Converting Crawl Space to Basement 2

[Apache1]

Hello everyone, question here on converting a 3' tall residential crawl space into an 8' tall basement. Prior to making this post I have read through every thread I could find on this site about underpinning multiple times. Please consider the drawing below showing the dimensions of the crawl space that is to be converted to a basement. At my old firm we would spec out something similar to the detail below. However, after reading through old posts it seems that this detail is not favored by many of the engineers here. It seems that the standard recommendation is to underpin the existing wall/ftg with a new wall that matches the width of the existing footing (in my case, that would be 20" thick). My concern, which has been brought up by others previously, is what about the "hinge" between the existing footing and the new wall? I've seen some of you say that if there is enough dead load on the existing wall then it will provide the overturning/sliding resistance needed so that we don't need to worry about the hinge. I've also seen some of you say that the new wall needs to be quite thick (30" or more) to prevent the overturning/sliding.

Well in my case I have an entire 31'-4" length of wall that runs parallel to the framing so it won't have any dead load from above providing resistance. Also, the existing footing is only 20" wide. PEinc in particular has said that doweling rebar into the bottom of the existing footing is not a good practice. Even if doweling was acceptable, wouldn't the dowels need to extend 18" at least up into the existing footing and wall? Seems impossible that that could be done correctly.

To summarize: Does lack of any dead load preclude underpinning? If I underpin, what about the hinge? If I design an l-shaped cantilever retaining wall with load from the existing footing, what is the problem with doing that? I don't have a soils report. I appreciate your thoughts and comments.

 

[Greenalleycat]

I don't understand what you're worried about in terms of this 'hinge'...can you elaborate?

My definite concern with this is that it seems the stability of your house footing is dependent on surcharging your new 5' basement wall.
To me this isn't an underpin. Underpinning is digging under a foundation (discrete or continuous) to support the foundation on an identified firm bearing layer
We do a lot of this work with post-earthquake repairs here
In your case, my first thought was to look at using short piles under the footing to the same depth as your new basement foundation
This would ensure that the success of your house isn't dependent on your new basement wall

 

[PEinc]

I assume that the existing building with crawl space is stable for sliding and overturning. If you properly underpin the existing building, the upper, existing section should still remain stable. Worry about installing proper underpinning. Don't use the sketches you show.

http://www.PeirceEngineering.com

 

[Apache1]

Greenalleycat,

The hinge I'm worried about is that the new underpinning is now its own cantilever retaining wall beneath the existing ftg/fnd. Most of the details I have seen posted here show that the new wall matches the width of the existing footing, but the new wall doesn't have a footing itself. Since this new wall is a cantilevered retaining wall I would think that it needs its own new spread footing at the bottom (see below).

PEinc, are you assuming that the existing wall is stable since it's so short?

 

[XR250]

I either use helical piers or slotted-in CMU piers at 6 ft. O.C. +/- to carry the gravity loads. Then, the basement wall only has to be designed for normal, lateral earth pressure. I would def. let a portion of the footing bear on the new foundation wall. I see these fail all the time around here - albeit ones that were not engineered at all.

 

[Apache1]

Thanks, XR250. It's the lateral earth pressure that I am worried about. This new wall will be a 5' tall retaining wall which is why I think it needs a footing (to provide overturning and sliding resistance). I think the footing would need to be wide enough to provide this resistance that the gravity loads wouldn't need to be handled with piers, unless I am missing something?

 

[Greenalleycat]

Yes, the new 5' high wall will need to be designed for earth pressures
Does your basement have a ceiling over it? Can this be used as a diaphragm to span loads out to perpendicular walls?
Where is the earth level? Is it flush with the top of the 5' wall or does it sit up beside the existing 3' perimeter beam?

 

[dik]

...as Apache, but without the strip footing at the bottom. You have to excavate in about 4' wide 'chunks' and you want to fill the space as fast as possible, should work with 18" footing for small residential loadings.

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

-Dik

 

[Apache1]

Greenalleycat, the soil is 6" down from the top of the existing 3' tall wall. Yes, the existing crawl space has a floor diaphragm on it. I see this diaphragm as bracing the existing foundation wall, but not the new wall. Are you saying that the new wall is "braced" by the existing footing sitting on top of it? The existing footing and the new foundation wall would not have a connection made between them (the footing just bears on top of the new wall).

dik, how can I get away with a new footing that is only 18" wide? This new 5' tall wall is a 20" thick retaining wall so won't I need the strip footing at the bottom to counteract overturning and sliding as with a regular retaining wall?

