How do you know when the ground is settled enough for concrete?

[tomplum]

We had a sloped area of the yard that was partially an old building torn out which had just a gravel pad. Last summer, we filled over this with close to 3' of mixed dirt that had a fair amount of clay and expanded that area. Built a solid 60# block retaining wall around that. The goal is to build a 120 sq.ft. building with a concrete slab.

 

[s219]

You would have had to put the dirt down in thin layers (aka, "lifts") and compact each one in order to have any hope at proper compaction. Did you do anything like that?

Around here the clay soil is useless for any sort of base since it won't compact well once disturbed. I have to bring in a "compactable fill" material that is a mix of sand, gravel, and clay. I spread that in 1-2" layers then drive over each one a lot. I doubt it's perfectly compacted but it's been good enough to support concrete slabs for vehicles without any cracking.

The key is to understand the load distribution on the concrete slab and then verify the soil can support it. You should be able to estimate the ground load (in PSI or PSF) needed to support the slab, then do a very simple test to see if the soil meets that. The test would involve placing a weight over a certain bearing area and see if the ground supports it without compacting or sinking in.

 

[cqaigy2]

Not an engineer, but in my distant memory, seems like a non compacted fill was considered "compacted" after two years for some purposes. The compaction requirements would be dependent on the amount of weight to be supported. I'd imagine your 120 s.q. ft. building would probably be fairly light load. A 4" concrete slab with reinforcement will hold a bit of weight, of course 6" will hold more. Then the use of the building, if it was to hold a water tank that large, or rakes and shovels.

 

[tomplum]

No, we didn't compact it in layers. Though I thought that far ahead, didn't have the time or energy for that. The main reason for the slab is to keep a clean area. We will put a couple freezers in the shed and a stainless counter for like a brew area. I have enough pavers to do the whole area if I wanted, but then I would have to be smart enough to anchor a building and keep it rodent proof.

 

[bigtiller]

I poured a 100 sq. ft. slab on top of some tall Kentucky Bluegrass 15 years ago and everything settled evenly with no problems.

The contractor ordered concrete for 3 small jobs and one of them canceled so he needed a place to dump 1.5 yards of concrete. I nailed four 2x4's into a frame and laid it by the fire pit then went to help pour the driveway extension project.

I wouldn't worry about your 10x12? slab, just pour it and let it cure for a couple of weeks

 

[Hay Dude]

We had a sloped area of the yard that was partially an old building torn out which had just a gravel pad. Last summer, we filled over this with close to 3' of mixed dirt that had a fair amount of clay and expanded that area. Built a solid 60# block retaining wall around that. The goal is to build a 120 sq.ft. building with a concrete slab.

You in a big hurry? You can try puddling. It doesn't work in every scenario. What you do is flood the fill dirt with water. A garden hose will do. Just keep moving it around and soak the fill and it will settle. This will eliminate air spaces and air pockets in the fill. You will probably end up adding more fill. You'll need time to allow it to dry out.

From a construction article:
" You're correct that the dirt in the center of the garage area is solid fill. Mother Nature, over time, does an amazing job of compacting soils. Rainwater plays an important part of this process. Adding water to fluffed or disturbed soil will go a long way to accelerate natural compaction of soil particles.
When I was still actively building, I would go to great lengths to water the fill soil in utility trenches and around the outside of a new home to get the soil to settle faster. Many builders don't take this extra time."
I used this technique with a large garage I built and never has a concrete problem

 

[Handloader109]

All depends.
Yeah. I build a shop that was 12x24 in 1987 on a slope that had fill on lower side. red sand fill, and really not compacted. 4 inch slab. over the years a lot of the fill was dug out by animals and water. Slab never cracked.
Fast forward 20 years. Built a 30x30 metal building. Only one corner had any fill. and was better compacted. about 6 months after fill and pour. again 4 ft slab, this time a slight 6 in footing and sidewalk down long side. Slab cracked at fill line and all along the sidewalk slab joint. Better prepared but didnt matter. These two were 20 feet apart..... a lot is the concrete strength also

 

[Handloader109]

Oh, and my daughter is building house on good slope, cinder block walls filled with rock and loose fill with 12 inch footing, 24 inches wide. Walls filled with concrete, 4 plus in slab. it ain't moving

 

[Diggin It]

The goal is to build a 120 sq.ft. building with a concrete slab.

