The plans say 24"

[ICE]

There's quite a bit of work here. It looks pretty good. It's too damned bad that it has to come out. You see, they didn't see the detail that shows the footing 24" into undisturbed soul. So they are 16' to 18" deep.

I have encountered this hundreds of times. Yes I said hundreds. It is because the detail is small and the writing is tiny and the info is nowhere else on the plans. Hell, sometimes I have to search for it. I have asked that there be a big warning on the foundation page. Apparently that would be too much trouble, but doing all of this work over is no big deal.

 

[Yankee Chronicler]

Now ya done it!

I'm in a region where the frost depth is 42 inches. I long ago lost count of how many sets of plans I've looked at that calls for 42 inches below grade -- but there's no site plan (or, if there's a site plan, it only shows the property line and the building outline, no grading information) and the foundation plan doesn't specify an elevation for bottom of footing. The usual argument is "Well, the inspector can check that in the field when he does the footing inspection."

Except that when he's there for the footing inspection, there's a big hole in the ground, piles of dirt everywhere, and nobody has a clue what the final grading is supposed to look like.

It's all part of the dumbing down of the design professions. When I was a young architect on the boards (when we still drew with pencils -- or ink), if the structural engineer didn't show bottom of footing elevations on his drawings, we showed it on ours. Now, nobody wants to take the time to figure it out, so everybody on the design "team" kicks that can down the road and leaves the poor contractor to figure it out (or not).

 

[ICE]

Except that when he's there for the footing inspection, there's a big hole in the ground, piles of dirt everywhere, and nobody has a clue what the final grading is supposed to look like.

The detail usually states, "into undisturbed soil". Final grade might include inches to feet of fill, none of which counts towards the 24" footing depth.

If the detail omitted "into undisturbed soil" I pencil it in for them. There is occasionally a bit of creative soil placement in order to achieve the objective.... otherwise there would be lots of wasted concrete.

 

[jar546]

So here is the question to keep this in line with the thread but veer off a little bit.

If you live in a jurisdiction that has a 42" frost depth. Is that to the bottom of the footing or to the top of the footing where the foundation wall begins?

 

[Mark K]

So here is the question to keep this in line with the thread but veer off a little bit.

If you live in a jurisdiction that has a 42" frost depth. Is that to the bottom of the footing or to the top of the footing where the foundation wall begins?

The bottom of footing. The soil below the frost depth is not subject to cyclic frost heave.

 

[Yankee Chronicler]

The detail usually states, "into undisturbed soil". Final grade might include inches to feet of fill, none of which counts towards the 24" footing depth.

If the detail omitted "into undisturbed soil" I pencil it in for them. There is occasionally a bit of creative soil placement in order to achieve the objective.... otherwise there would be lots of wasted concrete.

I've never seen a detail or a code requirement calling for the earth beside or above a footing to be undisturbed soil. If you think about it, except in a case where the soil is very firm and footings can be formed by just excavating a trench of the required width and depth, that's impossible. I don't know if California edited it, but what section 403.1 of the ICC IRC says is, "Footings shall be supported on undisturbed natural soils or engineered fill."

 

[Yankee Chronicler]

So here is the question to keep this in line with the thread but veer off a little bit.

If you live in a jurisdiction that has a 42" frost depth. Is that to the bottom of the footing or to the top of the footing where the foundation wall begins?

Bottom of footing.

 

[MtnArch]

I've never seen a detail or a code requirement calling for the earth beside or above a footing to be undisturbed soil. If you think about it, except in a case where the soil is very firm and footings can be formed by just excavating a trench of the required width and depth, that's impossible. I don't know if California edited it, but what section 403.1 of the ICC IRC says is, "Footings shall be supported on undisturbed natural soils or engineered fill."

California Res Code says the same thing.

 

[ICE]

Footings shall be supported on undisturbed natural soils or engineered fill.

On is not in. The undisturbed soil as has been enforced here has been described by engineers as some reliance on friction between the soil and the concrete footing. Others have said that 24" deep into any soil thwarts water from getting under the slab.

Some jobs had consolidated soils that allowed a trench:





By the way, that green barrel was supporting a two story house.

Some jobs had zero contact with undisturbed soil:

 

[Yankee Chronicler]

I understand that "on" is not "in." If the approved construction documents say "in," that's a unique situation. I don't know exactly where you are in California, but the California Building Code doesn't say "in," it says "on."

R403.1 General​

All exterior walls shall be supported on continuous solid or fully grouted masonry or concrete footings, crushed stone footings, wood foundations, or other approved structural systems that shall be of sufficient design to accommodate all loads according to Section R301 and to transmit the resulting loads to the soil within the limitations as determined from the character of the soil. Footings shall be supported on undisturbed natural soils or engineered fill. Concrete footing shall be designed and constructed in accordance with the provisions of Section R403 or in accordance with ACI 332.

 

[tmurray]

So here is the question to keep this in line with the thread but veer off a little bit.

If you live in a jurisdiction that has a 42" frost depth. Is that to the bottom of the footing or to the top of the footing where the foundation wall begins?

Bottom of footing here to. The actual code requirement is to the bottom of the "foundation unit" which is defined as the footing, raft, or pile.

Logically, this makes sense. It would be a little strange to see frost heave occurring between the footing and the wall.

But we did have to educate a building department on this in the past...

 

[Yikes]

Notice the citrus tree in the background in the photo in post #1. I assume it would be easier to revise and resubmit the detail to reflect the frost-free soil condition than to remove the formwork and deepen the footings to 24”.

 

[Paul Sweet]

The 24" depth might be due to shrink/swell soil, although I'd expect it would have to go down 4 or 5 ft. to get where the moisture level is fairly steady.

I believe the 12" into undisturbed soil requirement is to get below roots and other organic materials. This requirement dates back to CABO days.

I show existing & finish grades and dash in the bottom of footings on the elevations and specify 24" below finish grade or 12" below existing grade, whichever is deeper.

 

[Yankee Chronicler]

I show existing & finish grades and dash in the bottom of footings on the elevations and specify 24" below finish grade or 12" below existing grade, whichever is deeper.

Do your elevation drawings (for houses) show at least an approximation of the proposed finished grade?

There are three or four house plan drawers who all the developers in town seem to use, and they ALL just draw a straight, horizontal line about 10 inches below the ground floor elevation to represent "grade," on all four elevations. In reality, there isn't a single flat lot left anywhere in town, but the department has always accepted these drawings and I'm not the residential plan reviewer, so I'm not going to make waves.

It was a shock to me, because at my former department all house drawings had a pretty good approximation of finished grade shown on the elevations. It made my life as a plan reviewer easier, and it made my life as a field inspector easier, too. Everyone knew where the bottom of footing was supposed to be before they stuck a shovel in the ground.

 

[Paul Sweet]

Most of the homes I've designed were built on sloping lots. I haven't done stock plans for subdivisions, and I have a rough idea of the existing & new grades even if there isn't a topo survey.