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]

@kissymoose, that's why I've been supporting building a whole new 8' wall in front of the existing footing..

 

[kissymoose]

Barring a design of the new 5' portion working as a retaining wall, I would prefer vertical steel wall restraints, say 6ft on center.

 

[Greenalleycat]

Even with that, I still don't see how you could reliably seal that gap for waterproofing and long-term durability guarantees

 

[Apache1]

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).

Agreed, which is why I feel like I keep asking the same question over and over. PEinc has suggested belling the new wall out at the bottom which does act as a toe, I guess. The only other idea I've seen is doweling rebar up into the existing footing, but that idea has been criticized by others.

@kissymoose, that's why I've been supporting building a whole new 8' wall in front of the existing footing..

My initial thought was to do it like you've described, but that idea has been roundly criticized in other threads. Underpinning has been suggested in all cases as the best method to take care of this problem, but as kissymoose said, no generally accepted solutions for the overturning of the new underpinning wall have been given, plus the issue of waterproofing. I'm kind of on the same page as you regarding this project, but it really gives me pause when many of the veterans here disagree with the idea of building a new wall on the inside.

 

[Greenalleycat]

If I was going to do the 'underpinning' option, I would be doing as you've mentioned and drilling into the bottom of the footing to try give some moment capacity of the joint
But I would need good confidence in the connection between the footing and the existing foundation wall above if that was the case
And, again, I don't see how any reliable waterproofing system can be achieved with this method

 

[PEinc]

Jayrod12, I use the example of an old rubble stone wall because it is usually a thin wall, no footing, questionable mortar, with usually just 3 to 4 inches of embedment below a usually 2 to 3 inch thick, lousy, unreinforced concrete, basement floor slab. An underpinned crawl space wall is probably no worse than a stacked stone gravity wall. The weight of the foundation wall, underpinning, and building load on the underpinning help prevent overturning and sliding. The building DL and LL are stabilizers for sliding and overturning. Conservatively ignore LL when checking sliding and overturning (LL may not always be there).

http://www.PeirceEngineering.com

 

[jayrod12]

Thanks PEInc. I assume then you are essentially relying on friction to transfer from the existing to new.

I guess I'd need to run the numbers a few times to get comfortable with it. I assume you've likely ran those numbers often enough to have a good feel for what typically works.

 

[le99]

I suggest the new slab and the new wall should be made continuous. Drill rebars to connect the existing and the new wall. You may need to come back to perform pressure grout to close the gap. The waterproofing shall be applied on the exterior face.

 

[PEinc]

jayrod12, most often, friction due to DL only is considered for resisting sliding of the existing footing off the underpinning. On occasion, I have used dowels. Don't forget to properly daypack between the footing bottom and the top of the underpinning.

On occasions, I have had to either tie back or soil nail a wall above the underpinning when there was too much lateral pressure on the existing wall.

http://www.PeirceEngineering.com

 

[LittleInch]

Ok, I'm no structural engineer, but can't you just do both?

I.e. take your first drawing at the top of this post and simply extend the foundation down to the same level as your new basement on the other side of a new wall which you can reinforce and tie into the new floor slab and the sides to provide your lateral support to the new underpinning?

Then you can seal the new under pinning before building your new interior wall all the way to the top and avoid water ingress?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[phamENG]

My typical solution (if it isn't stable on its own) is tie backs in the underpinning with a wale across the face. If it's a finished basement the wale can be covered. If it's not a finished basement, then who cares?

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.

Granted, I don't have to deal with these very often. There are a few areas near here with some hills where it can come into play, but it's uncommon. Most of our "underpinning" jobs are due to horrid soils and we're just trying to keep the building from sinking into the swamp. That's where helical pile underpinning can be useful.

 

[bookowski]

Apache - There's some good info in this presentation (along with info irrelevant to your situation). This type of underpinning is very common here. It sounds like you understand the potential problems. Without much dead load you'll need to find another way to brace that wall (run the calcs but you'll likely find the hinge is an issue as you've suggested) - internal walers or rakers (or both), a "liner wall" inboard, tiebacks (unlikely in small resi job), etc etc. Some people do tie adjacent underpinning piers together with rebar driven through the side of the forms of each. If you tie them together this way you can calc out how often you need to restrain it. Maybe you can do some vertical (new footing to u/s of 1st floor) elements every so many feet.

https://www1.nyc.gov/assets/buildings/pdf/NYSSPE Presentation Underpinning.pdf

 

[PEinc]

The lateral load on the underpinning and existing foundation wall is usually analyzed on a per linear foot of wall basis. Therefore, dowels between piers should be necessary only if you have some type of unbalanced loading on isolated piers. Generally, the piers all stay stable or they try to move together as a group.

www.PeirceEngineering.com

 

[Apache1]

Appreciate all the responses. I've got three options here that have been proposed that we're considering. Obviously not a finished product but I think they give the general idea of what's been discussed.

