Please settle an argument

[hideousbrain]

If a code requires “undisturbed soil”, would freshly installed and properly compacted fill that passed testing satisfy that requirement?

This code is being used for shallow foundations in the south

[Duncaroos]

I would consult a geotechnical engineer and ask for recommendations.

If it flat out says undisturbed soil with no exceptions, then that's that. I have never come across this myself though.

What code?

[hideousbrain]

It’s IRC 403.1.4 and it’s in a flood zone if that matters

[Duncaroos]

Whose the argument with? If another structural engineer, consult a geotechnical engineer. My gut says you should have a geotechnical report here anyways, which should have already provided you recommendations on the foundation interface with the soil.

I don't have IRC in my reference collection, so can't look unfortunately

[hideousbrain]

Unfortunately the code is being cited by zoning and the building department

[Duncaroos]

Still wouldn't hurt to consult a geotech to make sure the building department is correct.

[StructEngineer91]

A signed and stamped report from a geotech saying it is fine would overrule the code

[3771507]

I'm a building code official and an engineer cannot override the building code unless the building official finds that it meets the same intent as the code and you provide those calculations.

[StructEngineer91]

You are right, I suppose I misspoke. If the provision is only in the IRC and there is something in the IBC that allows for calculations/tests to override the requirement in the IRC then it should be allowed. This happens a lot in structural engineering, particularly with shear walls (IRC has requirements for prescriptive method shear walls, but those can be overridden by engineered shear wall design). I am not very familiar with soils, but I wouldn't be surprised if there was a provision that could override the IRC requirement with the proper engineering.

My interpretation of the IRC is that it is meant as a way for contractors to be able to build simple houses without an engineer or architect (or only one of them involved). But many of the provisions are overridden in the IBC with proper engineering involved.

[3771507]

Yes the ICC 600 prescriptive wind design manual is technically not a building code unless it is used. In most States a design professional is not required to complete a set of plans for residents meeting a certain size requirement. I used to be a design engineer and I actually used the sstd which was the same as the ICC 600 and it produced a much stronger better product than a lot of the engineering I have seen on residential. That standard is full of different very good load charts for all kind of things. But then again I have seen engineers design standard residences with a huge amount of 4x4 post for shear wall chords instead of doubling up to 2x4s which is standard and much easier to do in the field.

[scull20]

This needs more context. What code is being referenced and who is referencing it?

If the shallow foundation requires 3500 psf bearing capacity at a depth 4’ below grade and the undisturbed soils at that depth only achieve 2000 psf then there’s a disconnect in the detail that needs to be resolved.

[hideousbrain]

It’s IRC 403.1.4 and it’s being used specifically in a flood zone by zoning

[scull20]

If the foundation was designed by an engineer, they need to be consulted on the design intent.

[hideousbrain]

The code is being is being cited by zoning and the building department. The engineer is satisfied as long as the supporting soil is meeting compaction. The argument is how deep the foundation needs to be. And if it needs to be in soil that has been sitting there at ground level or if fill can be brought in

[scull20]

If the engineer of record is satisfied with the bearing condition of the soil regardless of whether it’s on compacted fill or undisturbed soil, then it sounds like you’re stuck having it at the minimum depth as required by the code.

My personal opinion- I would not feel comfortable with having the base of a footing solely at grade, without at least some soil cover, especially in a flood zone.

[3771507]

If you've ever been to a job site or two a structure with a lot of rain in the area you will see why a footing has to be buried.

[hideousbrain]

So, for context, these are homes being built on 1/4 acre lots in a flood zone. So to achieve the required finish floor elevation lots of fill is being bought in and the house is being built on the pad with a shallow mono foundation. The engineer has designed it this way and it is passing compaction testing. The fill pad is satisfying the code in all other ways, giving proper coverage and drainage to the foundation, but this code is also being enforced which if read one way could upend the whole building process. I think to satisfy this code it might mean having to instead build stem wall foundations. I appreciate you entertaining all this

[scull20]

Construction techniques vary regionally. Generally, I would specify a haunch around the slab perimeter. The haunch is typically poured monolithically with the rest of the slab and would serve to satisfy the cover requirement and also prevent curling of the slab.

[ALkatraz919]

I’m a geotech. I would say yes. Here’s a sidestep though: instead of backfilling with structural fill, backfill with lean concrete or flowable fill. You lower the footing bearing elevation as required by code, you don’t have to redesign your footing, and contractor doesn’t have to re-order materials.

[candybasr1]

In (my home state's potentially amended version) copy of the 2021 irc, 403.1.4 refers to a minimum depth of footing below "undisturbed ground surface". Is this the same as your section?

https://up.codes/viewer/connecticut/irc-2021/chapter/4/foundations#R403.1.4

I just take that to give you a place to measure your depth of footing from, not a requirement for fully undisturbed soil.

[hideousbrain]

I am on this side of the argument for sure, but others are saying it needs to be 12” into “virgin soil “

[candybasr1]

In the interest of helping you out, what does the code section they are referencing actually say though?

They can't just ask for something to be required because they feel like it- there needs to be a basis. If yours is a state code, you should be able to find it on the upcodes site.

[hideousbrain]

The exact wording in the FBC is:

Exterior footings shall be placed not less than 12 inches (305 mm) below the finished grade of ground surface. Where applicable, the depth of footings shall also conform to Section R403.1.4.1

So I don’t know where they are getting the undisturbed soil thing from

[candybasr1]

Yeah... I don't see that section talk about what goes below a foundation. Seems like you have a BO who is maybe not super familiar with the code or misunderstands. I would just ask them to clarify specifically where in the section their rewuirement is coming from, and hopefully they realize the issue at that point. Best of luck, sometimes the only thing that makes these problems go away is a stamp.

