Retaining Wall Failure

[jheidt2543]

I've never seen a retaining wall failure this complete and thought I'd share a few photos with you all. The details are starting to emerge, but before I share them, I thought I would post a picture to arouse your curiosity. The nightthime failure followed a heavy rainfall; with approximately 200' of wall tipping over. I was not involved with the design or construction, just an interested observer in my home town.

This photo was taken the morning after and shows about half the wall looking West. I will post the original design detail and some soil information in the next few posts.

 

[jheidt2543]

Here's a photo looking East of the other half of the wall.

 

[jheidt2543]

And here is the original design detail. The upper wall is the one that failed, the lower all is still in place.

 

[BigH]

Impressive - looks like they backfilled with clay - the drawing didn't indicate the nature of the backfill? Would be interested to see what the house foundations are like - given your geographical location, they would probably be pretty deep, eh? I'll admit I haven't seen a retaining wall like this - other than when one of the RE walls in Calcutta failed due to basal shear. We did have a brick wall in our China complex fall over when we got hit by a flood - about 1.5 m of water on one side of brick wall and nothing on the other = topples (to be expected). I'll be interested to see the soils data.

 

[SlideRuleEra]

Very interesting. Looks like the western portion of the failure must have happened very suddenly since the sidewalk adjacent to the wall came more or less straight down. Perhaps a little slower failure on the east side, where the sidewalk tipped over with the soil under it.

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[dgillette]

Do you know what gave? I can think of several possibilities, but the first that comes to mind is bearing failure at the toe of the base, perhaps in fill that was not so well compacted, then softend by all the rain. I would also look at the #4s at 12".

Have to wonder about the effectiveness of the drains.

 

[JAE]

It appears in the second photo that the horizontal reinforcing bars are missing - a clean break from what I can see.

Did the stem fail or did the footing roll over?

 

[jheidt2543]

OK, Here is a close-up photo of the failed wall at the East end showing on the right side a construction joint and on the left side the fractured wall. No rebar visible either vertical or horizontal. Also, no waterstop in the construction joint (this will be noted in the next photo).

 

[jheidt2543]

And, here is a close-up photo of the wall section at about mid-height during demolition. Note vertical #4 rebar location. Also, look at the waterstop - there is no construction joint at this location??? Why they put waterstop here and not at a construction joint raises a question as to if the contractor new what is was for (I don't think it doesn't pertains to the failure, only the contractor's understanding).

Soil properties to follow later.

 

[dgillette]

I don't understand why the waterstop is there at all. Why not a strip of filter fabric to let the joint be a drain?

 

[Ron]

Back to JAE's question. It looks like a rollover...was it?

Looks too clean to be a shear or sliding failure.

 

[jheidt2543]

I'm still trying to get a copy of the soil borings so I can provide the soil paramenters for everyone. But, the wall rolled over, the footing stayed in place, virtually as level as the day it was poured. It looked to me like the rebar pulled right out of the footing.

If you look at the rebar in the last photo you will see it is in the middle of the wall not in the tension face. When I ran a sample calculation, I couldn't get the #4's at 12" of the original design to work in the clay soil (using average clay soil paramenters) and if the bars are in the middle of the wall, it is much worse.

I "redsigned" the wall and came up with #5's @ 6" for the bottom 24" of the wall, then #5's @ 12" for the rest of the way up the 8' wall.

Also, according to ACI, there is supposed to be two mats of bars in a 12" wall.

 

[JAE]

I think if you review ACI you don't need to put two mats in a wall where it is always compressed on one side...there are many cantilever retaining wall designs out there that don't use reinforcing on two faces. Someone might correct me on that but I think I've gone through that discussion before. I know that Chapter 14 requires reinforcement on both faces for 12", but I think there is an exclusion/out if it is a retaining wall.

They obviously didn't construct the wall to meet the design details.

 

[jheidt2543]

Just to finish out the discussion, here is what I have found regarding soil boring information from the borings done prior to construction.

Soil Classification & Average Blow Count from the
closest 9 borings:

4’ below grade SM 14.1

8’ below grade SM 18.9

Lab Analysis Results:
Moisture Content: 18%
Dry Unit Weight 116.1 pcf
Wet Unit Weight 137.2 pcf

Liquid Limit LL = 40
Plastic Limit PL = 23
Plasticity Index PI = 17


Sieve Analysis from closest three borings:

D85 D60 D50 Gravel Sand Clay

2.83 .306 .227 20.0% 56.3% 32.8%
1.94 .294 .205 7.1% 52.2% 40.7%
1.30 .341 .268 4.3% 65.7% 30.0%

 

[oldestguy]

My old Dutch born structural engineering prof made a statement that still sticks with me. "If they at least put steel in, that sometimes is sufficient, even if in the wrong face."

I have seen wall failures with severe leans with steel in the front face only, but they still held earth backfill.

It goes to show that a favorite statement of mine is true here:

"If something can go wrong in construction, it will".

 

[BigH]

and mine, "oldestguy, I just knew you were an optimist!"