Flawed concrete slab construction advice

[BiTurbo228]

Hi folks,

Wondering if anyone might be able to help with a bit of advice on my garage base. I've got a few unusual constraints on the design, primarily that I can't really dig into the ground (which is clay over tree roots...not ideal).

I have 10-11.5" of depth to play with, and was wondering how people might structure the base and substrate.

• Option 1: typical 4" of compacted ********, with a 6" reinforced slab
• Option 2: 2" of compacted ******** with an 8" reinforced slab
• Option 3: all-in with a 10" reinforced slab

I suppose the question is: 'If you only had a limited depth, would you prioritise getting a probably-not-deep-enough base of ******** down, or making sure the concrete slab is as strong as it can possibly be?

Pre-empting the response of 'just get an engineer involved'. That may well happen anyway, but I'm more interested in general principles.

 

[PCustoms]

Clay over tree roots?

Not the place for a slab. Excavate it out and fill/compact or pick another location

 

[Rusted Nut]

A slab on grade needs a good base, clay and tree roots don’t count. If you can’t excavate it out, maybe a structural slab.

 

[jkeyser14]

Any concrete you put over clay and tree roots is going to crack. It needs to be dug out.

 

[coldh2o]

A slab on grade needs a good base, clay and tree roots don’t count. If you can’t excavate it out, maybe a structural slab.

This. If you absolutely can't excavate to a sound subgrade, then you need a structural slab - consisting of some or all of adequate rebar, grade beams, post-tensioning, etc. Just going with thicker concrete will have minor benefit.

However, that's only going to give you a good slab that won't crack (much). You still have the issue of the unstable subgrade, and your whole structure is subject to movement and differential settling. Piers or piles may be an (expensive) option.

Definitely requires engineering - geotechnical and structural.

 

[Viper98912]

I guess I'm not understanding why you can't dig into the ground with clay and tree roots? Are you doing this by hand or with a small bobcat? If you are, then get a bigger bobcat and/or backhoe.

 

[billconner]

Even with a structured slab, won't the tree growing cause problems eventually?

 

[Rusted Nut]

Even with a structured slab, won't the tree growing cause problems eventually?

Good point!

 

[dcg9381]

Hi folks,

Wondering if anyone might be able to help with a bit of advice on my garage base. I've got a few unusual constraints on the design, primarily that I can't really dig into the ground (which is clay over tree roots...not ideal).

Here with an unstable base will drill concrete piers and create a "table" for the top of the concrete to sit on. My garage sits on piers that are about 10' deep. Not cheap or easy though.

I wouldn't place a slab over roots though, it's like building concrete on a ground-contact wood base. It's going to rot eventually. If they don't rot, roots are strong and can displace concrete over time.

 

[BiTurbo228]

Thanks folks, and yeah I appreciate it's far from an ideal set of circumstances.

The reason I can't dig is that we're in a 'Listed' property, which is a legal designation here in the UK for houses of historic interest (the house itself was built in the 1500s).

More accurately, I can dig as much as I like, but I need to get archaeology involved. I've got a few quotes in, and the costs are rather severe. Not astronomical, but well enough to put other forms of ensuring a structurally sound concrete base into strong contention. The costs rise with the amount of groundwork done, though not proportionally, so it's very expensive for what I'm doing at the householder level but probably acceptable for bigger developments.

I think I'll have the same issue with sticking 10ft piles into the ground as well. I don't think the archaeologists would be pleased if I stuck one through a 10C Anglo-Saxon burial...

I'm not overly concerned with settlement of the slab or structure off-level. I'm resigned to the fact that the tree roots and ground expansion will bully the slab around a bit, and am adjusting my timber superstructure to accommodate that (slip-joints where it abuts to adjacent structures, and extra racking support to deal with the studwork taking loads off vertical).

What I don't want to do is have a slab that cracks clean down the middle of it.

I've done some preliminary calcs treating the whole structure as a simply-supported 1-way slab (an extreme case, as a better way of describing it would be a rigidly supported 2-way semi-supported slab). That gives me a thickness of 7" (including the required reinforcement) so I'm at least halfway confident that if I go overboard and do an 8-10" slab it'll be up to the job (or 8" thick with 10" thickened edges maybe).

