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

[mgstevens]

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...

While a "Jumping Jack" might take you over an edge, a plate compactor will not. The plate compactor is the right tool for an under-slab compacting job. Use it with water, and you'll get acceptable results. I built up slope for a carport slab (7' in places..) and we dry compacted every 6-8". Three feet is a LOT, but with water to consolidate the lower material, and the compactor packing the first 6-12", you'll get the best results, short of having done it right when the fill was placed. As for people saying "go for it without.." I would proceed that way only after considering how long you want the structure to be reliable.

 

[Username Taken]

How about spreading a load of sand over it? Use a tractor and some come-alongs

Sand won't compact.

We scrape out a building site and pour it a few days later. But, that's over sand. If there's clay in it or if we have to build it up to Flood Elevation, we use a compacter.

But, every place is different. Different soils, different climate. I'm not there but if I were, I'd compact it down a bit, dump some sand on it, form it up, let it sit for a day or three (hoping for rain) and call the Ready Mix company.

But I'm not there. Whoever is there would have a better idea of what he's looking at.

 

[Diggin It]

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

Why would anyone spend thousands on concrete,

120 sq ft, figuring a 10' x 12' slab, 5" thick, concrete calculator website says 83.13 80# bags. They run under $5/bag and you can get them for less in bulk.

We're talking $500 or less here.

Concrete Calculator

This free concrete calculator estimates the amount of concrete necessary for a project and can account for different shapes and quantities.

www.calculator.net

 

[Hay Dude]

120 sq ft, figuring a 10' x 12' slab, 5" thick, concrete calculator website says 83.13 80# bags. They run under $5/bag and you can get them for less in bulk.

We're talking $500 or less here.

Concrete Calculator

This free concrete calculator estimates the amount of concrete necessary for a project and can account for different shapes and quantities.

www.calculator.net

Don’t know many people that are into mixing 85-80lb bags of concrete mix. I mean that’s like 2 pallets, but the bigger point is who would want to pour it over garbage and then have to rip it out and start over?
Ground prep and reinforcement is too important

 

[Garson]

I worked in the construction business doing building foundations. Your situation is worst case with original ground on one side and 3' of loose fill on the other. Yes, your concrete slab will crack if poured on this material, but worse still, it will easily settle 6" on the loose fill side. No matter how large your compactor is you cannot compact deeper than about 8-10".

You can verify the loose fill has not settled sufficiently to pour concrete on. Take a piece of 1/2" rebar 3 feet long and drive it down on both the solid ground side and the uncompacted side. If there is considerable difference in compaction you will feel this instantly. When your rebar is 2' down on both sides a similar tap with a sledge hammer will likely drive it 1/4" on the compacted side, but 1/2" on the loose fill side. Then you have a problem that can only be fixed by removing the material and compacting it in 4-6" lifts depending on your compactor strength.

The other option is to put a lawn sprinkler on the ground and keep it wet for a full year. Along with heavy rains this will make it much better and possibly worth the risk. Heavy rains are a great compactor over time.

We worked with one garage where the situation was identical to yours. A few years later the 24' x 24' garage sunk a foot on the loose fill side and the concrete was not pretty. If this happens, your work and expenditure has just begun. Good luck.

 

[crashz]

Compactors will easily handle a 1ft loose lift thickness as a standard lift. Their effectiveness diminishes the deeper the soil lift, but it still has some effect. Therefore some compaction will occur in a cone like shape beneath the machine even at 3ft deep. Will you achieve 95% of the maximum density? No. But you are miles ahead by compacting now versus not compacting at all.

If I were you, I'd rent a compactor and thoroughly compact that area as well as a significant margin (say three feet on each side extra) a minimum of four times. Four passes. Three will typically render the soil fully compacted +/- 95%. The fourth pass is to compact deeper. Moisture is a concern: too wet and you make mud, to dry and the soil won't compact properly. My rule of thumb is anytime you see dust its too dry. Good luck!

 

[fishdrivel]

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.

If you are worried, I'd get somebody with a nuclear density gauge to check or,
pre load the area with a large pile of dirt or,
Use a jumping-jack tamper to compact or,
fill a dumptruck and drive back and forth on it. Don't get too close to the wall!!!!!

 

[Username Taken]

120 sq ft, figuring a 10' x 12' slab, 5" thick, concrete calculator website says 83.13 80# bags. They run under $5/bag and you can get them for less in bulk.

We're talking $500 or less here.

Concrete Calculator

This free concrete calculator estimates the amount of concrete necessary for a project and can account for different shapes and quantities.

www.calculator.net

Unless it's code, why bother with a 5" inch slab.

A 4" slab is roughly 80 sq feet per yard. A yard of concrete from a Ready Mix shouldn't be much more than $100. Figure 1-1/2 yards unless you're gonna have to have a footer. Is it CBS construction? If not, I don't think I, personally would bother with a footer. Even if it is Concrete Block, I would probably pass on a footer because.... I'm lazy and footers are over-rated in my neck of the woods.

I'd order the concrete with the fiber, too. I know wire is more gooder but, c'mon guys...... It's a shed.

So, for the slab, I'm thinking $200 including beer. If one of my friends is helping, maybe an extra $50 for beer. Rebar shouldn't be that much. Again, if it was me, I'd just dig down a couple inches around the perimeter and use that as a pretend-footer, into which I would put my rebar (Make sure it's off the ground a couple inches, they got 'chairs' for that) And tie it. Yes, just tie it. This is all depending on codes and code enforcement in your area. Some people don't bother pulling a permit. You can usually get away with it unless some wokey makes a call to the County

Remember to set any anchor-bolts (eye hooks, whatever) you might need. If any

 

[Hay Dude]

puddling is the answer

 

[RancherGuy]

No, we didn't compact it in layers. .... I have enough pavers to do the whole area if I wanted, but ...

I'm not convinced that I fully comprehend what you are planning. Pavers? Is that for the building foundation?

What is most important is that you have uneven ground conditions from one side to the other. Having filled it loosely less than a year ago is not enough time for natural compaction. And don't believe anyone that a vibrating plate will compact at depth and do it evenly with the uneven situation, but it will help. You also did not state what was under the added fill. If it was more vegetation than grass, then that too means settlement over time. One more variable is that retaining wall. If it was not built well with proper back drainage and good lean, in just a few years you will have more expense, perhaps after causing damage.

Everything that needs to be said has been said by someone. If you want to go quick and cheap, then build now and worry about the impact later. But if you want something to be durable over time, and don't have time to wait a year for thorough watering, then compacting in lifts and be sure that wall is well built is the way to go.