The county wants me to dig a 8 foot wide trench for my 30" wide concrete footing. They claim this is done to remove any soil issues. To me it's a waste of money but I have to do it per the county.

So even the footing is 30" wide the rest of the 6 feet width needs to be filled in with ABC.

The county is also requiring I dig 12" below the footing and add 12" of compacted ABC and then place the footing on top of the compact ABC.

I was always under the impression that footings should be on undisturbed soil.

What kind of soil are you dealing with?

Usually the footing rests on undisturbed soil, unless you have an unusual soil condition. 8' does sound like a lot of extra digging and a lot of specified fill. Sounds like you have a building department that is worried about liability, so they set arbitrary rules OR engineered design. IE: Do it our prescribed way or show us a plan with a PE stamp on it... Some times they have a good reason for these rules, sometimes not so much. As an example, one of the special inspections(I have to hire an engineer to inspect and report to the county) I will need is for the roof diaphragm. Come to find out it isn't a structural thing, it is because the county won't allow their employees out on the roof for liability reasons...

What building code standard is your building department currently operating under?

12" of gravel will spread the downward force making the footing effectively wider. And prevent the surface from getting torn up during construction. Might even distribute moisture more evenly, reducing differential expansion (if there is clay). Or it might hold moisture under the footing, increasing expansion.

Yes, the code calls for undisturbed soil or engineered soil... So you might want to consider just building on an engineered pad (e.g., roll in some 3/4 minus to desired height and top off with gravel) and avoid having to dig to create the footings. You will still have to back-fill the footings, but you will end up with a building having a foundation that is elevated above the local terrain, which is often preferable for water catchment and drainage. You could also then fill the interior with self-compacting pea gravel, place under-slab insulation, and simply pour a slab in lieu of having to suspend a floor and having a rat crawl space. We commonly use this approach in our area which doesn't have soil issues...and if you are pouring a slab, you should always consider embedding PEX in it to create a low cost and efficient hydronic radiant floor heating emitter...

The county had some areas that had expansive soils/clays so it freaked out due to foundation issues. It then cast a wide net and made all new builds over build the foundations. In my area the soil was tested by geotechnical engineers and I had none to very low plasticity. Around 4-6 plasticity which is very low.

They still want to see the over excavation and 12" of MAG SPEC 3/4" minus ABC under the footing and then the width around the footings must have MAG SPEC 3/4" minus ABC around it to protect the footing.

Yes, normally if you hire a soil engineer and get a good soil test report, you can avoid the standard remedy expense for expansive soil. Sounds like you are dealing with an extra cautious and conservative building approval authority.

I would be talking to some engineers. The PE stamp wins over prescriptive requirements and in this case might be able to save a good bit of money. The county has a universal comprehensive approach but there is likely a more cost effective solution for your particular case.

The county made this new rule where one has to test their soil and the testing runs $2k

Even in cases like mine, when the soil is NONE to VERY LOW in expansion, the PE ALWAYS makes the recommendation to over-excavate and add at least 12" of ABC at the base of the footing. According to the PE, they do it to protect themselves from future foundation issues so even on very good soils, they will always mandate 12" of ABC at the base of the footing.

Most people think it's a waste of money and a complete scam but it is what it is.

Typically, how much expense does it add to the foundation work to add 12" of ABC below the footing? It can't be that much more $ since the backhoe work to dig 12" deeper is not that much time and ABC out here is around $16 a ton delivered.

Lbear, where are you from? I had to search the web for what "ABC" was. Around here (central PA), we call it Crush'n Run or sometimes 2A modified.

Interesting how different parts of the country name what is basically the same product. Out in these parts (Boise ID), Lbear's ABC is referred to as "road mix" or sometimes "3/4 minus". Also here, this type of fill is priced by the cu. yd. not by the ton.

In any case, I can't for the life of me envision the need for 8' of footing fill. Assuming a 160' footing perimeter (40' x 40') your looking at ~47 cu. yd. of fill. Assume a weight of 1.35 tons/cu. yd, your looking at an extra $1,000. Do they also require the fill to be compacted? That will add some cost as well.

According to the PE, they do it to protect themselves from future foundation issues

Would be interesting to know exactly how they think it does this (vs just using a wider/thicker footer).

3/4 minus must fully compacted (heavy rolled and shaken) before placing concrete.

Properly compacted 3/4 minus creates a solid foundation base that should not settle significantly over the life of the building. Compare this to questionable soil quality that could result in settling and foundation failure. It is always safer, but more expensive to go the 3/4 minus path. However, if the soil is properly tested and shown to be acceptable for the building design, this expense can normally be avoided. However, there appears to be some disconnect in this regard with this specific building approval authority. I suspect that this may be the consequence of previous soil testing data and subsequent foundation failures having been problematic in this region. I doubt there are any villains here...just a sensitized and now extra cautious and conservative building approval authority.

Posted By sailawayrb on 21 Aug 2016 02:27 PM
3/4 minus must fully compacted (heavy rolled and shaken) before placing concrete.

Properly compacted 3/4 minus creates a solid foundation base that should not settle significantly over the life of the building. Compare this to questionable soil quality that could result in settling and foundation failure. It is always safer, but more expensive to go the 3/4 minus path. However, if the soil is properly tested and shown to be acceptable for the building design, this expense can normally be avoided. However, there appears to be some disconnect in this regard with this specific building approval authority. I suspect that this may be the consequence of previous soil testing data and subsequent foundation failures having been problematic in this region. I doubt there are any villains here...just a sensitized and now extra cautious and conservative building approval authority.

Out in the SouthWest (Arizona, New Mexico, Utah) they call it ABC or Aggregate Base Course

It's basically 3/4 inch minus material that consists of rock smaller than 3/4" and fines (sand). It compacts well and is used in road base, foundations and anything that requires a solid base underlayment.

My question is which soil is "safer"? Undisturbed soil (low plasticity) or disturbed soil with compacted AB?

Good question Lbear. I would say undisturbed soil having zero plasticity is best...zero chance of having any compaction issues. However, about the only place you find soil this good is on bedrock or very rocky soil not far from bedrock. ABC or 3/4 minus properly compacted will have very low plasticity, but still something greater than zero. Once you can get into the low plasticity range, you are golden and the only consideration is the economics of getting there. If your soil test was good for your building load, in my opinion you should have been allowed to proceed using undisturbed soil.

Posted By sailawayrb on 24 Aug 2016 11:37 AM
Good question Lbear. I would say undisturbed soil having zero plasticity is best...zero chance of having any compaction issues. However, about the only place you find soil this good is on bedrock or very rocky soil not far from bedrock. ABC or 3/4 minus properly compacted will have very low plasticity, but still something greater than zero. Once you can get into the low plasticity range, you are golden and the only consideration is the economics of getting there. If your soil test was good for your building load, in my opinion you should have been allowed to proceed using undisturbed soil.

I hope the county will allow me to just build on native soil or at least just do the 12" AB under the 30" wide footing. Doing an 8' wide area that filling it with 12" AB is just overkill since my footing is only 30" wide.

That's an entirely reasonable and sensible argument... Good luck!

That's an entirely reasonable and sensible argument... Good luck!

Adding 54" wide, 12" thick gravel will put much less pressure on the soil than just 30". Agreed, 8' wide gravel is overkill.

On a slab on grade shallow footing (18" deep), is it typical practice to run any plumbing lines (sewer, well) through the stem wall (ICF)?