12" Concrete Wall Underpinning

[CorrectHorseBattery]

Hi everyone,

We have a project where we need to underpin the basement walls of a +-100 year old two storey brick house. The existing walls are approximately 5' high from existing SOG to u/s of joists and are believed to be 12" wide unreinforced concrete walls with no footing (based on some test cores). The client wants to lower the SOG 3' to make the basement usable. I do not know the soil conditions, but I imagine it is crappy 100 year old backfill beside the current wall and some kind of native soil below. I am familiar with designing structures but not underpinning specifically, but my colleague, who is designing the underpinning, plans to just underpin the wall 3' high with 12" of unreinforced concrete (4 foot segments, 3" of drypack grout, etc.)...

That wall thickness seems very thin to me for resisting the lateral earth pressure on the wall. Particularly on the two walls of the house parallel to the floors joists with minimal gravity load to keep the wall in compression. My impression was that underpinning was usually at least 2, 3, 4 feet thick? Have you guys seen this before?

Thanks!

 

[PEinc]

Your concerns are valid. The underpinning needs to be checked for bearing, overturning, and sliding. A light building (especially with joists parallel to the underpinning) coupled with thin 12 inch underpinning will not provide much resistance to sliding or overturning. Deep underpinning piers usually have minimum dimensions of about 3 feet x 4 feet so that a worked can fit inside the pit being excavated. If you can underpin from outside the building, you can make the underpinning more than 12 inches wide without encroaching into the basement. Bearing probably will not be the biggest problem. I suspect that sliding will be. Therefore, you may need to extend the underpinning a few feet below the proposed basement level to pick up some passive resistance. Making the underpinning wider than 12 inches outside the building and pouring concrete up to near the outside finished grade can help with both sliding and overturning. However you deepen the basement, make sure that you have proper grading outside the building so that surface water runs away from the building, not toward the building's foundation walls.

http://www.PeirceEngineering.com

 

[CorrectHorseBattery]

Thanks PEinc, I have seen a number of your helpful responses on this topic and really appreciate your feedback. That all makes sense. I will discuss again with my colleague and hopefully come to some resolution.