Yet another building on fill thread

[Streetcar]

That is not entirely true. It is however why material is placed in lifts and not mass fills. The effectiveness of compaction is lost at a certain depth based on its weight and soil type. The soil could still settle even if it has been "proofed" on the surface.

My statements were based on previous statements that the material was being placed in lifts.
If placed in lifts over the stated 12" it is possible for the surface to bridge over uncompacted material and settle later. Excess moisture can cause problems for a clay soil, even if it was previously well compacted

 

[GLyford]

Perhaps a higher density of piers, tied together with a "grade beam"+ extra rebar where you expect higher load concentrations (wheel tracks, lift pads) and a little more spread out (or even none at all) in the general shop/workbench/storage areas?

It's not like you're going to jockey all the vehicles into all the corners of the building, you're mostly just going to drive them straight in from the doors, right?

 

[EddieWalker]

Be careful with over engineering and complicating the process. When building, always figure out what it will take to accomplish the task in the most cost effective method using time tested, proven methods. While there is always some benefit to more materials in your footings, there is a point where you are just throwing money away. If the soil is of good quality, compacted with proper drainage, there is no reason to go into a complex foundation design. Spend the time and money on getting the dirt right, and the rest is easy.

Eddie

 

[Cord]

Hey guys -

I am building a 50x80 gambrel barn/shop using structural insulated panels (thermocoremo.com). The trouble is that the building site needed to be brought up about 4'. While the required footing will be resting on virgin soil, the 6" thick interior concrete slab will not be.

An early thought is to incorporate a concrete brick ledge on the inside perimeter of the stem wall (which is going to be constructed using insulated concrete forms) and then let the slab rest on that. As this only supports the perimeter, though, I was toying with the idea of installing 10' O.C. 12" diameter concrete piers for the slab to be further supported on. Though the first two feet of fill was installed in lifts, compacted properly, and even has benefited from a freeze/thaw cycle, the remaining two feet was taken from a wet part of the property which made it impossible to put down in proper lifts. Essentially muck just dumped out of the dump truck and was 'spread' in as thin of lifts as possible (sometimes 12" or more). When it got dry enough a few days later, a vibratory roller was used. Given the slope of the original site, as much as 3' of fill was placed this way. Sigh.

The question is whether I should at least wait another freeze/thaw cycle after final grading of the newly elevated pad before building or whether there is another way. Of course, I am not at all convinced that waiting 'just' a year will be sufficient. The pad will have two inches of roller and/or plate compacted 1" clean limestone gravel, then two inches of pink rigid foam, and then six inches of 4000psi concrete reinforced with 20 pounds/yard of Helix, a 'micro-rebar' that when used in this dosage is equivalent to #4 rebar placed 12" OC (http://www.helixfiber.com/sites/g/files/g605716/f/files/Application Guide - Slab on Grade_1.pdf).

Thoughts? I desperately want that building up, but don't want a floor that will need to be torn out later either. Of course, I want the floor to be able to support a 12,000# automotive lift and the periodic driving on it from a 21,000# LULL and similarly heavy tandem axle dump truck. Am I completely crazy or just mostly?

A couple of freeze thaw cycles will not be sufficient to settle poorly compacted material. You have two choices to do the job right:

1. Build the foundation walls and then bring in crushed stone and use it to backfill the hole. This material should be consistent in size (3/4 to 1-1/4) and could be compacted in 12" lifts with a walk behind compactor.
2. Use a non-organic dirt material which is free of large rocks and compact it in shallow lifts using a ride-on compactor. The bigger the compactor you get, the thicker the lift can be. These can be rented from a commercial outfit.

With a proper moisture content and some good material option #2 will work and won't settle. You'll spend more on the compactor, but will save on material. You'll have to do the math to see which way you will want to go. If you do go with option #2, try to proof roll the area using a fully loaded dump truck. If it makes ruts you did a poor job of compacting and have to start over.

 

[Cord]

Quote Originally Posted by Streetcar
The heavily loaded dump truck will allow you to proof the soil.
If the soil does not pump or deflect significantly when the truck drives over it, the soil will support the concrete slab

That is not entirely true. It is however why material is placed in lifts and not mass fills. The effectiveness of compaction is lost at a certain depth based on its weight and soil type. The soil could still settle even if it has been "proofed" on the surface.

Streetcar was talking about proof rolling the fill and his statements were spot on correct. The truck is not being used to compact the material, that's the job from a drum compactor, but to verify the compactor has done a sufficient job. You're statements that the compaction is lost in mass fills is also correct. Shallow lifts must be utilized, usually 12" is the standard when using heavy equipment, but shallower lifts are a sure bet when a geotech isn't on site.

