Shallow Footing Converted to Pile Cap

[phamENG]

So I've got a weird one. Designed a renovation using spread footings. Contractor just bought a new toy, so he decided to put in a single helical pile under each of two point loads without telling anyone. Just found out this morning, so I'm still gather information, but I'm wondering what everyone thinks about ignoring the helical pile and justifying it based on the fact that the spread footing I designed works? If I try to look at it as a pile cap, 1) the concrete design will fail and 2) it'll be considered unstable as there's a single helical pile under it. My concern with ignoring it is that it's there and could cause a punching failure.

The city said go ahead, but have an engineer say it's okay. I'm not excited about it. I may tell them they need to add two more for each point load and put in a proper pile cap, but I'm not excited about that either.

 

[TLHS]

I don't like that at all.

The problem is that you don't have the deflection compatibility with your assumed load path. Your helical pile is a hard point and your math for the shallow foundation assumed movement of the soil to get both bearing capacity and lateral capacity. You're going to either have to rationalize out how it's going to move with the pile now so you can see if it'll still activate what it needs to activate, or you have to switch to designing it like the force is going into the pile.

There's ways to design composite foundation systems where load is partially taken by piles, but I suspect that you don't have enough information to do that properly, and even if you did do you have enough local capacity in the structure that you are comfortable that you won't have a punching failure or something before there's load sharing. Also, that's generally done when you're in soft soils where you will expect the piles to move significantly too, and you have a regular grid of piles that share load consistently and a reasonably rigid foundation. This situation is not really ideal for that. It feels a lot more like your local point load will mostly just find the stiffest load path, which is the pile, unless the piles are really wussy and won't stop the footing movement, but in that case I'd likely design the footing to the expected capacity of the pile, which it sounds like you can't make work.

Can you get an idea of the comparable stiffnesses of the systems? Spring stiffnesses for each? If so you can try to model the situation and see what happens. But then you'd need to think about what the potential soil movements are in the future. Will the bearing strata have non-elastic settlement, erosion or other movement and to what degree, because the footing won't follow the ground until the pile's moved that much too.

 

[Celt83]

"Contractor just bought a new toy...."
Any confidence the pile got installed correctly?

Foundation poured already?


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[phamENG]

TLHS - all great thoughts and I'd love it if I could. But this is one of those "small project, big headache" deals. Available data is essentially nil. Thanks for confirming my instinct on it.

Celt - no, fortunately they haven't. I should be getting photographs of existing conditions and installation data (torque, depth) as well as the manufacturer sometime today for evaluation. As for correct installation - no idea. I'm hoping (in vain, I'm sure) that the install data will come from some testing lab that observed it.

Cheapest option will probably be recommending that they remove the plate from the pile, dig down 2 feet, cut it off, back fill with stone, and pour the footing as designed. Though I'm sure he sold this to the homeowner as an improvement, got the homeowner to pay for it, and this will cause a whole pile of legal trouble for somebody....just so long as it isn't me...

 

[Celt83]

"no, ..."

Tap a geotech and see how far they would need to go back in and overdrive the pile beneath the footing bearing elevation to limit the influence.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[phamENG]

Good idea. Thanks.

 

[JStructsteel]

Contractor, please have your engineer prove the spread footing still works, and submit to me for approval.

 

[phamENG]

JStructsteel - doesn't work like that on these little residential remodel projects. I could force that, but then I'd have to deal with another engineer, the back and forth, the eventual fix, and probably never get paid for any of it. I'd rather just fix it and be done.

 

[kipfoot]

Cheapest option will probably be recommending that they remove the plate from the pile, dig down 2 feet, cut it off, back fill with stone, and pour the footing as designed.

I think that this sounds like the best answer. Simplest..to explain and execute. Cheapest to implement, just sunk cost for the contractor. Quickest...no outside people involved to spend time coming up with other solutions, no more of your time brainstorming for alternatives to redesign.

 

[jayrod12]

I may be willing to play ball on this one depending on the overall framing scheme. Depending on the depth of the helical piles, they do act as an end-bearing type foundation and may potentially not be bearing significantly lower than the footing is in terms of materials. So you could potentially just make the changes to the concrete work and utilize the piles.

 

[KootK]

Cutting the pile down and replacing the footing may well be the best approach. That said, there are situations in which I would willing to consider leaving it be with a few tweaks. How close is the sketch below to the real / possible conditions that you have? Given that you mentioned concrete punching failures, my guess is likely incorrect in some dimension.

 

[dik]

Does the new construction stabilise the top of the pile to prevent rotation? and is the pile capable of supporting the load?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[ukengineer58]

check the pad for settlement if it doesnt settle to much pile will never receive the load. Check the theoretical pile load for punching shear and shear if the base. If both conditions are ok youll probably be ok. Or tell them to cut it down.

 

[phamENG]

dik - pile is capable, but the new construction was never designed to stabilize a pile, so it would take some gymnastics to prove that it might work.

KootK - finally got some pictures. There are two point loads spaced about 3 feet apart, and the contractor drove a single pile between the two with the helical plate sitting above the cage. So it'll punch if it doesn't crack in half in negative bending first (they put in the rebar for the spread footing). Now I'm thinking the easiest solution would be removing the plate, putting a PVC sleeve around it to the top of concrete, and then just pouring around it...

Either way, I'm not getting good info from the site so I'll be heading out there tomorrow to figure it out.

Thanks, everyone, for your thoughts.

 

[KootK]

With that being the condition I'm definitely opposed to it, barring the ability to design the pile cap properly.

I'd considered the sleeve option as well and like that. I was confused initially because it sounded like it was one pile directly under one load.

This is a pedantic concern but, even if you take the pile down a few feet, it's probably still a hard spot unless you fill above it with something compressible.

 

[phamENG]

KootK - I thought that's what they did, too. But it turned out to be worse. I agree on the concern - that's why I thought of the sleeve. Didn't want to when I thought there was a point load above, but now I'm certainly open to the idea.

 

[BAretired]

I prefer two more piles, each as close as possible, and aligned with the point loads, with a pile cap straddling all three. Otherwise, the moment from a single misplaced pile could fail it in bending.

BA