I ran out of scoria here and would estimate for my project ~½ cubic metre of scoria at least was required.
Step 7
Completion and Lessons Learned
I think it's helpful to point out things that I thought I could have done better and things that didn't go to plan, as often this is where we learn!
I think I could have done the drainage a bit better. I was surprised how much scoria I needed and I could have got some more, though it was tricky as I didn't have the soil behind the retaining wall when I did the drainage so it was difficult to restrict the scoria to just 30cm behind the wall. Maybe this used up a bit, so probably better to somehow backfill with soil at the same time as filling the drainage behind the wall so that you don't run out of scoria before it gets to the height that you want. The soak well seems to work well so far after some rain with no water pooling and hopefully it will help give some water to the plants on the lower level too.
Painting the sleeps with the timber protector was another issue. If you paint 10cm above the soil line as directed then it is visible. I like the look of it actually but my wife now wants me to paint the wall to cover it! So either paint the protector only to the soil line, enjoy the black base, or get ready for more work!
The depths of the post holes was pretty straight forward but finding information on the hole diameter and whether to slope (batter) the posts was not. Thanks to the Bunnings community for the good advice.
Lastly if you have any doubts about whether you need council permission and/or engineering input then do make sure you check. A nearby neighbour has a wall that is falling over despite being very new which just looks terrible and it's so much easier to avoid this problem than try to fix it.
Here is how it turned out. It's taken a long time over many weekends so we're really happy to finally have the retaining wall in place to allow the area behind to be filled in so the lawn could go down. Bring on summer!
