Increase capacity of existing foundation 3

[mte12]

Can this be solved practically?

For an existing pad footing buried in the ground, supporting a column, where the load increases from say 1000kN to 2000kN (in compression), how could one or both of these problems be solved.
1. Crushing in base plate. Because base plate is relatively thin, compression load cannot be transferred to grout/concrete.
2. Bearing capacity of foundation.

Criteria is no vertical settlement of column, and load transfer to be guaranteed.

 

[Once20036]

I think that the first problem is pretty straight forward with some creative thinking.
I don't see any possible solution for the second question and "no vertical settlement" criteria. Everything moves.

 

[phamENG]

1) If there's enough grout there now, shore up the column and slide a new baseplate underneath and weld it off. If not, shore up the column and cut the bottom off, welding a new baseplate on.

2) Impossible. As Once20036 said, everything moves. You can dowel into the existing footing to make it larger, but the soil under it has already consolidated while the other soil has not...so some settlement is guaranteed.

 

[TLHS]

Item one can likely be solved with stiffeners.

Item two is heavily situationally dependent.

Things to ask:
-Does the existing footing have reserve capacity because of design choices made (e.g. is the footing larger than necessary to match other footings, is it larger because of practical sizing constraints)
-Were the original geotechnical assumptions conservative
-Does the performance of the existing indicate that there is likely reserve capacity, assuming it is deflection governed (this lines up with the above)
-Could additional geotechnical investigation potentially result in higher justifiable capacities?
-Can you find additional capacity by looking at settlement and ultimate conditions separately

I'm definitely comfortable finding 10-20% in a typical footing most of the time, and it sometimes has significantly more reserve capacity than even that. Smaller ones tend to have significantly more reserve capacity because there's a minimum practical size and people tend to round up on things. Doubling capacity is a pretty big ask, though.

What about concrete strength in the footing?

 

[phamENG]

Even if the footing has an additional 20% capacity (or 100%, since we're discussing it), the soil still hasn't seen that load. It may not fail, but it will likely consolidate and cause the footing to move down a non-zero amount.

OP needs to get real about the criteria.

 

[TLHS]

Oh certainly, just figured the guys above me had that part pretty covered! Stuff is always going to move.

 

[phamENG]

Sorry - I don't trust people on the internet. Too many threads go something like this:

Responses 1-60: you shouldn't do that!
Response 61: well it might work if you hold your breath and stand on your head in a hailstorm during a full moon, but remember everything 1-60 told you.
Response from OP: great, thanks, #61! We'll keep going with this!

Confirmation bias is real...

 

[KootK]

1. Crushing in base plate. Because base plate is relatively thin, compression load cannot be transferred to grout/concrete.

As TLHS suggested, my first stop for that would be stiffeners on the base plate if space allows.

2. Bearing capacity of foundation...Criteria is no vertical settlement of column

If the soil is suitable for helical piles, you might install several about the perimeter of the footing and then pick up the underside of the existing footing will some manner of bracket that can be preloaded. In this sense, the existing footing would become a pile cap of sorts and the usual checks for that would apply. This should get you about as close to zero settlement as any right thinking client would have right to expect. This would have to be done skillfully, however, lest the pile install and preloading operations wind up inducing the very movement that you hope to preclude.

It might even be feasible to use a similar solution where you grab the pier instead of the footing.

Is this solution practical? Hard to say. Practicality, like beauty, is in the eye of the beholder / costing matrix. Would this be a practical way to add some new equipment out at CERN? Probably. Would this be a practical way to open up the kitchen at your grandma's house? Probably not.

 

[TLHS]

Going down the "I want to install screw piles for some reason" pathway just for fun and in a way that does not help with the original question really...

That would create an interaction between deep foundations and shallow foundations that can sometimes cause problems. Because the shallow and deep foundations have different stiffnesses and act at different locations, you can end up with unexpected results. You can accidentally restrain things like soil expansion, screw up load distribution that assumed reasonably even elastic foundation response, cause problems with assumed seismic rocking actions, and all sorts of fun stuff.

