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 missed that link, thanks for sharing. Not sure that method is really used here, interesting to see though.
If doing that in short sections then yea, option A could be a goer. Still have the issue of the mid height hinge but that's a case-by-case review of soil height and existing footing construction, definitely a solvable issue

 

[phamENG]

PEinc - nice pictures. What some people think is safe just blows my mind.

 

[Apache1]

By the way...your concern about the overturning resistance of the underpinning is warranted...Link

Oh gosh...now I'm thinking I should not take on this project...

What's missing from your A that is related to my comments on B/C is some method of at a minimum ensuring full bearing...

So something more like this...

Ok, so waterproofing on Option A?? Membrane on the inside of the basement that goes down to a drain in front of the extended toe? Membrane on the exterior (which would require digging around the outside of the home)?

 

[Greenalleycat]

Apache, I think if this were me I would be looking for a contractor with specialist experience in both the construction and waterproofing of these types of works
Complex but manageable problems that would be best led by the crews who have done it before, in my opinion
Your design is the icing on the cake to give them the specifics once the methodology is sorted

 

[phamENG]

Like I said, search for Blind Side Waterproofing. You'll still have to be a bit creative and consider all the various paths that water could take since it'll be broken up a bit...or you do the piers, a layer of blind side waterproofing, and then another wall inside. But I'd work with the architect on that.

 

[Apache1]

Like I said, search for Blind Side Waterproofing

Ah, I was thinking the blindside waterproofing would work with Option B and C, but not Option A. With Option A I'd think the waterproofing would have to extend up the exterior side of the existing footing and wall, which would require excavating the exterior face of the wall - which would eliminate the need for blind side waterproofing.

 

[phamENG]

Pull too hard on that string and your whole sweater is going to unravel.

You need waterproofing behind your new wall, so if you're eliminating blind side water proofing that means you're excavating inside the crawl space to remain to apply the waterproofing to the backside of the underpinning. That excavation will need to be sloped back for safety and carried to the ends of the walls. What does that do to the perpendicular walls? You'll have to underpin those and do the same thing. Eventually you'll be left with the whole building up in the air while you try to build a new foundation. (Not a terrible idea - I've done several of that type of project, though the goal was elevating the house and not putting in a basement.)

 

[PEinc]

Apache1, if there is ground water above the bottom of your planned excavation subgrade, get rid of it before you start digging underpinning pits or you will have economic and performance problems. You should not try to underpin below a water table. Investigate if you actually need waterproofing behind the existing wall and underpinning. Before pouring concrete into any , properly shored (and dewatered, if necessary) underpinning pit (Option A), you can place bentonite panels against the pit shoring boards and maybe even some long, triangular, bentonite tubes in the rear, vertical corners of the pits, against which the concrete can be poured. Trying to install some type of WP membrane that needs to be lapped and sealed will be harder and more expensive. You could also consider installing a narrow (about 12" wide) geo-composite drain board, like Miradrain, vertically against the outside face of alternating underpinning piers and then run it out under the piers to an in-basement collector pipe system, or French drain, and sump pump.

http://www.PeirceEngineering.com

 

[Apache1]

You should not try to underpin below a water table. Investigate if you actually need waterproofing behind the existing wall and underpinning

There's no groundwater here to speak of. But if the owner has sprinklers or it rains then won't I need waterproofing?

Pull too hard on that string and your whole sweater is going to unravel.

Yeah, but I hope you see what I'm saying. If the owner has sprinklers or if it rains then any water that is up against the existing wall will make its way down the wall, down the existing footing, and into the seam between the bottom of the existing footing and the top of the underpinning. Blindside waterproofing would only be able to go up as high as the top of the underpinning, maybe a bit higher, but not far enough to prevent water from getting behind the waterproofing itself. Am I straining at gnats here? The waterproofing just seems ineffectual to me if it can't go up to the height of the grade.

 

[Enable]

The wall above is restrained by friction. I can get away with that here because our frost depth is only 8", so it's rare to find a house with a bottom of wall footing more than 24" below exterior grade, and in many cases the crawl space is depressed so there's even less being "retained" by the existing wall once the exterior soil is stripped away.

Thanks for bringing frost depth into the discussion as I think it's important to mention that frost jacking can be a real concern when solely relying on friction between old / underpin to resist lateral pressures. We often concern ourselves with frost heaving but if a surface is irregular, as the case of a stone rubble wall is apt to be, the foundation can be jacked up by expanding water into those irregularities even if the footing is above the frost line. This can substantially reduce the DL friction between the old / underpin.

Apache1 the appropriate waterproofing solution here is to realize there isn't one. Talk to blind-side manufacturers and install one of their systems, but realize it's going to leak. The play is to install blind-side along with an interior weeper system.

 

[PEinc]

There's no groundwater here to speak of. But if the owner has sprinklers or it rains then won't I need waterproofing?

