Concrete Settling: How to avoid

[ron in sc]

The part of my property where the garage will be built slopes significantly to the rear. It will be backfilled and compacted. I'm wondering what other precautions can be taken to prevent settling of slab. The area in front of the garage which is concrete is cracked badly, it's 28 years old, and will be redone too.

For example would wire mesh or rebar help. I don't think they normally do that where I live, but I'm not sure. Is there a type of backfill material which is good, rock, clay, sand?

 

[Dave Carney]

Around here we use limestone screenings, it's essentially a granulated/dust form of limestone. Once it's packed with one of those vibrating compaction tools, it's hard as hell and won't further compact.

 

[PAToyota]

As an architect, I would always specify reinforcing in concrete. Concrete just doesn't resist tension... Considering the cost of the concrete and the cost of labor to place it, the little bit extra that you pay for the reinforcing is incredibly cheap insurance. Go for at least 6ga. wire mesh in "sheets" - the roll stuff is worthless.

 

[tintoptexan]

Keep in mind that reinforcement does not keep the concrete from cracking. It keeps it from shearing and seperating. Nothing can replace proper compaction in the proper lift thickness and the soil at its optimum moisture level. What type of siol are you dealing with? If it is high in clay content, then you might look into lime or cement stabilization to offset the swelling and shrinking. Anyway, concrete is one of the few things where more is usualy better. Good luck, Joe

 

[1320stang]

Install piers.

 

[kbs2244]

No argument on the need for mesh. The more the better.
I think you have two choices on the floor.
If you go with just a "floating slab" of 4 to 6 inchs thickness, you should go at least 4 to 6 feet downhill of your walls and build a retaing wall of some kind. The material and style is up to you, but the function is to get a level platform to put the slab on. Dampen and compact as you fill in. The longer you let it settel before you pour the better.
Or you can go with a permiteter foundation, like used on a home. Here the wall sets on footings and goes up as high as needed. You fill in the space that will be under the slab, as above, and pour the slab inside the walls. This is how it is done on an attached garage since the garage is considered part of the whole structure and has to resist settling at a different rate than the house. You will not use as much fill, but it is a more complex job, needing wall forms, etc and it will use more concrete.
For the fill, sand is a no no. Too fluid. In your area crushed shells are great. They pack down like the limestone mentioned above. I have seen shell drivways that are as nice as paved.
In either case, think about having a drain pipe installed dureing the filling. You have a perfect case for a "drain to air", where the pipe just exits at the lower ground level.

 

[brownbagg]

there only one way to prevent settling. compact the backfill.

 

[Krodad]

Use as much compacted fill as you can...here in the cold climates with freeze/thaw, I do at least 10" of granular fill, mostly to help control heaving. It's a royal pain in the **** due to all the fill I have to remove, but it pays off in the end. Ditto on staying away from sand. It's cheap and easy and done for that reason. It's not the best way to go. In your case, probably 4" would do, but you will not be doing the best job you can if you don't rent a compactor. Compact the dirt before the fill, then compact the fill. when you think you've done enough, go over it again.

For smaller projects, I like to use galvanized cattle panels as wire reinforcement. You will also want to use at least some #4 reinforcement if your slab is of any size...certainly stub 24" bars across the saw joint locations. Wet cure if you can and you'll help minimize shrinkage cracks. Make sure you chair up the steel...it does you no good if it's laying on the fill or way at the top. Minimum of 1 1/2" concrete coverage around the rebar. Concrete is not cheap any more, but go for 6" if you can. You'll just feel better about it 5 years from now.

 

[wilbilt]

What type of siol are you dealing with? If it is high in clay content, then you might look into lime or cement stabilization to offset the swelling and shrinking.

We have a lot of clay around here. The stuff definitely swells and moves when it gets wet. The school district I work for built a new school last spring/summer. Due to the clay content, the building locations actually had to be excavated, the clay removed, and "normal" compactable soil brought in.

I was involved in the project (planning network, video, security cameras, cabling, etc.), and had a chance to read through the soil engineer's report. It was one of the most interesting things I've read in a while.

It just so happens that Google Earth has images of the site while in the preliminary dirtwork phase. It is interesting to note the contrast in the soil color after the clay was replaced.

39°28'23.10"N 121°32'2.32"W

 

[Hurricane]

ive seen some real bad advice here already. you almost NEVER need to add moisture to dirt for good compaction. 99% of the time, you want to get the moisture OUT of the dirt for good compaction. if your talking about minus rock, then moisture is good to a point. im in the excavating business, so this is one thing i know for sure. another big misconception is "sand is not a good fill", completely wrong. sand is 100% compaction the second it hits the ground, BUT it must be contained. like mentioned it is very fluid, so if its not contained it will move. clean rock is about 99% compaction as soon as it hits the ground, though soil engineers around here have started requiring it be dumped from a height of 5' when used to fill holes in the last few years.

make sure that the site is thoroughly stripped of the top 4"-6" and separated from the rest of the material. its full of topsoil, grass, roots etc that will decay over time causing the material to settle

theres a real easy way to be sure youre getting compaction, and thats not by listening to random advice online, but contact a soil engineering company and they can come out to your site, and check compaction for every lift of fill you put in. whether rock or dirt, they can test it. they have what we call a "nuke", portable x-ray so to speak, they drive a pin in the ground and slip this in the hole and it measures soil density. they are on all major excavating sites here. the good thing about it is that if you do get settling, they are responsible. engineered fill has a higher rate of compaction than natural ground


1 other option that we use for a multitude of reasons (high plastic soil, high moisture, etc) is soil stabilization. lime is spread across a lift and then mixed in with a large tiller or can be mixed in a pile and then spread. if you treat soil with lime, it gets so hard that you cant drive a stake in the ground. it gets hot when in contact with moisture in the ground and "cooks" the soil. in some subdivisions weve done, they forego the base rock underneath the street as long as the top 1' of dirt has been treated this way.

youll find that if you have the equipment available, its much cheaper to shape the land and use dirt to get the slab on grade than it is to pour a retaining wall and backfill with rock/sand. if you chose that route, than drain tile with weep holes at the bottom is recommended, especially if you use granular fill behind the wall

i could go on for a while with this stuff, if you have any specific questions let me know
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[ron in sc]

contact a soil engineering company

That's exactly what I'm going to do. I've contacted a couple this morning. They indicated they would do a soil bearing verification test. From that they can then say how the site will need to be prepared to prevent settling. With the cost of concrete and the garage I can't imagine not having this done.

