Concrete Foundation Underpinning Detail

[mfstructural]

I'm curious to get people's thoughts on this underpinning detail. I had an architect question why I'm cutting off the existing footing at the inside to extend the new concrete foundation wall up along the inside of the wall. First he asked how are they going to do that? I have personally seen one sided chop saws so I don't think it's a big deal to cut that off. I like the idea of having the new wall extend up the inside as it aids reducing moisture intrusion and also takes out some of the flexure in the wall. In essence, it functions like a retaining wall. Would you recommend having a footing/toe at the inside that extends below the slab?

 

[Tomfh]

What’s the construction methodology for your underpinning detail? How do you support the partially demolished footing while excavating and constructing the new underpinning structure?

 

[human909]

First he asked how are they going to do that? I have personally seen one sided chop saws so I don't think it's a big deal to cut that off.

It takes alot more 'having seen one sided chop saws' to explain this magic. That said, I am pretty ignorant about underpinning so forgive me if this is straightforward and I just can't see it.

This seems like a detail that is extremely hard to construct and I'd suggest needs a fair bit more information provided about shoring and construction order. Some might say that isn't an engineer's job. Though constructability IS an engineers job. It is little use designing something that can't be built or is extremely hard/expensive to be built.

 

[Trippelsewe]

I don't think cutting the footing is that big of a deal. If I was the builder I would saw cut the strip footing into slivers and chip away rather that cutting the face.

That said the temporary works on this looks a bit tricky, how are they going to excavate and build the wall? My goto detail involves first drilling bored piers at an angle from the outside to provide some temporary shoring. Excavation & construction on the inside then happens in sections so that there is still some passive pressure.

 

[mfstructural]

The construction methodology involves underpinning the foundation in 4' sections spaced at 12'. The soil under the 4' sections being underpinned is removed and concrete is bank poured below. Once cured, the next 4' sections spaced at 12' are installed. My main question is regarding the footing and not the methodology. I'm curious if people have included a footing/toe to resist the overturning associated with the lateral soil loads. I'm debating on adding that since the slab on grade won't be in place until the entire foundation is underpinned. in the meantime there won't be lateral resistance.

 

[phamENG]

That is different.

Typically with underpinning you don't do a footing. It's too awkward to dig that toe with any sort of accuracy and not have the soil above it cave in while...you know...a guy is in there with a shovel. If you need overturning resistance you get it by a) going deeper and turning into a sort of cast in place sheet pile (never done it, but it sounds neat) or you use some sort of tie back. Usually helicals, but could be a grouted anchor.

 

[AskTooMuch]

I don't any issues cutting it. It will be wire cut. Unfortunately, this type of cut are usually done by specialty contractors that specializes in cutting (and not just any gc) so that maybe over the client's budget.

 

[HTURKAK]

For underpinning of such perimeter grade wall , the common practice is , provide new wall with sizes around gravity retaining wall, without toe ..and deeper than the proposed new basement level..

 

[GeorgeTheCivilEngineer]

Typical approach here is to do mass concrete pins (phased manner with 48 hours between adjacent pins), then construct the reinforced wall internally so it can be properly tied into the slab and floor above. For shallow underpinning, just the mass concrete may be enough (edit - to create a gravity retaining wall). For temporary stability the earth dumpling (that is, soil in the middle of the room that will be excavated next) can be used to prop the walls/shuttering in the temporary case.

With OP's detail I don't think you'd get proper continuity of reinforcement - how would you place the first transverse steels? The complicated shape wouldn't be achievable and you're asking a worker to drill upwards into a masonry arching over their head? Have a care!

 

[HouseBoy]

I agree with the others who advocate working in stages/offsetting the underpins in a way that results in 3 separate pours.

I see limited value in the reinforcing steel.

The BIG ISSUE I typically need to wrestle with is how to instill resistance to the UNBALANCED SOIL.
I don't see how this detail will provide that.
There are some "tricks" that might be utilized such as extending the underpins sufficiently below the lower level slab so that a "toe" can be used to improve overturning resistance.
Still gotta get the concrete into the form too.

 

[PEinc]

In my opinion, the underpinning detail is a bad detail that does not represent typical underpinning.

http://www.PeirceEngineering.com

 

[phamENG]

In my opinion, the underpinning detail is a bad detail that does not represent typical underpinning.

OP - pay attention here. PEinc has quite possibly designed more underpinning projects than the entire forum combined.

 

[Trippelsewe]

Opinion on this detail below? Underpinning designed for gravity loads, retaining wall designed for K0. Excavation and construction of the retaining wall is carried out in stages with batters retained as required for passive pressure.

The negatives I can think of is poor bored pier bearing and loss of internal space. But so far builders have found this easy to carry out.

 

[XR250]

I would not want to be the guy drilling 12" up vertically into the old footing!

 

[human909]

I would not want to be the guy drilling 12" up vertically into the old footing!

That was certainly my first though! I almost did speak my mind about that but then I realised that I don't know the first thing about underpinning so I hedged my bets by simply mentioning vague comments about constructability.

 

[EcoGen]

I don't know much about underpinning but the one residential project I was involved in, a similar detail was used (without the the stem extending above existing footing). The underpinning designer specified dowels drilled up into the bottom of exiting footing. It might have been less than 12 inches but I thought doweling in from below would be pretty standard. But apparently people on this post think it's too difficult to do. Not sure what the alternative would be.

Regarding the overturning, I think someone mentioned it here, but in the calcs that I reviewed, the designer used the mass concrete weight to provide the required overturning resistance. Which means you may need to widen the underpinning to go further under the existing foooting.

Regarding sliding, the designer relied on the SOG but specified temporary HSS horizontal braces between the two sides being underpinned, to be removed after SOG is poured.

Again I've seen this type of underpinning detail specified in the past but not sure if there is an easier way of doing it.

EcoGen Consultants LLC
Structural Engineers
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