I'm wondering if you both are saying that the existing ftg that bears on the new foundation wall is providing bracing to the top of my new wall? How can that be when they aren't connected together with rebar or anything else? (note that PEinc has said in other threads that doweling rebar into the bottom of the existing footing is not a good practice).

Thanks everyone for your input so far.

 

[Greenalleycat]

I'm gonna be honest mate, I'm pretty confused as to the nitty gritty of the situation here, I feel like the sketch is missing some critical details.

If you sit your existing footing on top of the new wall then it will provide no significant benefit to the new wall unless they are connected
The nature of that connection is obviously up to your to design, but would need to include epoxied bars or other connection to allow moment to be developed across this junction for wall continuity (to span to the ceiling)
But, as you and others have pointed out, that could be fraught with difficulty and may well end up as something that only works on paper

My thought is that I would be looking to underpin the existing footing with piles or other at discrete locations
These piles would bear at below the depth of your new basement to avoid surcharging your new wall
The existing foundation would be required to span between these piles, so would need to be reinforced and have engineering design to check pile spacing
You would also want to consider the founding conditions of the existing footings to ensure you aren't going to make them perform relatively worse by doing so

Once this is done then you can look to excavate the new basement out (with propping as required to the existing footing) and create a new waterproof wall
You'd need some advice as to how to waterproof this though as you'd presumably only be accessing it from the inside to avoid undermining the existing footing too much

This new wall could presumably be much thinner than the 20" you've proposed - probably more like 8"-10" / 200mm-250mm, or even thinner if you can get a ceiling connection
This wall would be holding back 5' of soil pressures and so would need to be designed for those pressures
However, as you could now make this wall 8' high, you could bolt a ribbon plate to the top of it and tie it into the ceiling to avoid having a large cantilever footing
Hope that makes sense and that I haven't missed something obvious

 

[Apache1]

Greenalleycat, I think maybe you are looking at the original detail I posted (the L-shaped retaining wall) which the consensus around here - and in other threads - is that it's a bad idea. So what I'm proposing now is a standard underpinning detail (see below). In previous threads the advice has been to pour a new foundation wall in 4' wide chunks as dik says above. The new foundation wall should match the width of the existing footing, which here is 20". My question is that the details I've seen posted in here don't show a new footing at the bottom of the new wall. How can this new 20" thick foundation wall resist overturning without a footing? Hopefully that makes more sense. Sorry for confusing everyone lol

 

[Greenalleycat]

Yes, I was confused between the two images, sorry!

To me, underpinning only means to put a new support beneath a foundation to transmit bearing loads to a target layer deeper down
So, what you are doing with creating a new basement is not underpinning at all
Your question isn't about underpinning so much as it is "how do I create an 8' deep basement next to my existing footing"
That may be differences in local lingo though

As I said in my previous post though, I think my inclination is to support the footing on piles and construct a new wall to the side of the existing one
This maintains separation between the various elements and hopefully reduces risk
I don't really know how you waterproof the detail you posted above

 

[Ron247]

I have more questions than answers.

What is the existing stemwall made of? Hollow block?
How is the existing stemwall attached to the existing 20" foundation? Mortar only?
If you cannot connect the old upper stem with your new lower stem with a moment connection, the new foundation must resist overturning and sliding from lateral loads.

I attached 2 pictures of one I saw that was excavated 20 years ago to make a basement. Excavated but never got around to building the basement. What amazed me besides it survived 20 years while you can see erosion under the footing in the first picture is that the guy's wife and daughter hauled all the dirt out with 5-gallon buckets. This is well over 900 sf. Eroded over 60% of the footing width.

 

[Apache1]

Greenalleycat - Haha yeah so the "underpinning" detail I posted is similar to details shown in other threads about lowering a crawl space for a basement. In fact, dik posted one in another thread that looks just like mine except without a footing. The consensus in various other threads was that underpinning the existing wall and footing with a new wall is the best method for lowering a basement.

dik - I'd be interested to get your opinion on the footing question. You said an 18" footing should work, but I think it would need to be wider for a 5' tall retaining wall.

