Rotated Segmental Retaining Wall 3

[PittEng88]

Hi All,

Our company recently received a submittal for a segmental retaining wall, that will be approximately 10-12 feet in height. Typically these submittals are pretty straight forward. However, the design engineer decided to rotate the entire wall, including the foundation base, 5 degrees on top of the 5 degree rotation he had from offsetting the blocks (see attached). Also, his calculations do not reflect this rotation properly, he is still using the equations as if the wall were vertical.

Besides the fact that his calculations are incorrect, can the wall even be constructed like what he is showing? Has anyone else ever come across something like this before?

 

[CELinOttawa]

Um, I'm no Geotech... But isn't that wall even safer when leaned back towards the fill?

What's your concern? Are you used to doing straight walls with a backfill and thinking this will get a traditional backfilling?

 

[dcarr82775]

Bit of a pain to build, but I don't see anything bad about it. There is geogrid right?

 

[PittEng88]

My concern is the global stability of the whole thing. I am no geotech either, but I feel that inclining the wall like that may cause the entire system to give way.

No, there is no geogrid. The engineer rotated the wall this way to avoid using it.

 

[XR250]

Seems risky to me w/o geogrid

 

[dcarr82775]

The tilt doesn't concern me for global stability. That tall a wall with no geogrid is what bothers me (unless the blocks are substantial)

 

[MotorCity]

If it does not look right, its probably not.

One issue I have with it is that if a void ever developed behind the wall (say due to ground water, erosion, etc.), a local failure of a single block or a few blocks might result in collapse of the wall. In other words, the wall relies on the soil for stability.

In traditional wall construction, a vertical retaining wall should not fail if there is no soil behind it (i.e. a free standing wall). I assume the blocks are not connected to each other in any way.

While it may appear conservative since the wall "leans into" the soil, I don't think its a good idea for the reasons I mentioned above. There is a reason things are built a certain way.....it works.

 

[EngineeringEric]

That is an 'interesting' way to handle the sliding shear between blocks, sliding uphill & friction. The lean into the wall again isn't a concern, the fact that the tilt was used to handle lateral earth forces is a concern, and on a 10' wall. I am not sure what mechanism is being used to handle the loads (besides self weight and friction). If you are not comfortable with the calcs and it seems suspect, reject it. If the calcs seem to work but you don't like it as a concept then call the Engineer. He/She should be able to convince you that the design is adequate with a factor of safety and has longevity.

And as a side note: I think that is highly un-construct-able and the requirement for 5 degree tolerance is something only to be achieved on paper... What happens when contractor installs at 4.7 degrees? or over time it tilts away from the wall, progressive falling-over imminent?

 

[CELinOttawa]

I agree with the (forming) consensus that such a wall should absolutely have Geotextile between the blocks, or have the blocks rebar anchored and grouted together. I've seen both, but regardless of which gets used, it is not a simple leaning stack of heavy blocks. Personally I wouldn't do that for anything above four feet.

 

[PittEng88]

@MotorCity, that's a great point. There are shear keys on the blocks to help connect them. However, if a void does form like you mentioned, those shear keys won't be much help.

@EngineeringEric, that's exactly what I was thinking when I first looked at this. I that he is relying too heavily on that rotation to counterbalance the lateral forces and like MotorCity said if a void would happen to form behind the wall that mechanism is gone.

This is actually the engineer's 3rd submittal for this wall. The first one was just horrendous, it was riddled with mistakes and errors. For the second one, he submitted his calcs with his bearing pressure way over the allowable. Now he is trying to reduce down his bearing pressure by rotating the wall.

 

[precast78]

If it is a gravity segmental wall, you will need way thinker wall than that.

 

[EngineeringEric]

And that is sealed? all three? Well stick to your guns and keep that reject button checked. Sorry that you are going to be blamed for slowing down the project and other non-engineers will blame you since the design is from an Engineer.

 

[PittEng88]

Yup, all three were sealed! My boss actually ended up calling the designer to see if he could shed some light on his thinking. When my boss told him that his calcs do not necessarily follow the Segmental Retaining Wall Design Manual from NCMA, the designer responded by saying that the manual is more of a guidance as opposed to an actual design code. He also stated that he has designed numerous walls this way and that they are all fine.

Even if the Segmental Retaining Wall Design Manual is not considered an actual code, to me it is still an accepted standard of practice and should be followed.

 

[CELinOttawa]

There is always some bonehead willing to do it on the cheap, and produce substandard garbage. I'm forever stunned at some of the stuff I see...

 

[MotorCity]

......and let us know where the walls are located that they designed, I want to make sure I stay at least 12' away (I think that was the wall height, right?)

 

[hokie66]

I agree that the wall probably doesn't work, but not because of the slope. It can be constructed that way, and taken to the limit, the blocks just become slope stabilization.

 

[PittEng88]

Well this wall is for a housing development in PA....but anyway I digress. Thanks everyone for your input, it was very helpful!

 

[PEinc]

PittEng88, you should not post the same question in more than one forum.

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