Compaction Methods on backfill that wasn’t compacted as it was done

[Mitchy5150]

Hi All!

I haven’t really posted much here, but here it goes!

I’ve started a 32x36 garage build, standard construction with attic trusses.

I’ve had the foundation and walls poured by a local company, but they said they don’t do the backfill, that is up to the flatwork company. Well ok, but my problem is that my friends and I are the flatwork company!! We’ve got a group that help each other out with pours, mainly driveways and garage floors. I have never done any of the prep work myself, just helped out.

I managed to get a concrete contractor that I know to do me a favor and do the backfill. I asked them to compact it, but I was not there when it was done, and I don’t think they compacted it at all, but i am not even sure how to tell. Most all of the fill is sand (I live on a lake that was a former sand pit in the 60’s, so most everything is sand with about 12”-18” of topsoil on it so that grass will grow)

The backfill was done in early January (it never really froze hard in Nebraska this year) so it has had some snow and rain on it which should have helped settle the ground, but it doesn’t look like it has settled much, if at all.

so my main questions:
1. How do I tell if it’s compacted or not?
2. If it’s not, how do I compact it now that there is 4 feet of fill already placed? (I have heard of somebody basically doing a water injection with a 1/2 copper pipe and a garden hose?)
3. Since it’s mostly sand, do I even have to worry about it?

Other info: stem wall will about 6” above finished floor, I plan to do in floor heat, so I plan on 2” foam board and a 5” thick floor.

Thanks all!!

Mitch

 

[larry4406]

I think you need to hire a geotech to assess your situation.

May need to switch to a structural slab with grade beam. This type of slab design transfers the slab load to the perimeter walls and footing; the dirt fill is only used as a one-time mold to pour the concrete.

 

[LXCam]

As Larry pointed out. Hire a geo testing agency to perform a compaction test. It’s not something you can really do and it won’t break the bank.

 

[Leaflessshadetree]

Compaction on backfill isn't as critical as fill under a slab. It will settle over time.

 

[kaymccampbell]

If you want an unofficial test, to get an idea what you're up against, before the professionals show up, then water it. Take a hose and leave it run in one small area for an hour or so. If you get a sinkhole, then you're in for fun.

I have seen where uncompacted sand and gravel fill can be settled by supersaturating it, and using a concrete ******** to settle it. Or, you can just water the **** out of it, and hope it settles in a few months.

 

[Chris705]

Maybe ask the friend to describe how they compacted the backfill? If he says they spread out the fill in 12” lifts and used a plate compactor or a jumping jack on each lift then you know that they did a decent job….

If the heel of your shoe sinks down 2-3 inches when you walk on it definitely not compacted. Driving a truck or tractor should leave slight to little tire impressions. I wonder if the ground was frozen prior to placing the backfill? The slow water method should not be used till after threat of freezing has past.

 

[41plym]

I rented a 1200# vibratory compacter to compact screenings and it worked awesome. I don't know how it would work on sand though.

 

[428PI]

I'm just refurbishing my shed (built in 1970 over a barn foundation built in 1913). I removed the concrete and had to do some foundation work so dug down below foundation (only a foot below slab base) with backhoe and when I put dirt back into it I compacted with a hand compactor in 1-2 inch backfills. When it was level with rest of dirt I used a HF ******** compactor after I watered down dirt. I than backfilled with 2 inches of limestone and compacted, watered it down, compacted again and went another 2-3 inches fill and repeated a bout 3 times. Hope it's enough as the concrete slab edges will be above old foundation (which really is above frost line and no footer). Tying it altogether so it acts like one floating slab. Now HF has 100 bucks off the compactor. GRRR!

 

[Walkers]

You can get 90% compaction on sand just by wetting it down.

 

[gizardlizard]

Agree with Walkers. If you have time, wet it good once a day everyday. Sand compacts really well.

 

[larry4406]

Take a piece of 1/2" rebar about 3-4' long. Bend about 6-8" 90 degrees at the end to make a handle, it will look like a cane.

Holding the handle on your probe, push vertically down and see how far you can push the end into the fill.

Not real scientific put if you can go more than about 1/2-3/4" your fill is loose.

 

[Sturgeon]

Isn't sand and gravel pretty close to pea gravel as far as compaction ratios? Compact it all day long, it's not going anywhere and compacting on sand the machine is going to disappear. Did one personal garage useing pit run and watered the heck out of it as I went. It was solid as a rock , turned out really well. Good luck.

 

[Mitchy5150]

Thanks for all the replies! I am going to run some water on it today, I’ll report back with results!

 

[bb29510]

sand, flood it with water

 

[PoorUB]

45 years ago my dad put in a fill for a garage slab. It didn't get compacted either. We lived by the river and he hooked up a pump and sprinkler and ran it full time for days. Let it dry out for a week and built on it. The floor had a few cracks, they always do, but the slab stayed level. I washed it down a few months back and it was still level.

 

[zkdiesel]

Take a piece of 1/2" rebar about 3-4' long. Bend about 6-8" 90 degrees at the end to make a handle, it will look like a cane.

Holding the handle on your probe, push vertically down and see how far you can push the end into the fill.

Not real scientific put if you can go more than about 1/2-3/4" your fill is loose.

I can shove a 1/2” 5’ tile probe to the handle in virgin ground time and time again looking for tile. Don’t think your test is any good

 

[larry4406]

I can shove a 1/2” 5’ tile probe to the handle in virgin ground time and time again looking for tile. Don’t think your test is any good

Interesting.

Most of the geotech's on the day job and the County inspectors use this method. Rarely does one see a nuclear density tester on the job; I only see this when there is work in a public right of way for road excavation work.

 

[velocipede]

Years ago I saw a crane repeatedly hoisting and then dropping a huge concrete block on unconsolidated fill at a construction site. The choice was drop the block until geotech guys were happy, or let the site sit dormant for ten years while water and gravity did the job.

 

[Chris705]

That’s called dynamic compaction.