If there has not been any water problem, why would you expect one to now develop?

With respect to frost heave, has there been frost pushing the existing crawl space wall? What is the depth of the current foundation wall vs. the design frost depth? It should not be any worse than it has been for years. Therefore, should there be any significant concern when the basement is deepened?

http://www.PeirceEngineering.com

 

[ThorenO2]

Great insight from everyone on here for an extremely challenging engineering problem - annoying that homeowners think it’s an easy one and architects think they can handle it. You really have to have a mastery of engineering principles and absolute confidence to take these projects on. One thing that I would find necessary for any design, and which would be money well spent by the owner is to get a soils report and Geo’s input.

 

[EdStainless]

I worked on some old houses that had been modified like this. A couple of them were done by hand (digging a full basement and mixing cement).
The best of them had dug a new footing at the desired depth just inside of the existing footing. Then they put columns in every 5" or so, usually brick or block right to the bottom of the existing footings.
And then formed and poured a continuous interior wall.
The result sort of looked like a hybrid of your option A and C.
I am not saying that these were done right. None of them were actually engineered, just built by the farmers that lived there.
But they had been standing (with no issues) for 60+ years supporting large 2 story homes.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Apache1]

Thanks everyone for the input, pictures, sketches, etc. I've learned a lot from this thread, and I hope some of you have as well. And I also hope it will help future engineers many years down the road. I know I've learned a ton of stuff from threads nearly 20 years old!

I wanted to post an updated detail and note a few things I'm still thinking about:

- I've run some numbers on the underpinning wall and it appears that I will need a 3' wide "footing" at the bottom as shown for soil bearing and overturning. Thinking the wall and "footing" can be a monopour?
- With this 3' wide footing the numbers seem to say that I don't need any reinforcement in the wall. Am I ok to ignore the minimum reinforcement requirements from ACI?
- I've shown a 4" gap at the top of the wall where the forms will need to be angled out to provide a "slot" for the concrete to pour into. Many underpinning details show the edge of the existing ftg being cut off to allow for a slot to pour into. I don't feel good about doing that since I think doing so would create eccentric loading on the existing footing from the existing wall.
- I've shown a note for a weeping tile system that is to be installed at the toe of the wall. I have noted a blind side waterproofing system at the back of the pit. Hoping these two systems combined will provide adequate waterproofing.
- I'm discovering that having negligible dead load coming down from above is a definite drawback with underpinning design.

 

[Greenalleycat]

Some thoughts from me

- 100% you put steel in this, even if just for shrinkage sake.
Shrinkage cracks in the wall mean your waterproofing is at risk of compromise. Plus they will be unsightly.
Even ignoring that, I would not want to leave steel out a basement wall just to save a few bucks...consequence of failure vs cost incurred now vs cost to repair later ya know

- Will they do a separate wall + footing then slab pour, or is it better to do a slab + starter bar pour then do the wall separately?
This is probably a contractor question more than anything
If they do slab + starter pour then you eliminate the potential weaknesses in the unconnected slab to footing junctions and give your footing much more capacity and redundancy

I'm going to be honest that I don't like this detail though
Given your soil line is above the dry pack line, the success of your wall seems to be largely dependent on your contractor's grouting ability when filling that 4" gap
I know there are shitty 4" stone walls etc etc that show that things can hold up even when the only structural systems evident are thoughts and prayers
But we aren't designing those, we are designing robust systems that are supposed to have guarantees of success (or near enough to)
I don't like relying so much on that grout pack - it seems like a failure plane or conduit for water that has zero redundancy if workmanship isn't good enough, if materials fail naturally over time, or if unexpected forces come into play
There should be reliable steel and concrete forming your load paths and waterproofing

To be open though, this isn't a problem I've ever tackled personally as it would never be done in my city due to EQ, swamps, and high water tables, so perhaps I fear needlessly
Particularly as seismic designs govern our thinking here

 

[PEinc]

Apache1, underpinning piers are relatively massive and, therefore, usually are not reinforced due to very low compressive and bending tensile stresses. Also, the bending tensile stresses get reduced by at least the DL of the building and weight of underpinning concrete. Remember, unreinforced concrete underpinning should be stronger than a poorly mortared and always unreinforced, rubble stone wall of the same size. However, you do need to check your belled-out toe for any required reinforcing steel. You don't want it to break off if it is longer than it is thick.

http://www.PeirceEngineering.com

 

[Greenalleycat]

@PEinc, is this really an underpinning pier, or is it a basement wall that has a secondary function to support an existing foundation beam above? To me they are distinct things but perhaps they are the same in your lingo?

 

[PEinc]

Definitely underpinning. Do it all the time.

http://www.PeirceEngineering.com

 

[Greenalleycat]

Sweet as. Regional difference then

 

[PEinc]

See recent project pdf.

www.PeirceEngineering.com