Browse Forums Landscape & Garden Design 1 May 07, 2023 8:50 pm Hi all, I'm building a retaining set of stepped retaining walls on the front of my slopping property. My question: is it reasonable to cut away 400mm of earth directly infront of my patio slab and replace that earth with 400x400x1000 sandstone blocks? I have attached some basic diagrams to show the idea. Ignore the 1.5m high course of blocks at the bottom. This will be 1m to suit council regs. It's worth mentioning that the land here is hard packed sandy soil that is full of rock. Can't dig a hole anywhere in the yard without hitting 300mm+ boulders.. Thanks legends! Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅ Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅ Re: Retaining wall under patio slab 3May 10, 2023 4:38 pm ![]() it short, yes, just make sure the slab is still well supported, it might crack if not we’ll built and the work is sloppy. Awesome thank you. Also in pursuit of learning and my own curiosity, what's the long version of your 'in short' comment. I'd love to hear the theory behind your approval of my design 🙂. Structural engineering always interested me, and I nearly did it at uni, but went with electrical instead. Is it a go ahead because the sandstone technically leaves the 45 degree rule of thumb in place when looking at the force of the patio slab bearing it's weight on the earth below it? Re: Retaining wall under patio slab 4May 10, 2023 9:54 pm I understand your drawings are not structural drawings and only indicative of basic concept but it needs some refining. It's good practice for the top of any retaining wall to sit higher than what it's retaining so there is no possibility of earth escaping away and undermining the retained structure. Your top block should sit higher than the bottom of your patio slab. Then each of the retaining walls below should sit higher than the bottom of retaining wall above. You don't want any of your structures (slab or retaining walls) being undermined so it's critical that you get that right. In your example I'd be using a 500 high block at the top. Your sandstone block retaing is normally as many blocks deep at the bottom as it is high. So if it's 2 blocks high it will be 2 blocks wide at the bottom. If it's 3 blocks high it should be 3 blocks wide at the bottom. Accessible Carpentry & Cabinets accessiblecarpentry@gmail.com accessiblecarpentry.com.au https://www.facebook.com/pages/Accessible-Carpentry-Cabinets/583314911709039 Re: Retaining wall under patio slab 5May 11, 2023 8:19 am ![]() I understand your drawings are not structural drawings and only indicative of basic concept but it needs some refining. It's good practice for the top of any retaining wall to sit higher than what it's retaining so there is no possibility of earth escaping away and undermining the retained structure. Your top block should sit higher than the bottom of your patio slab. Then each of the retaining walls below should sit higher than the bottom of retaining wall above. You don't want any of your structures (slab or retaining walls) being undermined so it's critical that you get that right. In your example I'd be using a 500 high block at the top. Your sandstone block retaing is normally as many blocks deep at the bottom as it is high. So if it's 2 blocks high it will be 2 blocks wide at the bottom. If it's 3 blocks high it should be 3 blocks wide at the bottom. Is there a reference for this or a building/engineering guide that I can read into more about this? Reason I ask is that all the sandstone block retaining walls I've seen built have never been more than 1 course wide. The higher the wall the larger the batter angle I've noticed...which may be the reason why only 1 block wide is allowable? Re: Retaining wall under patio slab 6May 11, 2023 10:18 am ![]() ![]() I understand your drawings are not structural drawings and only indicative of basic concept but it needs some refining. It's good practice for the top of any retaining wall to sit higher than what it's retaining so there is no possibility of earth escaping away and undermining the retained structure. Your top block should sit higher than the bottom of your patio slab. Then each of the retaining walls below should sit higher than the bottom of retaining wall above. You don't want any of your structures (slab or retaining walls) being undermined so it's critical that you get that right. In your example I'd be using a 500 high block at the top. Your sandstone block retaing is normally as many blocks deep at the bottom as it is high. So if it's 2 blocks high it will be 2 blocks wide at the bottom. If it's 3 blocks high it should be 3 blocks wide at the bottom. Is there a reference for this or a building/engineering guide that I can read into more about this? Reason I ask is that all the sandstone block retaining walls I've seen built have never been more than 1 course wide. The higher the wall the larger the batter angle I've noticed...which may be the reason why only 1 block wide is allowable? What you are building is called a gravity retaining wall. Gabion walls also use the same basic principles. If you think of a dam wall it's thicker at the base as that is where the greatest pressure is. The extra thickening of the blocks is at the back of the wall (under the back fill) so what you end up seeing is just a flat wall. The blocks you are talking about are very heavy and require lifting with a bobcat so are not diy. The contractor you get to do the job should be well versed in what is required to build a compliant wall This is a basic drawing of how a wall should look. Starting from the top you just come down however many blocks the height of the wall will be and that gives you an idea of how thick it should be. Notice how there is one block founded below the ground level to prevent movement. Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅ This is a picture of the back of a walk before backfill. Notice how it's thicker at the bottom. Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅ Accessible Carpentry & Cabinets accessiblecarpentry@gmail.com accessiblecarpentry.com.au https://www.facebook.com/pages/Accessible-Carpentry-Cabinets/583314911709039 Yeah my reasoning is just that those house posts are designed for vertical load not lateral. I just didnt want to risk interfering with the existing house… 4 12471 Thank you again Simeon.. I will call my certifier for that. Have a good day 4 8965 To me, that's what it looks like: poor backfilling and/or compaction. The width is about 1000mm maybe. That conduit in the photo is for the NBN. The water is about… 4 11786 |