 

[5030tinkerer]

Excess moisture can cause problems for a clay soil, even if it was previously well compacted

I'm helped on the moisture front I'd think by the fact that the building site is the highest thing around it. The site has a 4 degree sloped drop off on the west, south, and east for 800+ feet. 100 feet to the east is my home, the grade of which is 3' higher than the building, but there will be a 2 degree downward slope for a 40 foot concrete apron from the building, which will intercept a 3.6 degree slope from the 56' long apron from the house. I could potentially raise the building pad up another 17" to get matching 2 degree slopes from each building, but doing so would necessitate making my frost walls another foot deeper to ensure that I hit virgin soil on the spread footing.

I am considering raising the building the 17" and NOT extending my frost wall despite the virgin soil concern though, given that the earth 2.5 feet below current grade (four feet below future grade) has been there more than two years already, was properly compacted unlike this fill I am using now which is only 'somewhat' properly compacted, and the fact that I am more than just a little concerned about hitting an electrical service that runs under the pad which SHOULD be deep enough, but hitting it is beyond just bad. Ultimately, I want the ability to have a basketball hoop on the far end of the concrete pad and don't want people to 'feel' like they are playing on a slope. I'm not at all sure that 3.6 degrees would 'feel' that way, but know that we've had visitors have trouble on the more slippery days even getting through the 4.4 degree slope on the county's road with their 2WD vehicles.

Removing all of the 200+ dump trucks of fill already placed to get this area built up again strikes me as more expensive than just installing piers with perhaps grade beams at this point.

Am I on track?

 

[5030tinkerer]

The other option, I suppose, would be to install a new electrical service that goes around the building, abandon the existing one (two 200 amp feeds), build a basement under the building and install a formed beam system for the floor on the main level like what the folks at liteform (LiteForm | Concrete Deck Forms - LiteForm) offer, but the price now just WAY up.

 

[RedNeckRacin]

Quote Originally Posted by Streetcar
The heavily loaded dump truck will allow you to proof the soil.
If the soil does not pump or deflect significantly when the truck drives over it, the soil will support the concrete slab



Streetcar was talking about proof rolling the fill and his statements were spot on correct. The truck is not being used to compact the material, that's the job from a drum compactor, but to verify the compactor has done a sufficient job. You're statements that the compaction is lost in mass fills is also correct. Shallow lifts must be utilized, usually 12" is the standard when using heavy equipment, but shallower lifts are a sure bet when a geotech isn't on site.

I agree to a point. I was stating that "proofing" the soil as you guys are putting it, only checks the surface and not the entire fill depth like you seem to be implying. Th e pressure of the truck is spread out easily with enough depth that it looses effectiveness. This is exactly why they put in 12"-18" lifts based on material and roller size for examples.

I also agree that moisture is pretty critical for compaction and is hard to eyeball for a novice. Over saturation after compaction also causes severe problems.

 

[5030tinkerer]

So what would you guys do in my situation? I've got about 1400 yards of dirt already spread, some by me, some by others, that is not properly compacted but the footings will go to virgin soil or extremely close to it. Ride on vibratory roller compaction DID occur, just in likely too high of lifts. Remember also that there is an electrical service about 66" below current grade.

Options are:
1. Install a pier or pier and beam system to support the floor along with an interior to the foundation wall brick ledge and go.
2. Use more rebar/microbar to strengthen the slab and pour anyway.
3. Build the foundation stem walls, backfill around them as usual, and repeatedly flood the interior to promote settling.
4. Do a combination of the above.
5. Remove the 1400 yards and replace it in proper lifts, compacting as we go now that things are drier.
6. Hire a geotech and have the compaction measured to get his recommendations since maybe things are better compaction wise than we think they are.
7. Let it settle a freeze/thaw cycle and then do one of the above options.
8. Other

For tools, I have a full sized 42,000 excavator (an old Koehring 6620), an old IH175C track loader with a 4n1 bucket, a tandem axle dump truck, and a ride-on vibratory pad foot roller. I also have the phone book to 'just' hire one of these solutions done, but that strikes me as the spendiest option of them all! I'm tempted to pay the money to have a geotech report done.

I am kicking myself for not ensuring that the dirt was placed properly to begin with, but such are life's lessons. I guess I had to pee on the electric fence for myself this time. Sigh. At least we're having the conversation before things get REALLY expensive.

 

[Cord]

I'd get a small cat in there and scrape the site clean. Start pushing the material back in the proper lifts while simultaneously using a drum compactor. You can do it once right or do it twice. You already need to remove the improperly compacted material so the sooner you can start doing the job right, the less pain there will be. If that's not happening then I see two choices: Roll the dice and beef up the floor slab or hire a geotech.

Geotechs are not horribly expensive, but if you can compact the material well you can put that money back into the bank. The last contract I signed was $11,500 for 16 borings. You won't need as many holes, but the trip charge alone will put you in the $1500+ range.

I just finished a project which had improperly compacted fill. The geotech was guessing it had been compacted with a dozer. That material had been down for 25 years and only had 1500 psi of bearing capacity and the potential differential of movement was in the 3" range.