Concrete Retaining Wall Retrofit

[JennyNakamura]

Hi everyone.

I'm working on a concrete retaining wall that needs to have its original flexural capacity almost doubled do to a change in use. The wall is about 13 feet high and currently supports an unused field overgrown with weeds and has been there for about 65 years. The land has been sold and is being developed for retail and the retaining wall will now be supporting traffic loads and parking area.

Long story short, need to thicken existing wall from 14 inches to 20 inches and add new vertical bars at 12 inches on center. I can dowel the bars at the bottom into the existing foundation, but how do I get composite action between the existing wall and the new wall to ensure that the "d" I use in my new flexural calcs is really the new total thickness?

I colleague suggested doweling in #6 bars 6 inches into the existing wall with 180 hooks into the new concrete at 12 inches on center (horizontal and vertical) and make sure they hook around the new vertical rebars.

Will this work? Is there any books or research documents that discuss this? I surfed the net for a long time today but found nothing.

Thanks in advance...

 

[hokie66]

In general, the scheme you propose can be made to work for the stem, but I am curious as to why you think the anchorage requirement is greater in the new concrete than in the old.

Is the new wall to be thickened on the tension or compression face? If tension, how are the bars to be developed in the footing? And how have you evaluated the footing? How big is the footing?

Another approach might be to use soil nails or anchors to avoid having to upgrade the concrete wall and footing.

 

[JennyNakamura]

Hi hokie66,

The wall will be thickened on the tension face. We need to excavate to retrofit the footing, so that will be a perfect time to retrofit the tension face of the wall. Can't use soil nails because the exposed face of the wall is right up to the property line, and the adjacent property owner is opposed to the development and will not allow access to his property.

The footing checks out OK for strength but needs to be extended for additional bearing and sliding resistance.

Plan to develop the new vertical bars by drilling and epoxying into the existing footing. The existing footing is 20 inches thick, so hopefully with 17 inches of embedment we can find an epoxy that will provide sufficient bond strength to resist the expected tension/pullout forces at the new rebar anchorage points.

Not quite sure what you meant by "...why you think the anchorage requirement is greater in the new concrete than in the old."

 

[hokie66]

I meant...it is hard to see that bars glued in 6" will have the same capacity as bars with a 180 degree hook.

Another way would be to turn the wall into a gravity wall like a Reinforced Earth wall (I think the common generic term is MSE). You would drill anchors into the wall, and connect the reinforcing strips to the wall as you backfill. That way, you do everything from the back side, but don't need to extend the footing, which in itself is not a simple task. Maybe you wouldn't even have to go all the way to the bottom. Probably worth getting a specialist involved. To make that work, you have to have adequate granular backfill material available.

 

[JennyNakamura]

Regarding the 180 deg hook and 6" embedment -- yeah I thought about that too...still thinking about it.

Regarding the reinforcing strips -- great idea, I will definitely look into that!

Thanks for the insight.

 

[dcarr82775]

ACI318 Chp 17 provides some direction on how to get composite action between the new and existing.

We have done something similar to what Hokie66 mentioned using geogrid a few times. Works fine.

 

[EngineeringAdam]

Composite action shouldn't be a problem - as dcarr82775 mentions. But just make sure you follow the ACI requirements (roughen the concrete (e) to interface, etc).

I would be most worried about the epoxy strength of your tension bars - I would imagine this needs to be checked via Appendix D of the ACI. Since it isn't a seismic application you can probably justify an analysis based on uncracked concrete which will help your numbers.