Also, I counter-propose driven piles into firm rock as our zero-est settlement option, with jacks at the bottom of column to adjust in the future if necessary. Not quite sure how to compensate for the elastic movement of the rock and pile, though. You can spread load out even more with pile groups, but that just reduces magnitude proportional to the bearing area.

 

[KootK]

Also, I counter-propose driven piles into firm rock as our zero-est settlement option

I'd be worried about the driving operation causing the settlement that OP seeks to avoid unless the thing is shored rigidly somehow during driving. But, then, whatever that shoring would be probably makes sense as the final solution anyhow.

That would create an interaction between deep foundations and shallow foundations that can sometimes cause problems.

Pish posh... puritan solutions are the domain of new builds. The criterion here is minimized settlement. Deep foundations is that. Let's not let the perfect be the enemy of the good.

 

[KootK]

Criteria is no vertical settlement of column, and load transfer to be guaranteed.

As pham has intimated, this language is the particular dialect of the unsophisticated client. Proceed with caution and break out the bear suit. Your first order of business on this assignment is probably an aggressive campaign of client re-education.

 

[mte12]

Thanks for responses.
It looks like people are getting upset at the question. The first line in the original post does have a question mark at the end.
The last line should have been worded as "no movement of column and guaranteed load transfer following new works". Understand there will be settlement of ground with higher load, and interaction with new works.

The situation I'm sure has come up in the past.

KootK has mentioned helical piles. Others may have experience with diaphragm wall installation, for which I know little of, I'm sure it's costly and may affect existing works during installation. Perhaps a thick raft foundation above pad, etc.

I'm sure each one of us has the sense to interpret a response and think critically of it.

A thank-you gesture at the end simply means thank you for responding.

 

[StrEng007]

I don't think anyone is getting upset at the question.

However, you have asked a group of structural engineers, several of who I hold in high regard, for their guidance on how to best handle a situation. Most will assume themselves in your position and think about how they'd sign off on something like this. The friction you feel is that to which we all struggle with from time to time.

By now I'm sure you're seeing there isn't a straight forward solution on this. This is an exercise in experience, cost, constructability, and acceptable levels of tolerance for the work once it's been finished. It might be prudent to educate your client (as others have suggested) and work to agreeing on what constitutes an acceptable outcome once the work has been installed. Example: Even the most sensitive machinery has tolerances to which foundations can be agreed upon.

 

[mte12]

Thanks StrEng007, understand it's difficult to solve and should have phrased the question better.

 

[StrEng007]

Yeah, I've been there before. I've even had some really great solutions presented to me and struggled to see it clearly.

Step away from it, get a clear head and come back. Some of these folks make it look so easy.

 

[phamENG]

Some of these folks make it look so easy.

Everything's easy when you don't have to sign off on it. I can pontificate on here all day long...but when it comes time to put my seal on a drawing? Let the hand ringing begin!

KootK is probably the most honest and direct about the realities of balancing practice and engineering rigor.

mte12 - nobody's upset. "No column movement" is just as bad as "no vertical settlement of the column." We just want to make it clear that, whatever advice we give, you're probably not going to meet that criteria. "Column can have no more than 0.25" of vertical movement" or "Column must have the capability of being adjusted back to original position with +X/-Y tolerance" are attainable design criteria. But "no movement" is the realm of sky hooks, unstoppable forces, and immovable objects.

 

[Once20036]

But "no movement" is the realm of sky hooks, unstoppable forces, and immovable objects.

And perfect gusset plates

 

[phamENG]

Well said, Once20036. Well said...

 

[KootK]

KootK is probably the most honest and direct about the realities of balancing practice and engineering rigor.

I'm going to choose to take that as a compliment, in which case it has made my day. I would give you a star but, as you know, that would not be consistent with my personal philosophy of prudent star giving.

What "balance" I have is something that was developed, first and foremost, as a way to sort my own self out. My structural engineer wife is fond of saying that my "gift" in life is the ability to make any problem so complicated that I can no longer solve it.

 

[phamENG]

I'm going to choose to take that as a compliment

Good; it was meant as one.

my "gift" in life is the ability to make any problem so complicated that I can no longer solve it.

I may never be as smart as you, but at least I know we have something in common...