Bad Existing Footing Condition

[Lion06]

I have an existing building where the owner wants to make some relatively minor modifications to an existing CMU wall. During the site survey I went into the basement to look at the condition of the CMU in the basement. I encountered a condition that made me very nervous and has become the focus of my effort on this project.

The attached sketch shows the existing condition (in red) and my current thinking on how to address it (in blue). Basically, at some point someone dug part of the basement lower to pour a slab on grade and, in the process, left the footing exposed on one side with soil below the footing sloughed down. My current thinking is to pour a mass of concrete around the exposed soil to confine it and drill into the slab on grade to restrain it from sliding.

The building has been this way for at least 30 years, but now that I'm involved in the project I feel it's necessary to address this condition in its entirety. Has anyone encountered this type of condition before and how have you handled it?

 

[CELinOttawa]

Hello Lion,

I have seen this type of "new" footing before, and though I dislike it intensely, it is sometimes all you can do. I wonder if you need to staple into the existing footing, or at least roughen significantly... Otherwise I don't see the detail working other than based on gravity and the (very) minor tension capacity you can get from the new chemical anchoring of your vertical rebar.

FYI: Something similar is common residentially around Toronto for deepening old basements. It is even included as an "approved" detail in the TACBOC Standard Details book [see B01c].

 

[SkiisAndBikes]

Lion,

Sometimes referred to as a Dutch or Bench footings. Yes, City of Toronto has a standard detail for something similar. Like CEL, this is not my preferred method to address this situation. I would look at phased underpin of the footing, also contained in the TACBOC detail book.

 

[oldestguy]

You are not really making the best use of your new concrete. It needs to be dowelled into the old footing some way. As it sits, it provides a little resistance to the soil below the old footing shoving it out some and then settling. In the process of doing the concrete work set some PVC or even downspout pipes thru the new concrete pour connecting the soil below the footing to daylight, After your concrete has set. come along with a concert pump and force some concrete into that zone below the old footing to attempt some compactive effort on probably somewhat loosened soil. So you can see, with that pressure your added concrete has to have a secure bond to the old concrete, not just a roughened surface there.

I've done this at least two times that I can recall at age 86 and a concrete pump works fine. You don't need a grout pump. Just be very careful and immediately stop the pump if any problem shows up. I found it works to lift a sunken slab also, if that is present.

 

[Lion06]

Thank you for the feedback, gentleman. It's good to know that is an approved detail somewhere. I was considering pinning it to the existing footing, so given the feedback I will probably do that. One thing that wasn't clear in the sketch is the existing footing is not formed quite as well as I showed it. It's a very rough vertical face.

I like the idea of the pvc and pumping concrete in to compact the soil, but I'm not sure how I would convey the criteria at which they should stop pumping. I won't be performing the work, and I can't put a note on the drawings to "be careful and stop pumping if any problem shows up".

 

[oldestguy]

Perhaps state "The zone of earth under existing footing shall be compacted with a Portland cement grout to the extent that loose zones are compacted. The work shall be under the supervision of a person qualified to inject grout and shall have insurance covering possible damage from the work."

Or you might specify that the earth there should be compacted from the side immediately before placing concrete in the new work, progressively along the existing wall.

 

[msquared48]

I do not like the detail either, for many reasons, even though the statics could, technically, work, but under very limited conditions.

Were there any limiting backfill, earth slope, foundation drains, water table elevation, or height above slab dimensions to the "approved" detail in Toronto? Without these, and similar, specific limitations, I would never use such a detail, approved or otherwise.

Mike McCann, PE, SE (WA)

 

[bookowski]

Similar details are very common in New York to lower the cellar of rowhouses (referred to as bench footings here). The soil cut is usually specified between 1:1 to 2:1 depending on soil, and ideally the excavation and bench footing is done in sections to minimize disturbance.

 

[bookowski]

Found this article online showing typical benching of a footing in NY. 'Footing' is a bit generous in this condition as this is just a rubble wall that extends below grade. Same idea though.

 

[apsix]

I would dowel the new 'wall' to the footing and also undercut the footing by 100mm or so to get some positive support along the edge.

 

[Lion06]

I decided to dowel into the existing footing at 36" on center, into the slab at 24" on center and have them compact the soil immediately prior to pouring the concrete.

Thanks for all the input, guys.

 

[CELinOttawa]

Mike: It gets worse... The purpose of the TACBOC document (in which the detail appears) is to approve common details for builders and home owners. It does not have the types of logical and reasonable limits you're thinking of, and the use of the detail means the Building Officials often don't even require an engineer's involvement! *sigh*