Ron247 - Existing stemwall is concrete. It is a standard concrete foundation wall, concrete footing, with (allegedly) dowels from the footing up into the wall. Very interesting pictures, sounds like they must have had a good time hauling all that dirt out!

XR250 - I noticed that you started one of the other crawl space underpinning threads a while back. It sounds like you're advocating for the detail you posted in there which looks like mine above (the L-shaped wall next to the existing footing) except the new wall pours under the existing footing a bit? Seems like that detail was frowned upon in your thread?

PEinc - Interested to hear your opinion on the new wall. The new foundation wall will be 20" thick to match the existing footing, but then I think there needs to be a strip footing approximately 3'-6" wide under the new wall to resist overturning? Greenalleycat also brought up a concern about waterproofing this detail.

 

[PEinc]

[ol ]
[li]Underpinning piers do not get their own, larger footings. Doing so requires more excavation, more concrete, and multiple pours per each pier.[/li]
[li]If the existing wall is stable now, it should remain stable above the underpinning because you are not changing the lateral pressure, on that wall.[/li]
[li]Helical piers supporting the existing, short, foundation wall will not help with resisting sliding and overturning of the underpinning. Keep the building weight on top of the underpinning.[/li]
[li]If you have a sliding problem for the underpinning piers, extend them deeper.[/li]
[li]If you have an overturning problem for the underpinning piers, either 1) bell them out in front, under the new, deeper basement slab, 2) install the front face of the underpinning inside your basement similar to the front face shown in the second sketch of your original question, or 3), but less efficiently, extend the rear, outside face of the underpinning piers outside of your building by a foot or so.[/li]
[li]Don't look for underpinning information on YouTube. There is a lot of crazy methods shown by "experts" that MIGHT work, sometimes![/li]
[li]Finally, if an 8 or 9 feet deep, 120 year old, basement can have a poorly-mortared, 15 to 20 inch wide, rubble stone foundation wall without any type of footing below the stone wall, then your short block wall with concrete footing sitting on top of a concrete underpinning pier should be more stable.[/li]
Apache1, I do not assume that anything is stable. I check it for stability.
[/ol]

www.PeirceEngineering.com

 

[dik]

It depends on the soil and loads. 18" wide and 16" wide is common for many residential projects. The added weight of the concrete is only slightly greater than the soil it has displaced, so the load at the bottom isn't increased by much. If it was already in a state of impending failure, then it's a problem.

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

-Dik

 

[Apache1]

PEinc - I appreciate your detailed input. I also appreciate your warning about not looking at Youtube. When I took this project on I was researching how to make it work and many of the methods I was seeing online didn't look kosher to me. I wish there were some more trustworthy sources to look at for this scenario. Even here on eng-tips I am getting some wildly different proposed methods (helical piers, new fnd next to the inside face of the existing footing, new fnd poured slightly under the existing footing, underpinning with 20" thick fnd, etc.) Then there's the question of waterproofing that others have raised on other threads, with no answers. Unfortunately my personality is one that needs things to "calc out", so with this kind of project I'm having a difficult time since basically all of the proposed solutions don't calc out.

dik - I understand that an 18" wide footing would work for simple gravity loads. The problem here is that the new wall is a cantilevered retaining wall - it is unbraced at the top. Yes, the existing footing sits on the top of the new wall but that is not a braced condition. In my experience an 18" wide footing would not be nearly adequate for overturning or sliding on a 5' tall retaining wall.

 

[jayrod12]

PEInc,

Can you explain this

2. If the existing wall is stable now, it should remain stable above the underpinning because you are not changing the lateral pressure, on that wall.

I would assume the existing wall has some, albeit possibly minor, embedment below the top of crawlspace grade preventing the bottom of that wall from pushing inwards. By removing said backfill, you no longer have that resisting mechanism. And if you don't connect the underpinning to the existing wall, then you really have nothing preventing the existing wall from pushing inwards except friction on the top of the underpinning. Maybe that's enough, maybe not. But then wouldn't you also need to apply a point load to the top of the underpinning, in addition to the regular soil loading, from the existing wall?

 

[kissymoose]

I'm not too sure if a solution to this problem has been communicated yet:

Without a decent toe/heel, how is this "underpinning" wall resisting the lateral load from both the soil (triangular distributed) and the existing wall (point load @ top).

 

[dik]

OK

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

-Dik