Does that sound right?

Thank, Ron

 

[Hurricane]

just make sure they have somebody present when your doing the dirt work that will be able to test compaction. they have to sign off on it in order for them to take responsibility
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[KELLHAMMER]

I built a new house which was slab on grade . But it was anywhere from 2 to 5 feet above grade. So we basically, we built a perimeter foundation and filled the interior with 3/4" crushed stone. We ran a plate compacter across it as we filled every couple of feet. The stone is generally "self compacting" but as a extra measure we used the compactor. And it got some compaction from the tractor that was placing the stone. The 4" slab got a 6x6 steel mesh and fibers. We had an engineer and a architect on the project. Which I highly recommend if the project is of considerable cost.

 

[ron in sc]

The engineering firm that I'm going to have do the test said the procedure is soil bearing verification. I'm also going to go ahead and have them test another area where we are having an additon built on to the existing house. That will be built on crawl space but I figured it couldn't hurt to have that part tested too.

 

[Hurricane]

sounds like theyre just going to test how much the soil can bear as it sits. you need somebody there to check the compaction every foot as you fill to grade. if they dont know/have the equipment to do this, contact another firm
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[boiler7904]

Definitely need to have on-site compaction testing during that entire phase of the project. I just completed a project that sank about 2" because the soil in the middle of the buildup (6' down in an 11' buildup) was too dry at time of placement / compaction and was the right type of material. In my area, the clay supports 3,000 psi + no problem and foundations are built on it every day. During this project (major drought), a little black dirt (virtually 0 bearing capacity) got mixed in and missed - until the building started to sink.

 

[tintoptexan]

"ive seen some real bad advice here already. you almost NEVER need to add moisture to dirt for good compaction. 99% of the time, you want to get the moisture OUT of the dirt for good compaction."

This is a quote from an earlier post that probably needs clarification. There is actually "Optimum Moisture Content" for differing type soils. Some are sandy, some clay and some sand, clay and loam combinations. The best thing to do is get the proper classification of the soil, have a proctor run to determine the maximum compaction that that soil can achieve and then try to get at least 95% of that maximum laboratory achieved density. It's all based on how many pounds of a particular soil can be pounded into a 1 cubic foot area. There are many differing types of soil which means many different compaction efforts and "Optimum" moisture contents. Bottom line is, dry soil will not compact as well as the same soil that is at it's optimum moisture. Take sand for example, the best way to compact sand is by wetting it similar to what you witness at the beach when a wave comes in. Hope this helps, Joe

 

[ron in sc]

I need to decide which type of testing to have done. One type involves soil borings and the other uses something called a cone penatromator (sp?)

All the firms I've talked to will come the the site to monitor work and make sure it is up to the specs.

 

[tintoptexan]

Any lab should be able to guide you. Typically it is;
1.Have a proctor run on the soil to determine the maximum density.
2.Place the soil in 6 to 8 inch lifts and compact each lift.
3.test each lift using the sand cone method or nuclear density. Preferably the first test a sand cone and the rest nuclear. This is for comparison purposes. Nuclear is affected by hydrocarbons if present and are read as moisture by a nuclear density machine.

 

[tintoptexan]

I failed to mention, this is for back fill. If you want to see what the soil you are placing the fill on then you will need borings or density tests on the undisturbed area. On our sites we excavate and go back with compacted select fill.

 

[Hurricane]

This is a quote from an earlier post that probably needs clarification. There is actually "Optimum Moisture Content" for differing type soils. Some are sandy, some clay and some sand, clay and loam combinations. The best thing to do is get the proper classification of the soil, have a proctor run to determine the maximum compaction that that soil can achieve and then try to get at least 95% of that maximum laboratory achieved density. It's all based on how many pounds of a particular soil can be pounded into a 1 cubic foot area. There are many differing types of soil which means many different compaction efforts and "Optimum" moisture contents. Bottom line is, dry soil will not compact as well as the same soil that is at it's optimum moisture. Take sand for example, the best way to compact sand is by wetting it similar to what you witness at the beach when a wave comes in. Hope this helps, Joe

you are correct. i was referring to the blanket statement made earlier. while there is optimum moisture for different soils, 9 times out of 10 you need to dry the material to get "optimum". sounds like you are in the trade as well, so i shouldnt be telling you anything new here. unless youre filling in july heat, in the middle of a drought, you shouldnt need to add water. generally speaking, after you cut a foot, youre right back into dirt with moisture. so by the time it all gets rolled around, its fine. ive been doing this work for 7 years and i can count on one hand the number of times weve needed to add water to dirt to get compaction. rock on the other hand is a different story.
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[tintoptexan]

10-4 to that. We do a lot of soil work in my line of work. I've been with the US Army Corps of Engineers, Construction Division for 27 years. We have a tendacy to overkill sometimes, but always with good intentions. To me, construction is not much more than a series of problem solving. Hopefully we learn each time. Good luck. Joe