Existing Retaining Wall 1

[SteelPE]

I have a client who is looking to add onto their facility. Along one wall of their existing facility they have a 12'-0" tall retaining wall where the grade is higher on the outside than the slab on grade. The client is looking to place the addition along this side of their existing facility. The issue I have is that they plan on making the slab on grade for their addition 16'-0" higher than their existing slab on grade and they need to add 4'-0" in height to the existing 12'-0" tall retaining wall.

I do have the existing drawings and it looks like the wall was efficiently designed for the 12'-0" height (a bit too efficient for my liking, but one could argue I am a bit conservative). I have told the client over and over again that this is not a great idea. That they should place their new slab on grade at the top of the existing wall (12'-0" above existing slab on grade). I have run some preliminary numbers and it looks like they will be doubling their overturning load on the current wall with the added height.

How would you attempt to reinforce this existing wall for the additional 4'-0" of soil surcharge? I am at a loss on this one. Other than placing a new 16'-0" wall behind the exisitng 12'-0" tall wall I just don't know how you go about adding 4'-0" to an existing retaining wall.

 

[phamENG]

Some sort of tie back system at the top and carbon fiber to the face to give you adequate positive moment capacity? You'd need to then protect that from sunlight and UV degradation, but the bridge and infrastructure repair industries have products for that.

 

[SteelPE]

phamENG I can't say that I have ever designed carbon fiber reinforcing before.... and I am not sure how a tieback system would work when it seems like you would need to backfill the area in order to install the tiebacks (or maybe you construct the tiebacks 25-30' away from the wall where you can have a higher grade and run tension ties to the top of the wall.

I am also not sure how accepting the tenant would be to installation of this inside of their space. I didn't mention this before, but the existing building tenant stores medical grade materials in the building (sugar, salt etc) and they were not thrilled about some snow drift reinforcing that was proposed in the existing building.

I just had an idea of using geofoam to fill in the extra 4'-0" of height. Not sure if this would be feasible or not. I have asked the contractor to investigate this idea. Hopefully the results will be positive. I believe this would virtually eliminate the surcharge load on the backside of the wall.

 

[GeorgeTheCivilEngineer]

Dig out and replace some backfill (and up to the proposed level) with a very lightweight backfill such as EPS blocks.

 

[phamENG]

EPS/geofoam is another possibility as long as you're well above the water table and you don't have to worry about flotation.

I may have misunderstood the arrangement when I made my suggestion.

 

[JoelTXCive]

You might be able to add the 4ft.....

How thick is the wall and do you have steel on each face?

If so, then you can possibly add a tie-back & deadman in the fill behind the wall. This will flip the tension steel from the back face of the wall to the front face though.

Another idea is this: Do you have any extra space available on the site? You can construct a new 4ft tall wall that is offset back 10ft or so from the existing wall. Then get the geotechs to do a global stability analysis for the new terraced wall setup.

In the 'crazy, but still doable category', you can consider this... You can add vertical stainless steel straps to the front of the front of the existing wall. A now retired guy in my office did it ~15 years ago and the wall is still standing. I'll try and find a photo. He was thinking outside the box for sure.

 

[structSU10]

you could place permanent tie backs (steel rods) back to dead men that extend into the new building enough to be outside the active soil wedge. These could be installed at the existing grade level and be a thru bolt and plate on the inside face of the existing wall. Maybe the deadmen can line up with new interior columns and act as the footing.

 

[JoelTXCive]

I call this the Frankenstein wall. It was broken back and raised with a deadman & stainless straps about 15 years ago. Then, about 10 years ago the levee was raised so the terraced upper wall was added.

 

[JAE]

Is the 12 ft. retaining wall a part of the foundation of the existing buildings exterior wall? Trying to follow your description.

 

[StrucPatholgst]

I am a firm believer in not trusting an older wall to bear additional loads. And I would never stake my reputation on adding 4 feet of soils and the surcharge of a new building on someone else's wall.

If it were me, I would design what is actually needed, and try to adapt that to existing conditions. But in no way, shape, or form would I ever stack a new wall on top of an old one.

 

[JoelTXCive]

If you have as-builts or can conduct testing to ascertain the existing conditions. Then, structural analysis can be performed and raising the walls is possible in my book.

If you can't ascertain the existing condition; or you have a client that doesn't understand that you will be relying on the as-built plans for your current analysis; then I'm with StrucPatholgst and would not raise the wall.

 

[StrucPatholgst]

I agree with that. About 20 years ago I was a part of a project at an industrial site where an older sulfuric acid dryer was to be replaced with a newer larger one. The new tower was 40 tons heavier than the older one when fully operational. They wanted to re-use the existing elevated concrete base support as-is. We convinced them to x-ray the existing concrete and recalculate. Not only did that trigger a substantial reinforcement design, we also were able to locate stable concrete into which new anchor bolts could be drilled without severing any existing rebar. I personally felt like that entire assembly was mine, after all of the data came in. IMHO I think that's the level of rigor one needs to achieve in order to meet the standard of care.

 

[BAretired]

One possibility is to make the new "slab on grade" a structural slab with a four foot crawl space and leave the depth of soil twelve feet. In that way, you are not affecting the soil pressure on the existing retaining wall.

 

[SteelPE]

Thank you for your help. To cover themselves, the contractor is pricing construction a new wall behind the existing wall. This new wall would be designed to support all of the wall load and pressure.

Attached is the existing wall section. I know this EOR... but haven't talked to him in over a decade. From what I can tell, this wall is incredibly efficiently designed (a little to efficiently for my liking).

 

[SlideRuleEra]

To cover themselves, the contractor is pricing construction a new wall behind the existing wall. This new wall would be designed to support all of the wall load and pressure.

From what I can tell, this wall is incredibly efficiently designed (a little to efficiently for my liking).

If distance between new wall and existing wall is less than 24 feet:

"There is no free lunch."

 

[BAretired]

I propose a different solution. See cross section below.