I have a 20 meters long backyard with a 40 cm slope downwards in every 10 meters. I want to level the upper half of the garden by adding mix-soil. To divide the lower garden from the upper garden, I'll add a retaining wall, and a couple of wide steps. So the upper part of the garden will be in the level of the house and flat, while the lower part will remain as it is, as I have a different plan with it.
I found info on constructing a retaining wall and steps, but not much on levelling by purchasing soil-mix, and on how to do the whole task in the correct order. As a newbie, this absence also made me worry that I might be doing something unusual if not problematic! So let me know if you have the answer to any of these questions:
1. What should be the order? The retaining walls, then the levelling, and finally, the steps?
2. How deep should I dig for the trench of the retaining walls? Should the length of the walls be the same with the ground level, or slightly taller?
3. Can I do the levelling directly by pouring soil-mix and sod on the garden without digging the existing grass out? Or would that negatively impact the soil?
4. Is there anything missing in the broader picture that I should be planning?
Thank you, and please share if you know any relevant website or online information that I can use!
Welcome to the Bunnings Workshop community @ayda. It's wonderful to have you join us and many thanks for your questions.
What do you intend to do at the fence line on either side of the property? If you are levelling the top part of the yard you'll most likely need to retain the soil at the fencelines also. I don't think you'll be able to just pile the soil up against the fence. I presume you will be moving the shed but what about the large tree/s? It appears at least the lower one is going to require a fair amount of soil piled up around its trunk. I'm not sure how much it will appreciate that.
I'd be inclined to think that you should create the retaining wall and the steps first. If you work out the height of the finished wall you can then check with your local regulatory body as once you reach a certain height it will need engineering. You can have the height of the retaining wall the same as the level of new fill soil. You'll find it will compress down over time and end up lower than the wall. I can't see any reason why the old grass would need to be removed.
I note you have already found info on building retaining walls but I thought I would still include @Adam_W's article on How to build a retaining wall in case you haven't seen it. Adam and @Yanick might also like to join the conversation and share their expertise on this relatively large landscaping project.
Please let me know if you need further information or had any questions.
@MitchellMc hit on a few points but I'll roll over the ones that come to mind.
Is your access for the new soil going to be from the front of the house or can you come through the back? The reason I ask this is that if through the back it may make sense to have the fill brought in and dumped in a big mound before building the walls downhill from it and then levelling it afterwards.
I'm assuming here you are building a timber retaining wall.
The normal process would be to build the retaining structure first. I assume from how you are describing it you will have a continuous wall & then the steps will be attached to the front of this? And have you calculated the maximum height of the wall? If it gets over 500mm you may need council approval but this depends on the area so worth checking your councils regulations. You'll probably find it on their web-page with a title like 'exempt and complying development'. That council code for things you don't need a building application for.
Once the wall is built you will need to install drainage behind it. (I've attached a picture below to show you how that should be structured.) That will have to drain water to either a storm-water drain or somewhere on you property, like a garden bed, where it won't impact on neighbours.
A wall like this will very much need to be well structured and number 1 is ensuring the upright posts are a) - deep enough. Basically the same amount below ground as above. And b) - on the front of the horizontal sleepers (see the drainage diagram.)
You shouldn't need a trench for the retaining wall if building with timber. Just post holes so that each section of horizontal sleepers is supported at 3 points.
You will need to decide how you will end it too. You can't just bulk the soil up against the fence, you'll need to taper the wall back into the slope. That's another area to check with council as many have limitation on how close a retaining wall can be to the boundary without permission.
I wouldn't be worried about removing the existing grass. It will all be killed off by the soil.
This is probably enough for you to chew over for now.
Thank you both for great feedback! Dear Mitchell, the roots of the large trees are exposed, and I thought it will be better for the large trees to do the levelling and the addition of the soil (max 400mm). Also excellent point on the fences: I am planning to install continuous and high garden beds on both sides of the fences before building the wall and doing the leveling. Does that sound like a good strategy? Would you recommend a particular material to build the garden beds to make them strong enough for the soil addition?
That certainly sounds like a plan to me. Personally I would construct these side garden beds as retaining walls. You can then box in the side closer to the fence and make your garden bed. You'll need to make them strong enough to retain your new fill so I advise using the standard retaining wall principals and timbers described in Adam_W's article How to build a retaining wall. Although we are describing these as garden beds their purpose is still to retain soil. It would be worth checking with the council to see if there are any restrictions on the proximity of retaining walls to boundaries in your area.
Perfect; thank you both, once again! I'll send you the pictures of the garden once it is finished