Yet another building on fill thread

[5030tinkerer]

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?

 

[BuilderML]

The most important thing is that the footing for the main structure is resting on virgin soil. Many a times the inside must be built up once the foundation wall are poured. A few examples my garage is built up 3' in the front and 5' in the back. My inlaws garage had the be filled 8' in the back and 1' in the front.
One thing is to make sure your filling with good material nothing organic that will decay later creating voids. Where you plan on putting your lifts i would do piers under your lifts post. When i need to use alot of fill i like to flood the interior. At my inlaws we used a water truck. In my own garage once i had gutters up i diverted many leaders to drain inside the "filled" garage. Water is a very very good tool to use in "compacting" fill material. Sounds like a nice building your putting up.:thumbsup:

 

[BuilderML]

Another thing i would like to add is. I like to take a garden hose and run it down inbetween the foundation wall and interior fill turn it on and just let it run. This should be done in all the corners and all along the perimeter. Just an example, look at when a house is backfilled or any excavations is done for that matter, Once the sheathing is put on the roof and after it rains when you walk around the drip line of the roof you'll see the ground has sunk due to the cracks in the soil. Water and lots of it is your friend in setteling soil. Even areas that have been compacted in lifts could benefit from water.

 

[RedNeckRacin]

You can either pay to have a sheepsfoot compactor put the material in with 12" lifts with a compaction to refusal or you can wait several years for the materials to settle naturally. Its your money and time though!

 

[EddieWalker]

A plate compactor is just for homeowners to compact patio's and walkways. It's just about worthless for dirt work. As mentioned, a sheepsfoot is the ideal tool for building up a pad. It vibrates AND mixes up the clay. Proper moisture content is also important since it allows the clay to bind with itself. Too dry and nothing happens, too wet and it's just mud. Moisture content is critical. I worked a season running a water truck and would spend day after day hosing the dirt pile that was being mixed with a wheel loader that went to the fill areas on jobs. Sometimes it was an area being built up, other times it was for filling trenches. The operator gave the signals, I just held the hose and sprayed where he pointed. No way would they attempt to work the dirt without water. And when the tank was empty and I had to get more water, they sat and waited.

Do not use a dozer. If you have something really heavy, you can use that instead of a sheepsfoot. I have a full sized backhoe with a one yard front bucket. When I'm filling a hole for a stump, or building up a pad, I go over that soil with a full bucket and cover ever inch multiple times. I'll have my dad stand there with the hose keeping it wet.

It is better to build up a pad that slopes away from the building that gives you great drainage.

I would want my footings do go below the virgin soil. How deep you go would depend on your area, but no matter how much you build up the pad, the place you measure from for your footing depth is the top of the virgin soil, and go down from there.

Eddie

 

[5030tinkerer]

Thanks for all the replies, guys. It is GREATLY appreciated.

So what I am hearing is that the fact that my footing will be atop (and actually technically just a bit under to Eddie's point) virgin soil is the big life saver in this deal and to not sweat as much about the interior pad. That is HUGE news. Thank you!!

It has rained ALOT here in Iowa this Spring. Essentially I get 10 dump truck loads in, spread as best I can while its still muck or sticky (though trying hard to keep the yellow clay out of the equation using it just for external-to-the-pad slope drainage), wait overnight for it to dry some, then vibratory roll it. Then it rains for three days it seems. Then I wait for it to dry enough to be workable again, vibratory roll it again, check the elevation, sigh heavily when it's only gone up the slight bit it did (sure, the building footprint is 'only' 50x80, but the area to be raised is more like double that not to even mention the copious dirt needed to slope it out so that it can look more natural), and move more dirt/muck to start the process all over again. The dirt at this point is coming from an area that is generally wet where my pond is being expanded. Per the county extension office, the dirt is a Sharpsburg silty clay loam.

The tandem axle dump truck empty weighs something 25,000#. It of course has been driving over the pad over and over. The padfoot vibratory roller is a smaller one, though, at only 6' wide with a weight of about 10,000. I'd have to look up the centrifugal specs. It never 'feels' like anything is happening when I use it, but driving over the pad with the dump truck later tells the story with the lack of deeper than 1" ruts even with the truck loaded to the gills (the box is 14', but I am very guilty of over loading it on property).

The plate compactor would only be used for final compaction of the two inches or so of 1" clean limestone just before the foam went down.

All ideas on how to mitigate the risks of a sinking slab appreciated!

 

[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

 

[RedNeckRacin]

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.

 

[5030tinkerer]

From what I understand, compaction in lifts is the preferred route. If the lifts are too high, you end up with a compacted upper layer and an uncompacted lower layer.

With padfoot vibratory rollers, the lift height is evidently the length in inches of the pads. In my case, that is 6".

With the dump truck, I'd think it able to get deeper than just 6", but still not the height that some of the lifts were placed in.

It seems that the only way to get the compaction other than to wait for umpteen years is to remove all of the soil and re-place it in the proper lifts that should have existed from the start.

At that point, though, it's less expensive to just install piers on some interval and accept the fact that certainly some compaction has occurred. Ten foot on center 12" piers at 48" in a grid would burn 2 yards of concrete. No big deal. I'd think I could put in the piers at 5'OC intervals and really be set.

The question is how to do the piers as I had planned on having 2" of gravel and then 2" of foam under the concrete. What do you guys think? Are the piers the way to go? If so, should the top of the pier be at the bottom of the compacted gravel or should I lose insulation value and have the piers come up through the gravel and through the foam for a monolithic pad/pier pour?

 

[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