Retaining Wall Failure?

[USFEngineer]

I have a question regarding a masonry retaining wall that "appears" to underdesigned. Without taking the wall and footings out are there any ways to retrofit an existing masonry retaining wall?

The retaining wall is built 100% but has yet to be completely backfilled (approx. 75%). The contractor has already expressed his concern about the wall. Apparently the wall is showing signs of failure and he has raised the question about the design.

The wall was designed by an older engineer that I am aquainted with and he has contacted me to assist with any possible repairs/retrofit options.

Besides tearing the wall down and starting over I have very few ideas. Possibly adding another 8" cmu wall to the inside of the existing wall and adding to the existing footing?

The wall is at max. 6'-8" high and is 8" cmu w/ (1) #5 vertical @ 24" o.c. It is directly adjacent to a parking lot and driveway for a gas station in Florida. So there will be heavy cars/trucks. I have used the 250 psf surcharge over the heel of the footing as per AASHTO reg's.

Not only does the stem seem to be overstressed but the footing also appears to be undersized.

How can this issue be resolved without completely taking the wall down? Or can it?

 

[dik]

Is it possible to add 'deadmen' anchors and tie the wall back at the top?

 

[Qshake]

I think the key will be to make the masonry act as one unit which may be accomplished by post-tensioning. A concrete beam could be placed near the top to hold the vertical post tensioning and also hold the lateral tieback for the deadman if necessary. Once it's capped off, no one would be the wiser.



Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[USFEngineer]

Qshake

If a couple of the courses ontop of the wall were removed, and a concrete beam/cap was added, how would the post tensioning work? Would the existing verticals be used for the post tensioning or would additional strands have to be added to the wall?

dik

I guess it would be possible to use deadmen. But the top of the wall has a single k.o. block w/ (1) #5 cont. I would not feel comfortable relying on that for a tie back.

In addition, the footing itself is undersized. It should be at least 12" wider and 4" deeper.

 

[dik]

Is it possible to excavate behind the wall and use stabilized fill? so there is little pressure on the wall?

 

[Qshake]

It's not likely that the mild steel will be beneficial for tensioning but you shouldn't be looking for lot, just enough to keep the wall in compression through any lateral loading which might otherwise cause tension in the wall.

You might have to core into the openings where the mild steel is and anchor into the ground.

I'm not 100% certain but I think there are masonry applications for this. You shouldn't have to use a full blown rock anchor.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[eric1037]

Have you considered helical tie-back anchors through the face of the wall? The only trick is to get it to act as a unit, as was mentioned by Qshake. If the wall is fully grouted with reinforcement (and I assume it is), you could potentially use a system of walers on the front of the wall.

 

[DaveAtkins]

I haven't crunched any numbers, but conceptually speaking, you could pour a concrete wall against the outside face of the existing CMU wall, and pour a long toe footing on top of the toe of the existing footing.

DaveAtkins

 

[PMR06]

Can the parking lot slab be used to pin the top of the wall? Or, can more fill be added in front of the wall reducing the net backfilled height?

 

[jechols]

I had this same situation last month. A contractor built a 10'-0", 8" blk, retaining. He assured the owner that the wall was over-designed. Then the rain came and it failed. This wall was so under-designed that it failed between the temporary braces. The wall was supposedly reinforced with #4 each cell. However they tried to fill the wall with concrete in one big lift. The concrete didn't even make it past the first block in some places (I know this because I could see into the wall thru the 1" crack running down it). Also the contractor built the wall on a 2'-0" wide footing flush with grade. The solution we came up with was to add additional temp bracing and use the cmu wall as a form for one side of a new reinforced concrete wall. Also doweled into the in place footing and extended it.

Good Luck!

 

[USFEngineer]

I think the best solution may be to use the existing masonry wall as a form for a new concrete wall and pin into the existing footing to extend it. I have not seen the wall, but apparently it is in pretty bad shape.

The other issue and maybe it's not even an issue is that the contractor has been driving his heavy equipment right up against the wall. I am sure that it hasn't helped in anyway, but what recourse is there if this is the case. Is there any way to make sure that civil contractors do not use there grading equipment right against the wall. This has always worried me when designing retaining walls.