How to underpin front of concrete slab, that's poorly built on a slope and now overhanging it's hardcore sub-base?

[zhelevn]

Hello,

I moved into a house that has an end of garden workshop with a foundation that looked dodgy and had various tradesmen comment on the need to fix it before moving any workshop tools into the workshop.

It's on a slight slope and the shuttering for the concrete slab has not held up during the pouring. It's on hardcore, but it hasn't been compacted well. It's less than 10 cm thick, maybe around 7 cm mostly. Due to the slope, the front has hardcore gravel crumbling away from the concrete base, leaving the edge overhanging and unsupported. Ideas from any brickies/builders here on how to fix it?

I'm told by a knoledgable builder, it should have been retained by brickwork to begin with, not timber shuttering and to try to dig for fondations at the front (where it's sloped and base has overhang proud of ground level), build brickwork on top of foundations and to the side of the concrete slab (until it levels off with the ground) and pour concrete between the new brickwork and old concrete base? I may have not understood everything we discussed, but just wanted to clarify what my options are?

I will upload a photo tonight. Googling only reveals people underpinning huge full size log cabin buildings in America. This is a relativly small job, yet vital to start using the space as intended.

Thanks!

 

[noseall]

just wanted to clarify what my options are?

Build a wall.

 

[zhelevn]

Build a wall.

More or less as described?
Photos below

 

[Ivor Windybottom]

It looks like it's built on top of topsoil, so probably isn't going to last long whatever you do with the edges. It would be worth having a dig alongside it to see exactly what it consists of.

I'd either leave as-is and hope for the best, or take the shed down and make a new base from scratch. You could hire a crusher and crush it all to make a new compacted base deeper down.

Removing the shuttering may disturb it, it looks like the decking may be holding back loose infill.

Honestly I'd probably just do nothing and use it. If it cracks then worry about it another day.

 

[zhelevn]

It looks like it's built on top of topsoil, so probably isn't going to last long whatever you do with the edges. It would be worth having a dig alongside it to see exactly what it consists of.

I'd either leave as-is and hope for the best, or take the shed down and make a new base from scratch. You could hire a crusher and crush it all to make a new compacted base deeper down.

Removing the shuttering may disturb it, it looks like the decking may be holding back loose infill.

Honestly I'd probably just do nothing and use it. If it cracks then worry about it another day.

Thanks, I feel the urge to try to at least support the front overhang, as we want to build a decking up to it, so the new footing could be dual purpose for the decking too.

 

[cdbe]

If you're not up to building a wall, get some used concrete paving slabs off Freecycle, excavate all the loose material from under the slab and set them vertically about 50mm away. You'll need.to make a reasonable attempt to place them on solid ground (dig down a bit), set them on some semi-dry concrete and then back them up with wet concrete. Once set fill the void with runny concrete.

I think you'd need a jumbo bag of ballast and half a dozen bags of cement and a cement mixer.

You can bolt a ledger plate to the slabs to carry the decking.

I do like to recycle ex council slabs, I reckon I've got about a hundred in various places round my garden.

 

[zhelevn]

If you're not up to building a wall, get some used concrete paving slabs off Freecycle, excavate all the loose material from under the slab and set them vertically about 50mm away. You'll need.to make a reasonable attempt to place them on solid ground (dig down a bit), set them on some semi-dry concrete and then back them up with wet concrete. Once set fill the void with runny concrete.

I think you'd need a jumbo bag of ballast and half a dozen bags of cement and a cement mixer.

You can bolt a ledger plate to the slabs to carry the decking.

I do like to recycle ex council slabs, I reckon I've got about a hundred in various places round my garden.

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That's indeed a smart use of those slabs. So at 50mm away from the concrete slab, that's enough room to shovel in wet concrete mix between to prevent the rest of the hardcore to crumble away, and hopefully give some support to the edges? I think those slabs are around the right size to do that, if they are typically 450x450mm, I can dig a footing wall infront (and a halfway up the sides) lay a few scrap bricks down level, then vertically position the paving slabs and add postmix or concrete mix so they are maybe half way sunken/supported. Wait to cure. Then fill inbetween. Would you have the paving slabs be same height as top of the current concrete base (not as sunked into the ground) or same height as the bottom of the concrete slab (then once the filled concrete is dry it will be like a lip edge). Thanks.

 

[Ivor Windybottom]

The problem is that there's loose infill under it that's held back by the wood. As soon as you remove the wood it will fall out, and the concrete will be in the way of stuffing it full once you've built a wall in front of it. So you could weaken it by replacing it with something stronger.

The decking planks have probably got 20 years in them until they completely dissolve. By which time the base will probably have cracked in multiple places whatever you do (or don't) to the edges, due to the fact that it looks like it's pretty thin and on top of topsoil.

 

[Ivor Windybottom]

I too have done retaining borders with ex-council slabs. They look great, especially if they're well worn so the pebbles are exposed.

 

[Ivor Windybottom]

Actually if you are doing just the front then it probably isn't a big deal. Slabs on edge butted up to it would work well. Just ensure they're not higher than the concrete so you're not trapping rainwater that would end up under the doors.

 

[zhelevn]

Actually if you are doing just the front then it probably isn't a big deal. Slabs on edge butted up to it would work well. Just ensure they're not higher than the concrete so you're not trapping rainwater that would end up under the doors.

Thanks. Seems reasonable. I'll probably need to adjust my plan all depending on what happens when I remove some of the shuttering to allow for all this to be done.

As an alternative, what do you think about EasyPAD adjutsable footings? https://shedbasekits.com/product/design-your-own-foundations-with-easypad-1-5-tonne/
Do you know of other similar alternatives? The base is a little off level, I could jack it up with concrete blocks under the jack and then build blocks under, but these footing pads seem easier. Wondering if I can use 100 x 100 mm concrete posts fixed horizontally with these?

 

[Ivor Windybottom]

They have at least one big fan here, personally I don't like the concept. You're putting all the weight into a few small spots, so making movement more likely. Usually foundations spread the load over a larger area.

Either accept it as-is or rebuild properly. Re-use the shed, just unbuild it, put a better base in then rebuild. It looks like it's pretty much just stacked together anyway, so shouldn't be a big deal. Possibly more work in trying to mess about jacking it up etc.

 

[zhelevn]

They have at least one big fan here, personally I don't like the concept. You're putting all the weight into a few small spots, so making movement more likely. Usually foundations spread the load over a larger area.

Either accept it as-is or rebuild properly. Re-use the shed, just unbuild it, put a better base in then rebuild. It looks like it's pretty much just stacked together anyway, so shouldn't be a big deal. Possibly more work in trying to mess about jacking it up etc.

I guess it's not so off level to effect anything I will do in it, may be able to adjust for it with flooring.

I'm told it's all held together with 4 bolts and these log cabins are sort of T&G lego. Trouble is it's a 27 m^2 workshop. If a concrete base is £150 a square meter, that's £4000. Probably cheaper with individual footings/piers/columns then timber joist floor, but a lot of effort for something that isn't broken yet. I'm hoping if I stop it getting worst, it may last as long enough.

 

[Ivor Windybottom]

I got most of a concrete mixer truck pumped by a separate pump truck up a long drive for £1300. Don't know where you got £4000 from.

If it has moved then it will probably carry on moving. Then cracking.

Either accept it as-is or redo it properly, any middle option is probably a waste of time and money.

 

[aveatry]

Probably not the answer here but I have seen videos from USA where a concrete path to front doors have dropped and they drill holes and pump in expanding foam and it raises the whole slab.