retaining wall repair approach

[vato]

I have a poorly placed cantilever concrete retaining wall, 12" thick, 10 feet tall. The wall has a "cold" joint/pour line horizontally across it's entire length ranging in height from the bottom to a couple feet below the top. 2 layers of # 5 at 16" o.c. Not backfilled yet.
It is a remote location and demo/replacement are not an option. I'm thinking of another 8" concrete wall on the soil side with #5 at 16" ea way. I'm fine with the analysis side of this problem, hopefully. My concern/question is how to place this wall adjacent to the existing wall. Should it be bonded continuously at the concrete surfaces (which I think would be ideal), and/or doweled to the existing at some interval. (Basically a vertical cold joint between the walls where I need to transfer shear) My concern is the new wall shrinking away from the one I'm trying to help, making it difficult to actually perform as a 20" thick wall when complete.

 

[KootK]

I'm not clear on why the wall requires reinforcing. The cold joint is easily checked using shear friction and is unlikely to be a problem at all.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ron]

KootK...you don't have the whole story....

vato posted in another forum about 10% entrapped air in the concrete with a unit weight (density) of 135 pcf.

10% entrapped air is practically impossible unless there is honeycomb (lack of consolidation). I don't consider that to be entrapped air, but a significant lack of consolidation. He/she apparently has this condition.

Given that, he/she has a durability problem, not necessarily a strength problem. His/her compressive strength tests from cores came up to design strength, yet that does not address the durability issue.

Placing another structural wall behind the original will provide an additional structural comfort, but there are ways to handle this otherwise (epoxy injection, bush hammer and replacement, etc.) I don't know the local capability but that's what we would do here in a similar situation, right after we kicked the concrete sub off the job! If the OP is in the US, this should not be a big issue.

 

[KootK]

Ah, then I guess I'd need to know more about OP's intent with the backup wall. Is it to essentially replace the flexural bars with new reinforcement in a durable second wall? If so, I'd recommend:

1) flexural shear connection between walls with dowels for VQ/IT stuff.
2) careful attention to the connection between the new wall and the footing.
3) include lots of horizontal reinforcing in the new wall to control crack widths.
4) maybe match existing wall contol jointing.

Ron's proposal sounds like the way to go if, as he says, local capacity exists to execute it successfully.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[vato]

Thanks Ron and Kootk,

Really, thank you very much. I'm in a small town and the only structural engineer. Discussions on this forum are invaluable to me.

I was trying to simplify my questions by separating the issues on the wall. What I really need to do is separate myself from the project at this point, but that's not happening, yet, so....

It's a residential, owner/contractor neighbor friend hiring his own subs. I told him not to let the guy back on the jobsite immediately after seeing the forms pulled. And of course, I'm the only one concerned. The building department does not inspect final pours, only rebar (something that seems ridiculous but is very common to my surprise) The "professional" relationships here are messier than the concrete pour and should be another thread entirely. I am not invested financially in the construction or real estate side of this project so no conflict of interest there. I am being very careful not to assume any responsibility beyond my original design. But, since I raised the red flag as the EOR, it feels like my responsibility to salvage this mess somehow and give direction to the owner/contractor. So back to the wall.

I am attempting to resolve the issue without removing the existing wall. It seems like it would be easier to place another wall adjacent to this one than it would be to remove the honeycomb to good concrete and replace with non shrink/epoxy concrete, but maybe that's not correct. The "cold" joint is an optimistic description considering the honeycomb at the bottom of the pour on top of the "joint". The other side of the wall shown in the picture has more honeycomb and the "cold" joint is just as pronounced. Yes to freeze thaw possibility, it's 10,000 ft elevation.

The wall jogs so there is some redundancy in the system fortunately, since the walls actually span two directions, with two layers of rebar. So there is some capacity above zero for the existing construction. That's why the additional wall seemed reasonable to take care of strength issues and remove a majority of the assumptions required to justify any significant capacity of the existing wall.

The assumptions required to justify capacity feel like they are beyond reason based upon my experience.

So, after typing all of this, I think my recommendation is going to be:
1. Remove and replace
2. Removal of the honeycomb to sound concrete and repair with a non shrink/epoxy concrete mix.
3. Install 8" reinforced concrete wall on the soil side with sufficient shear reinforcement doweled between the new and existing per my new design.

There is a complete understanding on the owner's part that waterproofing has now become super critical to the performance of this wall.

I'm trying not to run away from this and help find a solution. I look forward to everyone's advice. Thanks.

 

[TehMightyEngineer]

Demo and repour. It will be cheaper and faster than a repair.

I had a similar project involving extremely long delivery times for concrete where the contractor pulled the forms and found similar results as yours. Contractor and EOR determined that a repair was the only feasible way to go. We were hired to design the repair and work with the contractor to implement it. We cored the wall at many locations and verified the structural capacity at the joints and honeycombed areas. We filled the cold joints with a high-strength grout. Sealed the joints. Wrapped the interior and exterior in a waterproof membrane. And finally developed an inspection procedure for them to follow to ensure the repair wasn't compromised. All said and done, I wouldn't do it again this way unless it was 100% impossible to repour. Though, my wall looked much worse than yours so perhaps it's not going to be quite so bad for you.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

I was imagining an isolated, cantilevered wall rather than a basement wall. The basement wall condition makes me a lot less concerned, particularly if below grade waterproofing is being looked at carefully.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.