Retaining wall foundations based on total height or height of retained amount?

[snsmurf]

Apologies for the long title but wanted to highlight the difference from a standard retaining wall foundation question. I've gone through nearly 10 pages of posts to try and find an answer but i've been unable to find an answer.

We have a two tiered back garden and are looking at replacing the retaining wall as it's crumbling in parts. We also have toddler in the house so want to make this tier a bit safer for her to be playing in come summer. The current wall is only to the level of top part of the garden and is around 550mm in height. In order to make it safer though we're planning on making the new wall around 1200mm in height from the bottom part of the garden so the top part of the garden will now have a 650mm high wall.

So my question is, what size foundations should I use? I've read the recommendations for foundations of retaining walls of various sizes but while it's going to be a 1200mm high wall, it's only retaining for the first 550mm. The rest is simply a acting as a standard garden wall as it's above the retained part. So to use foundation recommendations for a "1200mm retaining wall" seems overkill but it does still need to withstand someone leaning on it or a toddler being crazy.

For extra context, I was planning on using concrete blocks and facing bricks with ties on the lower part of the wall to create a double skinned wall that will look nice from the lower part of the garden and then switch to double skin of facing bricks when it gets above the retained part so it then looks nice from the house and the top part. The wall will be stretching around 8 metres in length across the garden and there will be some pillars added where the steps will be placed.

Thanks!

 

[23vc]

There’s no set formula, it’s more to do with the ground conditions and digging down as far as you need to in order to get firm ground. Very roughly, I’d say unless its unusual ground, the most you’d need to dig down to is 1m below the lower ground level at a given point, and make the trench width at least 200mm wider than the wall thickness. But realistically highly likely to not need to go as far down as 1m.

 

[Ivor Windybottom]

If you underdo it you'll regret it more than if you overdo it!

I built a retaining wall. A builder visiting next door described my mega construction involving two leaves of brick with a cavity of reinforced concrete as "Victorian". I took that as the compliment it wasn't meant to be.

 

[Dereekoo]

If the original wall is not toppling over then the existing foundations are doing there job and adding extra height but no extra lateral force will not affect the stability of the foundation. Perversely adding height/weight to the wall makes it even less likely of overturning as the the extra weight adds more to the overturning restoring force. When you say the wall is crumbling is it just a matter of mortar in the brickwork deteriorating or is the wall failing structurally. Here is a little light reading which may (or may not) help you understand the ins and outs of retaining walls

 

[snsmurf]

Thanks for the replies all and for the PDF. Will give that a read later.

Excuse the rubbish pictures, I only had a video of it and I can’t upload videos it appears.
The wall looks like a dry stone wall type build. I’ve never noticed any mortar. And the crumbling part is where all the stones have fully fallen out and soil and weeds have taken over.

We are also planning on taking it back to actually make more room at the bottom of the garden for a shed and garden office so the existing wall foundations will be out of position sadly.

 

[Ivor Windybottom]

You've got some lovely stone there, make use of it, either in your new wall or elsewhere.

Building with stone is a definite skill that needs experience - either drystone or mortared.

Give it a powerwashing before pulling down, as the green will only be on the faces that are currently visible, where the daylight is. What's inside should be reasonably clean, it will still need a wash but not as much of a blasting.

 

[snsmurf]

You've got some lovely stone there, make use of it, either in your new wall or elsewhere.

Building with stone is a definite skill that needs experience - either drystone or mortared.

Give it a powerwashing before pulling down, as the green will only be on the faces that are currently visible, where the daylight is. What's inside should be reasonably clean, it will still need a wash but not as much of a blasting.

Thanks for the tip!

It's not to the our taste though, It was there from the previous owners. So not sure if we'll have a use for it.
Is it worth sticking up somewhere to see if someone wants it? Are there any places to advertise it or is just facebook marketplace the best sort of place for it? Never had to sell stone before.

 

[Dereekoo]

In a few years time your toddler will be 5 and if anything like my grandkids will happily jump off my 600 high patio, yours will have more fun because they will have to climb up the wall and jump off 1000 high. Personally I would keep the rustic charm of the stone wall, re build in new position with slight backward batter, no foundations as such needed (lot easier/cheaper than laying brick) and if you want to protect the toddler for a few years put some temporary 2ft fence panels in at the top of the batter which you can take down as and when and have a open looking garden as opposed to a brickwall to look at. You could make the wall into a flowerbed area I attach a photo of my wall/bed area, I mortared the small retaining bit at the front, basically laid it at ground level bedded on mortar and battered the soil back with some of the stones placed to give it a bit of kerb appeal but I guess it depends on how good a bricklayer you are and what your attitude to gardening is, good luck

 

[Ivor Windybottom]

It's worth money if you can find someone who wants it.

But wash it then decide whether you want to develop some taste!

 

[Dereekoo]

Thanks for the tip!

It's not to the our taste though, It was there from the previous owners. So not sure if we'll have a use for it.
Is it worth sticking up somewhere to see if someone wants it? Are there any places to advertise it or is just facebook marketplace the best sort of place for it? Never had to sell stone before.

Well you could try selling it for a nominal amount but if you put it up for free I am sure someone will grab it and it will save you the hassle of disposing of it Nextdoor is a good place to start if you are on that forum as it tends to be more locally based/attractive but anywhere will do and see how it goes

 

[cdbe]

Clean it up and use for the back of your new wall and face it with something to your taste. You can literally throw it in with shovelfuls of sloppy concrete. If the ground behind slopes up you can level it a bit with the spoil.