Retaining wall repair rec

[MAEngr]

I am evaluating an exterior retaining wall that a contractor built to prescriptive codes. They came back after inspection and added two feet of additional retaining height which means insufficient rebar in the wall and inadequate footing sizes. The wall is starting to bow in the middle. The owner would like to not demo and rebuild the wall. My thoughts on repair would be to add a haunched wall butted against the existing wall with dowels and to add additional concrete/rebar to the footing. Is this the approach you all recommend? They would also be open to a buttress wall in the middle on the exposed side since that side is downhill and not visible. I don’t like this option as much.

 

[JAE]

We've used EPS foam blocks to deal with this in the past. Always worked out great - used about 3 ft. of soil/gravel on top with a thick membrane on top of the blocks. EPS will melt if exposed to petroleum substances.

 

[BridgeSmith]

One other thought I had was to excavate 9 feet of soil behind the wall and replace with geofoam to help greatly reduce the stress on the wall. While the soil is excavated I would also have the contractor epoxy any cracking on the heel side of the wall. This seems like a viable solution to me that doesn’t involve tearing down the wall. Thoughts?

If you can excavate behind the wall, the geofoam option can probably be made to work, but you consider reinforcing the soil mass - creating a mechanically stabilized earth (MSE) wall behind the block wall, also. We typically have the internal design done by the geogrid supplier, so we only deal with the external stability design.

 

[MAEngr]

How do you guys recommend attaching a concrete buttress wall spaced a certain distance attached to the toe side open face of a CMU retaining wall? Would it be as simple as dowel and epoxy into the cmu wall with your horizontal buttress wall bars or to attach angles to the corners where the two walls meet and use titen bolts? Or other recommendations? The cmu wall blocks are 8” wide.

The buttresses will be on a steeper hillside so the footings for them will also be significantly lower than the cmu retaining walls footing.

Here is an elevation view and a section view. the sketches aren't complete but show applicable info. The contractor is pretty adamant about not wanting to excavate on the heel side. I like this solution the least, but they want to price it.

Right now i am using code minimum soil values in arizona, we will have a geotech if this repair rec is picked. The CMU is solid grouted, so should i spec the horizontal bars to extend 6" beyond the buttress so that they can dowel and epoxy into the face of the cmu wall?

More context is this was was constructed to a 6' prescriptive design, but final construction has this wall at 8' of backfill so it is stressed and bowing 1-2". The buttress is to help alleviate the stress by providing intermediate support for the wall.

 

[MAEngr]

Another approach the contractor is open to is abandoning the existing wall and building a new 12" CMU wall on the outside face. This would require the footing getting beefed up to the appropriate width, and also a key to help with the sliding. Straight vertical bars doesnt seem like enough bar development into the footing, but is this a feasible approach?

 

[TRAK.Structural]

For the buttress idea - How are you going to excavate for the new buttress and footing 12' below the existing footing without the whole thing failing in the process??

 

[MAEngr]

That's a great question. I was going to leave it to the contractor's means and methods, but that could just be the bust i need to say this option isnt viable. I assume the contractor would try to provide some type of temporary shoring and add one buttress at a time excavating as minimal amounts of soil as possible until each are installed. but then again one at a time would end up taking months in order to let the concrete cure to its necessary strength so that seems like not an option. Any other ideas on how they would shore the wall until new buttresses were built?

 

[TRAK.Structural]

Any other ideas on how they would shore the wall until new buttresses were built?

Maybe it's possible, but on a wall that already has issues I wouldn't put my seal on a repair scheme like that, just my 2 cents tho.

 

[MAEngr]

The new wall built butted up against the front face of the existing wall, and increasing the footing width with a key seems like the best approach to avoid any excavation on the backside. However, the concern of how to attach the new wall to the existing is still present. It's a 12" thick footing. would something like #5's at 8" o.c. doweled and epoxied into existing footing with 8" min. embed be adequate?

 

[XR250]

The new wall built butted up against the front face of the existing wall, and increasing the footing width with a key seems like the best approach to avoid any excavation on the backside. However, the concern of how to attach the new wall to the existing is still present. It's a 12" thick footing. would something like #5's at 8" o.c. doweled and epoxied into existing footing with 8" min. embed be adequate?

I think you are making a mistake. I doubt coring 8"Ø holes, 5 ft- ish O.C. is going to have any effect on the existing wall. The repair you are specifying would be 20 times more costly if not more and way more risky. I'm not sure"means and methods" would keep you out of court on this one.

 

[MAEngr]

XR250, I am not following your response regarding coring 8" diameter holes at 5' o.c. I was talking about #5 vertical rebar being doweled into the existing footing at 8" on center. and the bars would be doweled 8" downward into the existing footing. i am wanting to connect a new CMU wall to the existing footing. The graphic above gives a visual of what i was referring to.

 

[XR250]

XR250, I am not following your response regarding coring 8" diameter holes at 5' o.c. I was talking about #5 vertical rebar being doweled into the existing footing at 8" on center. and the bars would be doweled 8" downward into the existing footing. i am wanting to connect a new CMU wall to the existing footing. The graphic above gives a visual of what i was referring to.

I am talking about helical tiebacks.

 

[MAEngr]

ah, thanks for the clarification. on that design repair rec i modified it since posting. i added back-to-back vertical c-channels at 4' on center so they align with the tiebacks being spaced at 8' o.c. plus one between each tieback, as well as back-to-back horizontal c-channels running from tieback to tieback. all of those channels should help pull the load from the wall into the channels and direct them into the tiebacks. i agree this approach is costly, and not my number 1 recommended choice. i recommended remove and replace, but they are looking for all potential options to price compare.

my last option i plan to offer is the new wall constructed in front of the old wall, but like i mentioned not sure about the vertical bars doweling into the existing footing if that would be adequate enough.

 

[XR250]

ah, thanks for the clarification. on that design repair rec i modified it since posting. i added back-to-back vertical c-channels at 4' on center so they align with the tiebacks being spaced at 8' o.c. plus one between each tieback, as well as back-to-back horizontal c-channels running from tieback to tieback. all of those channels should help pull the load from the wall into the channels and direct them into the tiebacks. i agree this approach is costly, and not my number 1 recommended choice. i recommended remove and replace, but they are looking for all potential options to price compare.

my last option i plan to offer is the new wall constructed in front of the old wall, but like i mentioned not sure about the vertical bars doweling into the existing footing if that would be adequate enough.

Sounds like overkill on the steel. If you install the tiebacks around 6 ft. O.C., I imagine you should be able to get a single MC6x18 channel flatwise running horizontal to work. That is usually what we do on basements around here.