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.

 

[phamENG]

PEinc - thank you for sharing that. I find it interesting that you're removing a portion of the footing to get a smooth wall. I suppose it would be "simple" enough to resolve that...but I haven't seen it done before. Ever have issues with the contractor removing too much? Ever specify a grinding of the last 4 inches or something like that to make sure that you get a relatively smooth and workable surface?

 

[Greenalleycat]

Thanks, PEinc, interesting to see some details in action. That looks more similar to details underpinning we do here. What was the purpose of that job - were they creating a basement space or just trying to level off ground on the lower side of the wall?

 

[JLNJ]

Back in 2004 Summit Country Day School in Cincinnati had a partial collapse of a 4 story school building (super lucky that it was on a Sunday and unoccupied). The contractor had excavated next to the building but did take into account of the lateral earth pressure behind the grade wall. The grade wall "kicked" out and the exterior bearing wall was lost. The people involved figured that the vertical load had a load path, so all was well. They failed to take into account that the horizontal pressures also need a load path.

Needless to say, lawyers were called to the front lines.

https://www.supremecourt.ohio.gov/pdf_viewer/pdf_viewer.aspx?pdf=621450.pdf

My point is: don't underestimate lateral earth pressure.

 

[Apache1]

100% you put steel in this, even if just for shrinkage sake.

Yeah, my intuition is telling me that there should be some rebar in the wall that ties to some dowels into the existing footing. PEinc, I hear what you are saying about usually not needing rebar due to dead load and the mass of concrete, but it seems that on many projects the underpinning piers are quite short (3' or less) or they are super tall, in which case tiebacks are used for lateral resistance. My wall height is around 5' tall and won't be using tiebacks. My pier is 20" thick though so I'm not sure where the rebar would be best placed. Single mat in the center could tie to dowels into the footing above, but probably wouldn't help much with shrinkage. Double mat offset 3" from each face?

Thanks for the detail, PEinc. Most of your projects sounds like large structures where a ton of dead load will be present to eliminate some of the things that I feel like I need on my detail (extended footing toe for overturning and bearing, rebar, etc.) as it's residential with essentially zero dead load.

JLNJ - Wow, thanks for posting. Sounds like they were digging for "an adjacent foundation" (per Wikipedia). Wish I could find more info about where the new foundation wall was being built, how tall, where they were digging, etc.

 

[PEinc]

Greenalleycat, the job was to build truck loading ramps and docks. Had to ramp down and provide the required frost depth. We do not usually need to bell out the front but they needed to trim the front of the footing to let the trucks get closer.

http://www.PeirceEngineering.com

 

[PEinc]

I try to make sure the contractor knows to remove only the piece of footing toe immediately within the underpinning pier that they are excavating at that time. Do not remove the entire footing toe or you could settle the building. Never had a problem - yet!

http://www.PeirceEngineering.com