Footing on a Footing

[Glennman CBO]

I recently showed up to a job site where they had excavated to the bottom of about 3 ft of loose fill so that they could get to the original grade. The excavation consisted of a series trenches approx 24" wide following the foundation footprint. They planned to fill the trenches with plain concrete (no rebar) about halfway up (18" to 24" deep), then proceed to build forms for footings and walls on top of that. The plans showed rebar in the footings.

The building is a 2 story town house.

Code 2006 IRC. There is no enginner involved, but there is an RDP.

I failed the inspection, told the pump truck driver it was not approved, and the owner had to send the cement truck back. I told him to get approval from an engineer. Obviously, they shouldn't have had the inspection scheduled at approx the same time as the inspection.

My supervisor told the guy to go ahead and pour, but not to do it again (which I obvoiusly didn't agree with).

If it's not OK to do it again, then why is it allowed now? That's another story.

However, it seemed odd to me that they would pour a footing that has rebar, on the top of a strip of concrete that doesn't have rebar, but I cannot say I have a code section to back me up.

Any thoughts?

 

[Paul Sweet]

They may have been using flowable fill or low-strength concrete as fill material. In either case the fill is much stronger than the ground under it. I don't see any problem with this.

 

[peach]

If the plans show re-bar, it needs to be there. It's a function of the soil whether or not the re-bar is required, but that's not the inspector's call. You did the right thing.

 

[RJJ]

Sometimes sub footings are place in areas of poor soil conditions.!?? However, I would agree you probably made the right call. Big question what was going on top of the second footing? Or was the top of the footing the first floor?

Any soil test done before construction started ? Must be other homes if a townhouse!

 

[peach]

I've seen designers (even registered design professionals) allow a slurry mixture with questionable (sometimes really bad) soil as a substitute of taking the bad soil out. These "footing/slabs" are not a replacement for the footer.. they are a replacement for the soil. The footing is designed to sit on the new "soil".. The slurry slabs can sometimes can be REALLY big... since they are a replacement to the soil!

 

[JBI]

If it isn't 'prescriptive', then Engineering is required... Seal and Signature as well. JMHO

 

[Glennman CBO]

Thanks all. The primary issue is that the contractor had called in the inspection at the same time as the concrete pour. There was no time to even research the issue, the pump truck was set up, and the cement truck was turing in the driveway. If I was the contractor and I had planned to do something that was out of the ordinary, I would discuss it with someone prior to scheduling the concrete.

 

[RJJ]

Glennman CBO: welcome to my world! I would have failed the inspection also. It should have been discussed prior and more then likely would be ok.

 

[brudgers]

Suppose they had excavated similarly for the bad soil and back filled with gravel.

Would you have required engineering?

 

[Yankee]

Suppose they had excavated similarly for the bad soil and back filled with gravel.Would you have required engineering?

R403.1

Footings to be engineered if not on undisturbed natural soils capable of supporting the imposed loading.

 

[cboboggs]

Yep, we require engineering if the footing does not bear on undisturbed natural soils.

 

[Glennman CBO]

On the pea gravel. If the soils at the sides of the excavation is loose fill, then how good is the pea gravel if it is able to push out the sides?

 

[peach]

don't use pea gravel on really bad soil.. there are soil stabilization methods that are "engineered" solutions to removing 20' of muck. Don't approve it on your own.

 

[SCBO1]

Glennman,

I was called to do a footer inspection and every thing was looking good until I turned the corner and there it was; a big "ol" rock. Fella's did you know you had this boulder in your footing, "ya" the concrete trucks on it's way. You might want to tell them to go back! Later the engineered design had 2" dow board over the top of the rock and stamped it.

"ok to pour boyz!"

 

[Glennman CBO]

That's great. The "footing" I was commenting on ended up with huge cracks in the bottom concrete (it was not reinforced). Also the footing (upper footing) ended up half way on the concrete portion and halfway on the dirt (in some places). We are still dealing with them.

 

[Mark K]

A major limitation of the IRC is that it doesn’t do so well when the assumptions made in developing the simplified rules are not true. When these assumptions are not satisfied you typically need to engage somebody with the expertise to decide how to deal with the situation. Typically the building inspector and the contractor do not have this expertise.

The concern is not with plane concrete used in place of dirt fill. Just look at it as an other way of filling in the soil so that a code compliant building can be placed on the top. The concrete fill will be stronger than the earth fill and you will not have to worry about compacting the soil. When used in this manner there is no need to reinforce the concrete fill. This is done all the time. You do not add reinforcing when you compact earthen fill.

The fact that they were encountering loose fill concerns me. In a situation such as this I would strongly recommend that the Owner have a geotechnical or structural engineer look at the foundation to determine if the excavation was deep enough into firm soil and whether other modifications were appropriate. Foundation settlement and related problems can be expensive to fix after construction.

The secrete to successfully using the IRC is understanding the limits of its applicability.

 

[peach]

concrete only know how to do 2 things.. get hard and crack..

Big cracks are usually soil/compaction issues.. little cracks are usually shrinkage cracks.. RE-bar helps with #1... WWM helps with #2.