As bookowski mentioned, I don't think we're going to look at using tiebacks on this project as it is a small residential job. We'd like to find a solution that doesn't require tiebacks.

Option A - Along the lines of what PEinc proposed. To get the overturning strength needed I've extended the toe of the new wall into the basement. le99 suggested waterproofing on the outside.

Option B - Suggested by Greenalleycat. In my mind this solves the waterproofing issue. The 3' toe at the bottom resists the overturning.

Option C - My original suggestion, and an idea that I've seen in other threads. This option is unacceptable to many engineers, but I haven't really seen an explanation for why.

LittleInch's suggestion sounds to me like a combo of Option A and Option B. Provide an underpinning wall (without the toe extension) and then put in a new full height wall next to the underpinning.

None of these options will require doweling into the existing footing since the toe extensions at the bottom provide the overturning/sliding resistance needed. PEinc seems like the resident underpinning expert here so I lean toward Option A. Option C would be least expensive. Option B seems unnecessary if Option C works.

 

[phamENG]

B and C would have to be built in alternating segments just like A, so waterproofing is far from a done deal with either of them. If you try to strip it all out you'll probably have failure in the soil below the existing footing and the building will come down. (See page 5)

You'd either need to do interior waterproofing or line the backside of your form with bentonite sheets (look up "blind side waterproofing") and use water stops between piers. Of course they'd have to be a hydrophylic strip (like Sika Swellstop).

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

 

[PEinc]

Thanks for the Page 5 link, pharmENG. I do a lot of projects with Keast & Hood. Great firm. Nice failure video also.

Apache1, I would not do Options B and C. The floor joists are parallel with the wall and are not sufficiently bracing the foundation wall. In addition, you would need to build these L-shaped retaining walls, which are NOT underpinning, in short sections so that you don't collapse the wall as mentioned in the following response from bookowski. Even if you could build the L-shaped sectional wall, you should calculate how much deflection or rotation the new wall will undergo. Rotation, deflection, and settlement mean building damages. Also consider the safety and legality of building short, deep, L-shaped, wall sections in unshored pits.

They were lucky that the soil was very stiff. However, the City and OSHA shut down the project because of the underpinning. The underpinning contractor claimed to have done an extensive number of underpinning projects. I told him he is a very lucky guy.

HOW NOT TO EXCAVATE FOR UNDERPINNING

http://www.PeirceEngineering.com

 

[bookowski]

This option is unacceptable to many engineers, but I haven't really seen an explanation for why.

My 2 cents on why I wouldn't do B or C. To me the biggest component of underpinning is execution. You are removing loaded soil below a footing (and I consider immediately adjacent to be under). It's a situation ripe for bad outcomes. How would you execute B or C with confidence - some soil will be lost, are they going to grout back into those voids? Even if you can get them to be careful and replace lost material it's now been unloaded. To get reloaded something needs to move. Doing this over and over around your perimeter seems like a crap shoot. I'm sure it has been done but it'd give me nightmares. I'm typically seeing underpinning on old crappy masonry buildings, maybe on a house under renovation you've got a little more stomach for some movement.

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, or better yet some preload. I typically see at a minimum rammed drypack in the gap between the u/pinning (stopped a few inches short) and the footing, more often I see wedges driven and then drypack, on some jobs/engineers I see jacking in pockets at every so many pins. You are handing off the load from one spot to another, you want to do that with as little movement as possible.

 

[Greenalleycat]

I have been advocating to do preliminary underpinning at discrete locations first, taking the loads down to below the depth of the new foundation
This ensures no surcharge of the house on the new wall and gives you reasonable confidence in the house foundation support during construction
Some lateral propping would probably be required but at least you don't have to worry about the whole footing caving in when the soil falls out
I don't really see how you build the continuous wall under the existing footing...pour the foundation first with starts out then try stick some temporary props under the foundation while your pour the wall?

 

[bones206]

The issue I see with Option B & C is that the existing footing will be imposing lateral surcharge pressure on the exterior of the new wall. That just exacerbates the overturning force on the new wall.

 

[phamENG]

Greenalleycat - it's usually done in short segments about 4 feet wide. Click the "see page 5 link" in my post above - it's a good primer on underpinning for architects and discussing the more common methods used.