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So I am getting ready to start a job where I will be building a new addition with a full basement onto a house that only has a crawl space. My questions is about excavating next to the existing house. I will be digging about 4' deeper than the existing foundation, and I am concerned about retainment of the existing wall. Has anybody done something similar to this? Do you have a particular way to retain the dirt and not undermine the existing foundation? Here is a detail off the plans to give you an idea.
 

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I've done something close... once about 15-20 years ago.

In principal, for a basement, I had to undercut an existing spread T footer/stemwall to get a full basement height for a slab basement.

There were some differences though.

One my original foundation was already 4+ feet below grade and on decomposed granite. I had to undercut it only maybe for a height of 18-24".

And it was only for about 1/2 my exposed wall length.

It was not eng or permitted..... (don't jump me.. just telling the truth..details not necessary)

In that case, I cut in under the old footer (16 spread T) and placed 2 or 3 metal screw jacks supporting the original footer.... then carefully dug out the rest slowly (it all held fine but I wasn't sure) and poured the new stemwall/footer right underneath the old/original one.

I just cemented the metal screw jacks in. I poured same day that I undercut it.

Now my earth/granite was pretty firm... I am not sure I would have needed the screw jacks.... and of course I had alot less (18-24") of exposed/ possible sloughing earth to contend with.

I still know the people and they have had no problem. Believe it was late 80's early 90's we did it .

Good luck

I'm going to watch this thread, as at the time I could not get any great ideas answeres as how to do it..... so we just went ahead and did it..... not sure what we would have done if we had had a problem.... I did have a bunch of extra screw jacks available though.
 

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We've done additions next to structures with basements, basically the opposite of your detail and in those cases taken the new footing to where there is no surcharge on the existing one. When looking at the loading on the footing one draws a 45 degree angle from the bottom of the footing in all directions and that is the surcharge area.

From that it's obvious that your existing stem wall and footing and the loads they are carrying are going right into the middle, more or less, of the new retaining wall. You're either going to have to shore and move the new wall and footing under the existing wall assembly as Mtn Remodel mentioned, or design the new retaining wall to carry the extra surcharge from the existing wall and footing assembly. Concrete and steel is strong stuff, you already are designing the wall and steel rebar is pretty cheap so if a few more sticks are needed it shouldn't be a big deal but you need to make sure that you get it calc'ed out correctly.

I'm not sure what impact the drain will have but that needs to be considered also as pointed out.
 

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I'm surprised the plan does not call for underpinning in this case...
 

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I don't see how you excavate that without the existing footing collapsing into the excavation. Looks like you need a temporary shoring system under the existing so that you can install a new footing/ stem wall to carry both structures.
 

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diplomat
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Highly dependent on soils, but I wouldn't touch something like this without building the full height wall at least 4' away from the crawl wall, or whatever dimention is needed so the excavation projects less than 45 degrees down from the existing footing.
 

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It's a very common situation in a city - not an addition, but new construction or renovation with a basement next door to an adjoining building with a shallow foundation. Even with precautions, settling problems are common.
 

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It's doable, whether with Peter's approach or some other. But I don't think the bottom half of the wall is going to look like that.[/QUOT]

It is doable, but my main concern would be is the possibility of the soil settlement which happens under the foundation load and after excavating next to the footing of existing crawlspace and by taking the soil out along the side, that load can cause the soil to fail in bearing and in shear load as well.
I'm surprised whoever designed that didn't take that into consideration.
Not to mention if the ground gets saturated with water, or depending on existing soil condition, by weakening the side, it can cause soils to move, collapse, etc and that will cause structural issues down the road.
You basically can create seismic instability in the soil if you don't underpin the footing under the crawl space along that new basement foundation IMO.
 

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I've seen similar detailed drawings before, I would never build that detail.

As stated above, that is a candidate for underpinning, or stay away from the existing footing.

mudpad said:
The so called "angle of repose"
One time I used that term "angle of repose" in a conversation with a GC I work for....He thought I made the term up, he had never heard of such a thing.... :laughing:
 

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It is doable, but my main concern would be is the possibility of the soil settlement which happens under the foundation load and after excavating next to the footing of existing crawlspace and by taking the soil out along the side, that load can cause the soil to fail in bearing and in shear load as well.
I'm surprised whoever designed that didn't take that into consideration.
Not to mention if the ground gets saturated with water, or depending on existing soil condition, by weakening the side, it can cause soils to move, collapse, etc and that will cause structural issues down the road.
You basically can create seismic instability in the soil if you don't underpin the footing under the crawl space along that new basement foundation IMO.
I agree completely. "Doable" doesn't mean it's a good idea, or that the result would be exactly as shown in that detail. There's plenty that we all don't know about this project - why it was designed that way (budget?), what the conditions are, etc. Lag screws at 48" OC raises a few other questions in my mind.
If I were handed these plans, I can imagine some circumstances in which I'd agree to do that, and some in which I wouldn't.
 

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I've done one basement addition adjacent to a crawl...

We temp jacked existing floor, removed the existing stem wall, and formed the basement wall in its place.
 
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I suppose another possible solution (probably/maybe not econmically feasable)....

...would be to helical jack /support that footer/stem.... and then cement it all in.

just an idea....
 

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One time I used that term "angle of repose" in a conversation with a GC I work for....He thought I made the term up, he had never heard of such a thing.... :laughing:
Yeah it sounds more like the position of some nude woman eating grapes in a renaissance painting:laughing:
 
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