The question I was thinking through is 'considering these less than ideal circumstances, is it better to prioritise extra concrete thickness or extra sub-base'? This is something that the engineers I've approached have been reluctant to advice on, which is understandable seeing as I'm off the beaten track and they don't want to take on the liability. It may be that I haven't found the right engineer, of course.

I'm lucky in that the previous owner of the property put an unknown thickness of ******** down over the immediate area of tree roots (3m from the base of the tree) before it was listed, so I'm not working with complete bare dirt. I'm also building a structure that is below the required floor area for building regulations to apply, so I'm within my right to put a 1" thick layer of plaster down on the ground if I really wanted to, but I'd rather make something that stands the least chance of breaking up possible.

 

[PCustoms]

If archeology is required to dig, how is it OK to just pour a bunch of concrete on top of what maybe buried?

 

[andyvh1959]

My slab for my 2020 build of a 24x28 shop is sold, no cracks, no spalling, no sweating. The 4" slab was poured in mid-Oct 2019 (up here in Green Bay WI) during a wet fall season. Under the slab is 2" styrofoam with a 6mil barrier. Under the styrofoam is about 6" to 8" of compacted/leveled recycled asphalt grindings. Under the grindings the top soil was scraped back to the sandy soil level, compacted/leveled. Behind the shop about 15' is a large mature maple tree and a huge now dead mature ash. Neighbor has mature trees east of my shop maybe 15' to 20', and his yard is about 1.5' higher than my yard. Don't recall we hit or dug up any majpr tree roots when prepping the area with a skid-steer. I had removed three small trees, roots and all, which is now about where the NE corner of my shop is. No problems.

 

[BiTurbo228]

If archeology is required to dig, how is it OK to just pour a bunch of concrete on top of what maybe buried?

I think you're expecting a greater degree of logical consistency than housing law can provide

My slab for my 2020 build of a 24x28 shop is sold, no cracks, no spalling, no sweating. The 4" slab was poured in mid-Oct 2019 (up here in Green Bay WI) during a wet fall season. Under the slab is 2" styrofoam with a 6mil barrier. Under the styrofoam is about 6" to 8" of compacted/leveled recycled asphalt grindings. Under the grindings the top soil was scraped back to the sandy soil level, compacted/leveled. Behind the shop about 15' is a large mature maple tree and a huge now dead mature ash. Neighbor has mature trees east of my shop maybe 15' to 20', and his yard is about 1.5' higher than my yard. Don't recall we hit or dug up any majpr tree roots when prepping the area with a skid-steer. I had removed three small trees, roots and all, which is now about where the NE corner of my shop is. No problems.

Thanks. Useful stuff. Looks like the groundwork layers are within the bounds of what I can achieve, though my trees are a fair bit closer. Your sandy soil may be a little better at load bearing than what I have too, given what it looked like when the neighbour did a bit of digging*.

*speaking of logical consistency in housing law, the digging they did was 5ft from where I'm building my garages, but because it's on a different property that's not listed (though was likely owned by whoever owned our house at the time it was built), they do not need to contact archaeology at all...

 

[PCustoms]

I think you're expecting a greater degree of logical consistency than housing law can provide

My bad!

Lol

 

[ConCretin]

The question I was thinking through is 'considering these less than ideal circumstances, is it better to prioritise extra concrete thickness or extra sub-base'? This is something that the engineers I've approached have been reluctant to advice on, which is understandable seeing as I'm off the beaten track and they don't want to take on the liability. It may be that I haven't found the right engineer, of course.

The thicker the concrete, the stronger the slab. Since you are constructing a structural slab and not relying on the base for support, adding a few inches of base material will have less of an impact than adding a couple inches of concrete.

 

[LopezBart]

Perhaps a ribbed slab would make sense in this application, since it would reduce the thickness of concrete on average while still providing a strong stable base. You could have 8-10" or more at the rib sections, and reduce that down to 6" between the ribs. The ribs function as grade beams. With a concrete apron, of course, you can keep your excavation depth to an acceptable minimum while being able to roll into the garage.