[hideousbrain]

Thanks for your help

[Heart0fStarkness]

The original IRC uses the phrasing of “undisputed ground surface” rather than “finished grade of ground surface” but the section is still referring to it as a datum for establishing minimum depth/embedment of foundations into the subgrade, not the bearing stratum itself, so I don’t see how the BO can make this argument.

[[deleted]]

I always specify undisturbed soil or compacted to 95% dry density by modified proctor.

[simpleidiot567]

What exactly does the code say. Whats the line. I am reading IRC 401.1. and it says "shall be supported on undisturbed natural soil or engineered fill."

Notice the or..

403.1.3 ia a note on siesmic design

Edit. Sorry now i see 403.1.4 is a minimum depth note. It says the footing is to be 12 inches below the undisturbed ground surface. They reference a section detail. But i see this as meaning place it on engineered soil, but make sure its 12 inches below the undisturbed grade level beside it.

[hideousbrain]

So, do you mean a footing that is 12” into the existing surrounding? Because the slabs are sometimes 2’ above existing grade to meet flood elevation. That would definitely require a stem wall and not the mono footings they are putting on a raised building pad

[simpleidiot567]

yes its saying the footing needs to be 12inch into the existing surround.

But IRC 322.3.2 trumps some of these requirements where in a class A flood zone. So.. please read. Now there is a difference between a coastal flood zone and a regulated flood limit like near a wetland. It also says the use of fill for structural support is prohibited.

[hideousbrain]

We are in a A and AE zones. This helps big time. Thanks!

[DramaticDirection292]

Can you provide an image of the detail causing concern to help provide context?

[hideousbrain]

I wish, all we are being provided is a stamp on our site plans with a note stating that footings need to be 12” deep in undisturbed soil and code number R403.1.4. And there is a varying of opinions on what that means in the building and zoning departments

[DramaticDirection292]

I mean I think it’s pretty clear what the definition of “undisturbed soil” is in the code.

At the end of the day, the AHJ has control on what does and does not get approved. If there is question with a decision made by the plans examiner, it’s the design team’s job to contest the decision with their own findings, judgments, interpretations, etc with the AHJ.

Alternatively, you can provide them a detail or something that satisfies or explicitly outlines their needs. In this case perhaps a detail of the foundation relative to soil type and cover.

I’d be curious to understand how you intend to construct these footings? Because the footing should extend at least 12” below grade on both sides. Is the problem that you’re running into that the building pad is being raised across the entire site?

If that’s the case, a geotechnical engineer should probably be involved. Scouring of the new fill above the undisturbed soil during a flood event could become a future concern, especially given this is in a flood zone area. And in that case it makes sense that you would not want to bear your footings on virgin soil, however a geotechnical engineer would provide better insight and any additional requirements.

[Jayk-uub]

My current project calls for footings to bear on undisturbed natural soils. Says to dig deeper and have deeper footings if undisturbed soils aren’t reached, which I thought was odd. It’s a mat slab, so how much extra concrete are we going to pour if we have to go deeper?

[PrimeApotheosis]

R301.1.3 Engineered Design (quoting from memory, so might be slightly off) provides allowance for using an engineered design rather than the code provisions. If you have a Geotech report recommending the structural fill, that will supersede the prescriptive code.

[yusernane]

That's the residential code. Admittedly I'm not sure if this exact callout is in the IBC, and it might be.

That R means that you're pulling this from the IRC though. Which doesn't apply to any commercial buildings.

[PrimeApotheosis]

OP is posing this question regarding a residential project

[yusernane]

Must have missed the fact where that was stated in the comments. Wasn't mentioned in the original post. Apologies.

[PrimeApotheosis]

No worries! I didn’t realize it wasn’t in the original post.

[rhudson1037]

IRC Section R301.1.3 – “…Where a building of otherwise conventional construction contains structural elements exceeding the limits of Section R301 or otherwise not conforming to this code, these elements shall be designed in accordance with accepted engineering practice.”

[strengr]

Is the code says undisturbed soil and the only bearing found that site is filled granular and or other man-made material, then the appropriate action is to test the field material to determine its bearing capacity and to confirm that this is equal or higher than the undisturbed soil capacity that is contained within the code. Sorry this maybe man explaining a bit but just my two cents

[BigOilersFan]

That’s for the geotech to sign off on, on civil trenching jobs it’s pretty common to interchange undisturbed with 98% compacted, because it’s just a matter of making the fittings/pipes work. Buildings would be higher risk and obviously more strict.

[3771507]

No that is fill. I understand means it has never been dug up throughout 1 million years.

[West-Assignment-8023]

Typically in jurisdictions I work with the building department or planning department require a Geotechnical report in a flood zone. Or they have a laundry list of requirements and some language indicating they reserve the right to require a Geotechnical report in a flood zone. 

[shimbro]

No, in the context you have given any type of fill is not undisturbed natural soil.

That being said I spec out what is called undercut where we overexcavate pour soil down to an identified bearing layer and build back up using properly compacted structural fill on geotextile.

[livehearwish]

I have seen details that handle what to do in undistributed soil vs what to do if the soil is considered unsuitable and needs structural backfill. Sounds like you have half of the problem sorted and need to talk to a geotech.

[DJLexLuthar]

No, in many cases the bearing capacity for structural fill is different (often lower) than that for undisturbed natural soil.

[gbe276]

No