The main reason for the slab is to keep a clean area. We will put a couple freezers in the shed and a stainless counter for like a brew area.

My opinion ... the size and expected load aren't significant enough to be concerned about. Embed wire mesh, and you'll most likely never notice an issue.

I guess you could get a load of crushed rock and rent a plate compactor for a weekend if you wanted to.

 

[tomplum]

Thanks guys. I had the thought of renting a jumping jack, but then there is vision of a sudden left turn and over the retaining wall...

 

[Hay Dude]

Thanks guys. I had the thought of renting a jumping jack, but then there is vision of a sudden left turn and over the retaining wall...

JJ won’t compact a thick layer of fill. Must be done in lifts.

 

[Firemanbuck]

Do you have a tile probe? If you can push the probe and it goes more than a couple of inches into the soil, it's not compacted. you want "firm and unyeilding" before you build. Some of the "clay" soils in the PNW are moderately expansive and can damage a poorly reinforced concrete slab.

Best advice has been stated already, remove, replace in thin lifts and compact before you pour.

Your block wall will also likely fail if you didn't account for the soil active pressure (Ka) particularly with a clay or silt soil with a low angle of internal friction. The soil will slowly but surely push the wall over.

/engineering geologist

 

[Renze]

Dont know what plate compactors you find on the average rental yard. But a Wacker DPU 6055 we used to rent at a contractor, has 60 inch (1 meter) compacting depth. If the soil is any good at all, the Wacker will solidify it. If not, it will sink and its better to dig out and replace.

This is standard prevention procedure when making a paved road or concrete slab, even on virgin soil, where i live. Any contractor who doesnt, is considered a hack

http://www.bequip.ca/images/BrochureDownloads/Wacker_DPU_6055.pdf

 

[Diggin It]

C'mon people, he's not doing a giant barn to house a fleet of heavy vehicles. He's doing a tiny shed to hold a couple of freezers and a table.


A couple of inches of crushed rock and a few mintues with one of these will be more than enough.

 

[Hay Dude]

Not if theres feet of lose fill under it (I don’t know if there is, but I’m just saying IF)
The compacted 4” on top will settle when the soft dirt under it settles.

 

[Egon]

We should get serious: Concrete piles set below the frost line supporting a grade beam.

 

[s219]

C'mon people, he's not doing a giant barn to house a fleet of heavy vehicles. He's doing a tiny shed to hold a couple of freezers and a table.

It's not the size of the building or the weight of the contents, it's the distributed pressure on the soil that matters. The bigger slab could easily be better! All very easy to test/calculate if there is any doubt.

 

[Hay Dude]

I think the bigger question that seemingly always goes unanswered in these concrete prep topics is:
Why on earth would anyone pour concrete on an "iffy" ground prep?
We dont paint over unsanded wood or drywall. We dont mow over rocks.
Why would anyone spend thousands on concrete, which by the way, is very difficult to rip out and start over on a poorly compacted or prepared surface?

 

[Steppenwolfe]

C'mon people, he's not doing a giant barn to house a fleet of heavy vehicles. He's doing a tiny shed to hold a couple of freezers and a table.


A couple of inches of crushed rock and a few mintues with one of these will be more than enough.

Some like to rain on a parade? Maybe???

 

[DonaldP]

We had a sloped area of the yard that was partially an old building torn out which had just a gravel pad. Last summer, we filled over this with close to 3' of mixed dirt that had a fair amount of clay and expanded that area. Built a solid 60# block retaining wall around that. The goal is to build a 120 sq.ft. building with a concrete slab.

I was working with an engineering firm for a commercial project that included an 80 x 150 ft slab for a restaurant.

After soil testing they found it to be not capable in its current form to support a floating concrete pad/building of that weight load.

What they ended up approving was a 6’ layer of runner crush. Basically gravel that would sit on the proposed site for 9 months and preload the site with the weight for the anticipated load. The project was a year out so that plan was as good as any.

It worked. They said that the base level compacted about 1 to 2 feet in that time frame and provided enough back pressure to support the foundation.
There have not been any noticeable cracks in the foundation and that was about 15 years ago.

In the end the preload material was used to bring the parking lot to grade and provide slope for water runoff. I think we used about 70% of the initial preload material.