Thread: 403.1.4

403.1.4 Minimum depth
R403.1.4 Minimum depth. All exterior footings shall be placed at least 12 inches (305 mm) below the undisturbed ground surface.
I would like to hear some debate on this requirement.
Does this apply to monolithic slabs and stem wall foundations or only to stem wall foundations or can they both be done as described below.
Can a mono slab be formed on the surface of the site, interior filled, then 12” of fill added before final?
Some believe both must be 12” below. Others believe if the 12” of fill is added later that the intent of the code is fulfilled.
Also what is the definition of undisturbed ground surface?

Originally Posted by Jerome Schrenker

403.1.4 Minimum depth
R403.1.4 Minimum depth. All exterior footings shall be placed at least 12 inches (305 mm) below the undisturbed ground surface.

It is frequently good to start at the IBC to better understand the IRC, and this is a case in point.

The IBC, which is a more restrictive code as it address occupancies other than just residential, states this:

- 1803.5 Compacted fill material. Where footings will bear on compacted fill material, the compacted fill shall comply with the provisions of an approved report, which shall contain the following:
- - 1. Specifications for the preparation of the site prior to placement of compacted fill material.

- - 2. Specifications for material to be used as compacted fill.
- - 3. Test method to be used to determine the maximum dry density and optimum moisture content of the material to be used as compacted fill.
- - 4. Maximum allowable thickness of each lift of compacted fill material.
- - 5. Field test method for determining the in-place dry density of the compacted fill.
- - 6. Minimum acceptable in-place dry density expressed as a percentage of the maximum dry density determined in accordance with Item 3.
- - 7. Number and frequency of field tests required to determine compliance with Item 6.
- -

Exception: Compacted fill material less than 12 inches (305 mm) in depth need not comply with an approved report, provided it has been compacted to a minimum of 90 percent Modified Proctor in accordance with ASTM D 1557. The compaction shall be verified by a qualified inspector approved by the building official.

"Undisturbed ground surface" is in many cases, possibly most or all cases, rather difficult to actually find and use. Thus using the IBC allowances for compacted soil serves to allow building on "disturbed ground surface" as though it were "undisturbed ground surface".

Does this apply to monolithic slabs and stem wall foundations or only to stem wall foundations or can they both be done as described below.

When using the term monolithic one must keep in mind that monolithic means one piece. So, yes, you can do the following with monolithic slabs, it is just more difficult and takes more concrete, more forming, and more reinforcement.

Can a mono slab be formed on the surface of the site, interior filled, then 12” of fill added before final?

Some believe both must be 12” below. Others believe if the 12” of fill is added later that the intent of the code is fulfilled.

The bottom of the footing is what much be a minimum of 12 inches below undisturbed soil depth and below frost depth or protected from frost damage (refer to R403.1.4.1).

The fill below the underside of the slab portion could be as high as one wanted it to be, however, it would be a bit difficult to have the bottom of the footing 12 inches below undisturbed grade, 60 inches of fill, and then form and pour the footing, the walls, and the slabs as a monolithic pour ... difficult, but it could be done.

Also what is the definition of undisturbed ground surface?

Ahhhh ... dat ees de question ... ... see comment about referring to, and understanding, the IBC and how it addresses this for more restrictive structures.

To try to define the term may be problematic - around here it is very typical to overexcavate and recompact to achieve suitable bearing values so technically the soil is already disturbed (as are many of us in our division)

If my memory is correct the only structure in the residential code than can have a 12" footing depth is a building under 400 sq ft? Anything over that requires a 42" depth or can be up to the local building official, ie- soil conditions etc..

I have heard this argument before. My opinion is, if the footing is resting on un-disturbed ground, even if it's on top of the ground and not excavated, that meets the requirement. Of course realistically all footings will be placed on disturbed ground because the topsoil must be removed.

Why would you not be able to fill to meet the frost depth. Is the soil hauled in more likely to freeze or let frost penetrate deeper than the existing soil? NO! in fact if the correct soil is used for fill, it's probably less likely.

I know an inspector that says the frost depth requirement is to the top of the footing, not the bottom. What do you guys think about that? How could that be, not all footing sizes are the same...

I have been told the definition of undisturbed ground surface is the existing ground before any construction has begun. I am not sure where that came from but sounds reasonable.

Originally Posted by Jerome Schrenker

I have been told the definition of undisturbed earth is the existing soil before any construction has begun. I am not sure where that came from but sounds reasonable.

Like I mentioned before, all soil gets disturbed, code also requires all vegetation (grass, sod, etc.) be removed. It's hard to do that without disturbing it. The word that is used often is virgin soil. You can remove the top soil and not excavate the soils that have always been present. Hence the term virgin soil. These soils would be assumed to have full compaction to their various pre-determined soil bearing capacities.