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

[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.